Studying Coherence in Ultra-Cold Atomic Gases
byDaniel E. Miller
Submitted to the Department of Physics
in partial fulfillment of the requirements for the degree ofDoctor of Philosophy
at theMASSACHUSETTS INSTITUTE OF TECHNOLOGY
August 2006
c Massachusetts Institute of Technology 2006. All rights reserved.
°

Author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Department of Physics
August 10, 2007

Certified by . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Wolfgang Ketterle
John D. MacAurthur Professor of Physics
Thesis Supervisor

Accepted by . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Vladan Vuletic
Lester Wolfe Associate Professor of Physics

Studying Coherence in Ultra-Cold Atomic Gases
byDaniel E. Miller
Submitted to the Department of Physics
on August 10, 2007, in partial fulfillment of therequirements for the degree ofDoctor of Philosophy

Abstract
This thesis will discuss the study of coherence properties of ultra-cold atomic gases. The
atomic systems investigated include a thermal cloud of atoms, a Bose-Einstein condensate
and a fermion pair condensate. In each case, a different type of measurement is performed.
However, all of the experiments share a common tool: an optical lattice which is used toprobe these atomic gases.
In the first case, we use an auto-correlation technique to study the interference patternproduced by a gas of atoms, slightly above the Bose-Einstein condensate transition temperature. A moving optical lattice is used to split and recombine the single particle atomic
wavefunction. Analogous to a Young’s double slit experiment, we observe high contrast
interference which is well described by the model which we develop. When we address only
a velocity subset of the thermal sample, however, the contrast is enhanced and deviatesfrom this model.
In a second experiment we measure the coherence of a diatomic molecular gas, as well as
the atomic Bose-Einstein condensate from which it was created. We use Bragg spectroscopy,
in which atoms exchange photons with a moving optical lattice, transferring momentum tothe atoms. This process can reveal the velocity distribution of the sample as energy andmomentum are conserved only for a specific velocity class. Based on this measurement, wefind that the atomic coherence is transferred directly to the molecular gas. We also discuss
similar preliminary measurements performed on a fermion pair condensate in the BEC-BCS
crossover.
In a third experiment we study a fermion pair condensate into a 3D optical lattice. Sucha system shares many similarities with electrons in solid materials which exhibit superconductivity, and can offer insight into mechanism which result in this behavior. We infercoherence from the sharp interference pattern observed in the expanding gas, after release.
Finally, we study the abrupt onset of dissipation observed in a fermion pair condensate, as a function of velocity, in a moving optical lattice. We equate this threshold with
the Landau critical velocity, and take measurements throughout the BEC-BCS crossover.
The critical velocity is found to be maximum near unitarity, where the loss mechanism ispredicted to crossover from phonon-like excitations to pair breaking.
Thesis Supervisor: Wolfgang Ketterle
Title: John D. MacAurthur Professor of Physics

to our futureAcknowledgments
It would be disingenuous of me to dedicate my thesis to any one person or group of people.
One does not endeavor to spend five years in a dark lab out of love or friendship. Along theway, however, relationships endure and grow stronger. My wife Ariana deserves a great dealof credit for her patience as she has sacrificed the most for the production of this thesis. I
look forward to a long life of paying her back.
It is difficult to spend time at MIT without developing a sincere appreciation for theopportunity of being a part of something truly exceptional. Many graduates will reflect
upon the Institute as a place where they consistently faced the most challenging problems,
were exposed to the most novel ideas and got to play with the most cutting edge toys. My
most fond memories, however, will certainly be of the people I met here. MIT attracts thebrightest people from all corners of the globe and all walks of life. More often than not,they are engaging and stimulating people at a truly human level, defying the stereotype
that accompanies the fields of science and technology. But, even the weirdest of weirdos are
fascinating to observe in their natural habitat.
I was welcomed into the New Lab by the tough love of Jamil Abo-Shaeer. The firsttime I slipped up, I was told that I was so wet behind the ears that it made him sick. He
delivered the line with a straight face, but he was laughing all the way. Jamil was relentlessly
inclusive, and his sense of humor and fun created bonds among coworkers who would haveotherwise rarely spoken. I hope it is not presumptuous to think that he passed the torch
on to me, and that I have honored him by doing more than just standing around, holding
my torch. Aaron Leanhardt is a man who excepts no excuses. Whether he is working in
the lab or catching a Hail Mary pass, he gets the job done while making it look effortless.
Together, Aaron and Jamil defined the work hard play hard culture of the group.
In the New Lab we encountered all the joys and struggles of a family. It has been areal pleasure to have worked with Jit Kee Chin for so much of my time here. Our skills
are perfect complements, which is to say that I have the machine shop and Google Earthunder control, while she handles the rest. As a show of my appreciation, I have even spelled
her name incorrectly, the way she likes it. Yingmei Liu has been a great person to know,
and I will always heed her fashion advice when choosing the color of my hat. Widagdo
Setiawan possesses an exceptional raw talent. He has been a real challenge to know, and I

don’t mean that pejoratively. I am glad to have become acquainted with Christian Sanner,because otherwise I would have thought that he had nothing to say. In fact, quite theopposite is true. His curious insight into all matters stands in defiance of the anesthetic
properties of our dark noisy lab. Aviv Keshet has a great sense of humor, even when thejoke is on him. I’m glad I have not been able to corrupt his naı̈ve idealism, if only because
it is the source of so much amusement. I have not spent much time with the newly arrived
Ed Su, but enough to tell that he will fit in well and contribute much.
Our lab has also been fortunate to include two adopted members. We sought out
Claudiu Stan for his unrivaled expertise in design, and could not have refurbished ourmachine without him. Marko Cetina we found in the hallway, like a stray cat. Whenlooking for an audience at 4 a.m. to discuss his most recent discovery, he knew he could
always come scratching at out door. His fascination helped temper our tunnel vision.
Beyond the lab itself, our corridor has always felt like a community, which continues togrow with the addition of new research groups. Tom Pasquini was the first friend I made in
the group. We met at our first group meeting together, and soon our acquaintance becamecloser than I had anticipated. I have enjoyed watching our lives follow parallel but distinct
trajectories. When Gretchen Campbell graduated and left me her squalid desk, I was gladfor the mess. She earned it. Gretchen had spent her graduate career cheerfully cleaning
up after others. I would like to offer her eternal reassurance that whatever inconsequential
email she is frantically hesitant to send, will not be construed as offensive, inappropriate,
or otherwise detrimental to her career or friendships. Micah Boyd was the most reliable
source of one-liners in the lab. Since his graduation, I have often found myself asking What
Would Micah Say? with a smirk on my face.
Martin Zwierlein could have gone on to practically any university he wished. The advice
I offered him, to stay at M.I.T., was of purely selfish motivation, and I hope he will not
hold it against me. Andre Schirotzek and I appear to be at loggerheads when it comes to
Middle East policy. However, we have been able to share a number of lighter interests frommusic to snowsports, as well as some laughs along the way. When I first heard Christian
Schunck play the harp I wondered how such beautiful music could come from such a dark
and brooding individual. All joking aside, Christian is truly one of the most uplifting people
to be around, and I will always miss our romantic breakfasts together in Varenna. Yong-Il
Shin has somehow managed to partake in the lab antics while maintaining complete dignity,and our younger members will learn a great deal from him. I have appreciated Jae Choi’s
loyalty as a fan of Dan Miller and the Dan Millers. He may hold the distinction of being
the only group member to have put on a significant quantity of weight since joining thegroup ... at least in the form of muscle. If I have succeeded in teaching Caleb Christiansen
one thing, it to not believe a thing I say. Caleb, you have been a terrible officemate.
Sebastian Will was one of those few German diploma students who got away. It was
our loss, as his gregarious personality was sure to energize any party. Peter Zarth returned
to Germany as well. I am sure he is happier in a land which values good bread and is more
permissive of nudity. Andrew Grier seems like such a nice guy, until he takes your legs out
trying to pull some hot dog skiing maneuver. Jon Simon has demonstrated that he has
what it takes when brought all the dignity of a frat party to my thesis defense.
Wolfgang Ketterle has been an inspiring advisor who has always been able to find thesilver lining. Carol Breen has always kept our hallway well fed, and provided a sort of nonphysicist oasis at numerous physics gatherings. David Pritchard is a fascinating individual
who refuses to constrain his curiosity to matters of physics.
I have neither the time nor the space to thank everyone appropriately. Please accept
my apologies and my appreciation.

Contents
1 Introduction

13

2 Quantum Degenerate Gases

15

2.1

2.2

2.3

2.4

Bosons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

17

2.1.1

s-wave Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . .

17

2.1.2

Gross-Pitaevskii Equation . . . . . . . . . . . . . . . . . . . . . . . .

18

2.1.3

Thomas-Fermi Approximation . . . . . . . . . . . . . . . . . . . . .

18

2.1.4

Healing Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

18

2.1.5

A nonlinear wave equation. . . . . . . . . . . . . . . . . . . . . . .

20

2.1.6

Bogoliubov Excitation Spectrum . . . . . . . . . . . . . . . . . . . .

20

The Non-interacting Fermi Gas . . . . . . . . . . . . . . . . . . . . . . . . .

22

2.2.1

The local density approximation . . . . . . . . . . . . . . . . . . . .

22

2.2.2

Density Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

23

The Feshbach Resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

23

2.3.1

Feshbach Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25

The Strongly Interacting Fermi Gas . . . . . . . . . . . . . . . . . . . . . .

26

2.4.1

The BEC-BCS Crossover . . . . . . . . . . . . . . . . . . . . . . . .

27

2.4.2

Unitarity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

27

2.4.3

Universality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

28

3 Optical forces and lattices
3.1

3.2

30

The optical dipole force . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

30

3.1.1

A classical dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

30

3.1.2

The AC Stark shift . . . . . . . . . . . . . . . . . . . . . . . . . . . .

31

The optical dipole trap (ODT) . . . . . . . . . . . . . . . . . . . . . . . . .

33

7

3.3

Optical Lattices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

35

3.3.1

Two-Photon Transitions . . . . . . . . . . . . . . . . . . . . . . . . .

35

3.3.2

Bragg Diffraction in the Two Photon picture . . . . . . . . . . . . .

36

3.3.3

Kapitza-Dirac Scattering . . . . . . . . . . . . . . . . . . . . . . . .

40

3.3.4

A light grating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

40

3.3.5

Band Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

43

3.3.6

Weak Lattice : Effective mass and interaction . . . . . . . . . . . . .

43

3.3.7

Bragg Scattering: Bloch State Interpretation . . . . . . . . . . . . .

45

3.3.8

Kapitza Dirac : a phase imprint . . . . . . . . . . . . . . . . . . . .

45

3.4

Bragg spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

47

3.5

Dynamical instability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

48

4 Experimental Apparatus

50

4.1

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

51

4.2

The Sodium laser system . . . . . . . . . . . . . . . . . . . . . . . . . . . .

53

4.2.1

The 899 dye laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

53

4.2.2

The laser table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

54

4.2.3

The Sodium MOT . . . . . . . . . . . . . . . . . . . . . . . . . . . .

55

4.3

The Lithium laser system . . . . . . . . . . . . . . . . . . . . . . . . . . . .

57

4.4

The Infrared laser system . . . . . . . . . . . . . . . . . . . . . . . . . . . .

59

4.4.1

The different infrared potentials . . . . . . . . . . . . . . . . . . . .

61

4.4.2

ODT Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

63

4.4.3

Lattice Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . .

64

4.5

The Lithium Feshbach resonance . . . . . . . . . . . . . . . . . . . . . . . .

66

4.6

The Q-section antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

67

5 Matter-wave Interference in a Thermal Cloud
5.1

5.2

68

A Young’s double slit for cold atoms . . . . . . . . . . . . . . . . . . . . . .

69

5.1.1

Matter wave interference of BEC . . . . . . . . . . . . . . . . . . . .

69

5.1.2

Trapped atom interferometry . . . . . . . . . . . . . . . . . . . . . .

70

Interference of Thermal Atoms . . . . . . . . . . . . . . . . . . . . . . . . .

70

5.2.1

Autocorrelation pulse sequence . . . . . . . . . . . . . . . . . . . . .

70

5.2.2

Interference fringe contrast . . . . . . . . . . . . . . . . . . . . . . .

71

8

5.3

5.4

5.2.3

Coherence length . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

73

5.2.4

Ramsey Fringes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

75

Momentum “Filtered” Interference . . . . . . . . . . . . . . . . . . . . . . .

76

5.3.1

Phenomenological Model for Enhanced Contrast . . . . . . . . . . .

77

Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

78

6 Degenerate Fermions in an Optical Lattice

79

6.1

The signature of superfluidity . . . . . . . . . . . . . . . . . . . . . . . . . .

80

6.2

The insulator state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

82

6.2.1

. . . . . . . . . . . . . . . . . . . . . . . . . . .

85

6.3

Recoherence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

86

6.4

Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

88Insulating Behavior

7 Critical Velocity in the BEC-BCS Crossover
7.1

89

The Landau criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

90

7.1.1

Phonon excitation spectrum . . . . . . . . . . . . . . . . . . . . . . .

91

7.1.2

Pair-breaking excitation spectrum . . . . . . . . . . . . . . . . . . .

93

7.1.3

The Landau criterion in the BEC-BCS crossover . . . . . . . . . . .

93

Measuring dissipation in a quantum gas . . . . . . . . . . . . . . . . . . . .

95

7.2.1

Stirring a BEC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

95

7.2.2

BEC in a moving lattice . . . . . . . . . . . . . . . . . . . . . . . . .

96

7.2.3

Superfluid Fermi gas in a moving lattice . . . . . . . . . . . . . . . .

97

Uniform lattice / inhomogeneous density . . . . . . . . . . . . . . . . . . . .

98

7.3.1

Loss dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

99

Centered lattice / homogeneous density . . . . . . . . . . . . . . . . . . . .

101

7.4.1

Critical velocity throughout the BEC-BCS crossover . . . . . . . . .

102

7.4.2

Critical velocity at different lattice depth . . . . . . . . . . . . . . .

104

7.4.3

Dynamical instability . . . . . . . . . . . . . . . . . . . . . . . . . .

106

7.5

Bragg spectroscopy in the BEC-BCS crossover . . . . . . . . . . . . . . . .

106

7.6

Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

107

7.2

7.3

7.4

A High-contrast Interference in a Thermal Cloud of Atoms

110

B Coherent Molecular Optics using Sodium Dimers

115

9

C Evidence for Superfluidity of Ultracold Fermions in an Optical Lattice 120
D Critical velocity for superfluid flow across the BEC-BCS crossover

10

125

List of Figures
2-1 Matter-wave Sum Frequency Generation . . . . . . . . . . . . . . . . . . . .

19

2-2 The Bogoliubov excitation spectrum . . . . . . . . . . . . . . . . . . . . . .

21

2-3 A Feshbach Resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

24

2-4 Feshbach molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25

3-1 The AC Stark shift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

32

3-2 Bragg scattering : a two photon process . . . . . . . . . . . . . . . . . . . .

37

3-3 Kapitza-Dirac scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

39

3-4 Schematic of a 1-D optical lattice . . . . . . . . . . . . . . . . . . . . . . . .

41

3-5 X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

42

3-6 Band structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

44

3-7 Bragg diffraction at the Brillouin zone boundary . . . . . . . . . . . . . . .

46

3-8 Bragg Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

47

3-9 Dynamical instability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

49

4-1 The sodium laser table. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

52

4-2 The lithium laser table. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

58

4-3 Infrared laser system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

60

4-4 An accidental lattice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

61

4-5 Band populations and energy gap . . . . . . . . . . . . . . . . . . . . . . . .

64

4-6 The 6 Li Feshbach Resonance . . . . . . . . . . . . . . . . . . . . . . . . . .

66

4-7 The Q-section antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

67

5-1 Double slit interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

70

5-2 Autocorrelation pulse sequence . . . . . . . . . . . . . . . . . . . . . . . . .

71

5-3 Thermal interference contrast . . . . . . . . . . . . . . . . . . . . . . . . . .

72

11

5-4 Reduced fringe contrast . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

74

5-5 Contrast emerges in time of flight . . . . . . . . . . . . . . . . . . . . . . . .

75

5-6 Fringe contrast for a filtered thermal cloud . . . . . . . . . . . . . . . . . . .

76

6-1 Dissipative collisions between momentum components . . . . . . . . . . . .

81

6-2 Interference of fermion pairs released from an optical lattice . . . . . . . . .

82

6-3 The Mott Insulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

83

6-4 Excitation of a Fermi gas in a lattice . . . . . . . . . . . . . . . . . . . . . .

84

6-5 Restoring coherence from a deep lattice . . . . . . . . . . . . . . . . . . . .

87

7-1 The Landau criterion for superfluidity . . . . . . . . . . . . . . . . . . . . .

91

7-2 Speed of sound and pair-breaking threshold . . . . . . . . . . . . . . . . . .

94

7-3 Configurations for perturbing a condensate . . . . . . . . . . . . . . . . . .

96

7-4 Time series of loss. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

98

7-5 Linear loss rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

100

7-6 Critical velocity at the cloud center . . . . . . . . . . . . . . . . . . . . . . .

102

7-7 Critical velocity throughout the BEC-BCS crossover . . . . . . . . . . . . .

103

7-8 Critical velocity at different lattice depths . . . . . . . . . . . . . . . . . . .

105

7-9 Bragg spectroscopy in the BEC-BCS crossover . . . . . . . . . . . . . . . .

108

12

Chapter 1

Introduction
Much of what attracted me to atomic physics resides in the the clarity with which
ultra-cold atomic gases exemplify the quantum mechanical nature of matter. Matterwave interference, for example, offers striking affirmation of the precepts with which
we first became acquainted in the classroom. The degree to which these systems areamenable to fairly simple theoretical description is encouraging to the experimentalist.
The Bose-Einstein condensate behaves very much like a single particle wave-packet.
The interaction of an atom with light is well modeled by the two level system. Verifying one’s own calculation in the laboratory is an empowering experience, and afrequent one in the field of atomic physics.
When I first joined this group, Bose-Einstein condensates dominated the research
agenda. A reliable and robust superfluid, this state of matter came to symbolize theprecision and control which are characteristic of this field. Throughout my graduate
career, however, the study of degenerate Fermi gases has been in the ascendency.
These systems provided a new perspective on phenomena which have puzzled physicists for decades. Our understanding of strongly correlated electron systems stands
to benefit from our study of atomic Fermi gases, which exhibit the same physics
but offer numerous degrees of freedom unavailable in solid state materials. Whileanti-ferromagnetism and d-wave superfluidity currently lie at the periphery of ourcomprehension, atomic physics is well poised to shed light on these phenomena.
The properties of ultra-cold gases which most interest us all rely on coherence.
Coherence is, in some sense, a measure of how a group of atoms will act in a coordi13

nated fashion, so as to exhibit a number of elegant wave-like properties. By studying
coherence and its breakdown we gain further insight into the mechanisms at work andthe limits of their applicability. In this thesis I will discuss a number of experiments,
all of which have sought to probe the coherence properties of an atomic gas, in one
way or another.

14

Chapter 2

Quantum Degenerate Gases
Introduction
In this chapter, we discuss the properties of quantum degenerate gases. The statistics ofidentical particles differentiates a quantum gas from its classical counterpart. This distinction is only meaningful when the gas approaches degeneracy, which is to say at low
temperatures and high densities where the occupation probability of the lowest lying quantum states approaches unity. A number of good review articles [66, 29, 44] and textbooks[70, 91, 13] can be found which thoroughly cover the basics of quantum gases. Here, we will
cover the most relevant concepts.
Identical Particles
The difference between bosons and fermions arises from a distinction in their symmetry. The
wavefunction describing a collection of identical bosons is symmetric under the exchangeof any two particles, and anti-symmetric in the case of fermions. The consequences of thissubtlety are significant enough that these two types of particle must be treated as entirely
different entities at low temperatures.
The Pauli exclusion principle follows from the anti-symmetry of fermions under exchange. Two identical fermions can not occupy the same state. Our earliest exposure to
this rule comes in high-school chemistry class, when we learn how electrons fill up energy
levels in an atom, producing the periodic table of the elements.
Bosons, on the other hand, can occupy the same state. In fact, a great deal of effort
has been expended in order to make them do so, by the tens of millions at a time, in the15

form of a Bose-Einstein condensate (BEC). A BEC is often described, colloquially, as a
giant matter-wave. In fact, BECs exhibit a number of striking wave-like phenomena which
exemplify what it means to be quantum mechanical. In addition, weakly interacting alkali
BECs are amenable to fairly simple theoretical treatment, bearing much resemblance to thesingle particle wavefunction of an introductory quantum mechanics course. This has madethe BEC an exciting and popular object of study over the past 12 or so years [4].
The atomic bosons which we condense are, just as all mass carrying bosons are [48],composite bosons: a bound state of an even number of fermionic subatomic particles. Similarly, we can create composite bosons, molecules for example, out of two fermionic atoms.
Recently, quite a bit of excitement within the atomic physics community has been directed
towards the so-called BEC-BCS crossover: the continuous transition between the physics
of atomic fermions and diatomic bosons, which is achieved through the magnetic tuning ofinteractions. We will elaborate on this phenomena in Section 2.4. Suffice it to say that theunion of two previously distinct physical regimes has been warmly received.

16

2.1

BosonsGenerally, the physics of many particles is far more complicated than that of the single or
two interacting particles. This is why in thermodynamics we look at bulk properties rather
than keep track of every particle. In many-body physics, interactions between particles
create correlations which require some fairly sophisticated mathematics to keep track of[88, 118]. In contrast to liquid 4 He, Bose-Einstein condensates of alkali gases are extremely
dilute, and the interactions are extremely weak. This allows us to make a number ofapproximations which greatly simplify the theoretical treatment.

2.1.1

s-wave Interactions

The interatomic potential is in general fairly complicated. Much of the detail, however, canbe swept under the rug at the low temperatures which we achieve. The two-body collision
process can be described in terms of partial waves [99, 25]. In short, the relative motionof the two particles is treated as a plane wave incident on some scattering potential. The
scattering off this potential can be expressed in partial wave components (s-wave, p-wave, dwave ...) which reflect a decomposition of the angular distribution into spherical harmonics.
In the limit of zero relative velocity, which we approach at ultracold temperatures, only thelowest order s-wave component contributes, which is spherically symmetric. In other words,
at very long wavelengths, the incident plane wave can’t resolve any of the structure of thescattering potential.
The long and the short of it is that we treat the interaction as a delta function pseudopotential
V (~r1 , ~r2 ) =4πh̄2 aδ(~r1 − ~r2 )
m

(2.1)

which is parameterized by a single quantity, the s-wave scattering length a. This assumption
is valid as long as a remains less than the average inter-particle spacing. The sign of acorresponds to the nature of the interaction; a is positive for repulsive interactions andnegative for attractive interactions.
Symmetry of interactions
It should be noted that identical fermions can not experience s-wave scattering. As wediscussed earlier, the fermionic wavefunction must have odd symmetry, which means only17

odd partial waves (p,f,...) can be involved. This has serious implications in the field of
ultra-cold gases, in which collisions are required for the cooling process. Identical fermions
essentially do not collide at low temperatures, meaning we must either use a mixture ofspin states or cool them sympathetically through interactions with another species, such as
23 Na. Similarly, bosons only experience scattering with even symmetry (s,d,...).

2.1.2

Gross-Pitaevskii Equation

The aim of mean-field theory is to replace all the interactions amongst a group of particles with an average or effective interaction, thus reducing a many-body problem into aneffective one-body problem. Adopting the pseudo-potential of Equation 2.1, the condensate wavefunction can be described by a nonlinear Schrödinger equation, known as theGross-Pitaevskii equation (GP equation)
"

#

h̄2 2
4πh̄2 a−∇ + Vext (~r) +|ψ(~r)|2 ψ(~r) = µ ψ(~r),
2m
m

(2.2)

2

a
where the chemical potential µ = 4πh̄m n0 , can be expressed as the interaction energy at

the peak density n0 .

2.1.3

Thomas-Fermi Approximation

For a BEC, we can usually neglect the kinetic energy term in Equation 2.2, with respect tothe interaction energy Uint = 4πh̄2 na/m. It then follows that a BEC in a harmonic trap
has the characteristic parabolic density profile.
n(~r) = |ψ(~r)|2 =m(µ − Vext (~r))
4πh̄2 a(2.3)

This approximation, however, fails at the very edges of the cloud, as the density approaches
zero.

2.1.4

Healing Length

Spatial variations in the density of size ξ carry a cost in kinetic energy h̄2 /2m ξ 2 . Equating
this energy with the interaction energy, we arrive at the natural length scale associated with18

Atomic momentum (pr )

0

1

a)

b)

0

1

2

Molecular momentum (pr )Figure 2-1: Matter-wave Sum Frequency Generation: justone example of the many phenomena which arise due toa nonlinear wave equation and which have direct analogs
in nonlinear optics. a) Atoms are prepared in momentum
states |0i and |1i using Bragg diffraction. b) The atommolecule coupling produces molecules with momentum |0i,
|1i and |2i, analogous to optical sum frequency generation.
The time of flight is 17 ms.

19

Uint .
1
ξ=√
8πna

(2.4)

ξ is referred to as the healing length. It sets the distance, for example, over which thecondensate density can change from its bulk value to zero, or equivalently, the size of avortex core.

2.1.5

A nonlinear wave equation

Equation 2.2 is a nonlinear wave equation. As a consequence, we can describe a number of
BEC phenomena in a manner completely analogous to that of nonlinear optics [76]. The
mean field interaction acts as a third order nonlinear susceptibility χ(3) , which can drive
processes such as four-wave mixing [33], soliton generation [34] and optical parametric
generation [17]. The coupling to the molecular channel, which we describe in Section 2.3,
is analogous to frequency doubling. In our lab [1], we demonstrated the sum frequency
generation of matter waves by combining atoms with different momentum (and therefore,
wavelength λ = h/p) into diatomic molecules, as shown in Figure 2-1.

2.1.6

Bogoliubov Excitation Spectrum

N. N. Bogoliubov first derived the spectrum of elementary excitations for a BEC [11]. Rather
than elaborating upon the derivation, I will simply state the main result. The dispersion
relation for elementary excitations is given by
s

Ep =

p2
2m

µ 2
p

2m

¶

+ 2 m c2

(2.5)

where the speed of sound c is given by the relation
m c2 = µ =h̄2.
2m ξ 2(2.6)

We can identify two regimes of the spectrum, as illustrated in Figure 2-2. At low momentum, the excitations are sound-like, with linear dispersion Ep ' pc. At high momentum2p+ µ. In addition to the kinetic
the excitations become free particle-like, where Ep ' 2m

energy, this excitation requires that we pay the chemical potential µ to kick an atom outof the condensate. This is a consequence of exchange symmetry: a distinguishable (ther20

Excitation Energy Ep / µ

5

4

3

2

1

0
0

1

2

3

Momentum p/mc
Figure 2-2: The Bogoliubov excitation spectrum (solid line). The elementary excitationsof a BEC are phonon-like as low momentum, following a linear dispersion relation (dottedline). At high momentum, the excitations are free particle like. The excitation energy
reflects the quadratic kinetic energy, offset by the mean field shift µ (dashed line).
mal) particle interacts with the condensate with twice the strength of an indistinguishable(condensate) atom.
The transition between these two regimes happens where the wavelength of the excitation λ = h/p is on the order of the healing length ξ. In other words, the condensate cannot support collective modes with a spatial variation λ shorter than the healing length.

21

2.2

The Non-interacting Fermi Gas

In the case of a degenerate Bose gas, we were primarily concerned with nature of the groundstate, since we could pack an arbitrary number of particles into that state, in the form of
a BEC. What concerns us most, in the case of a degenerate Fermi gas, is also the state oflowest energy to which we can add a particle. According to the Pauli exclusion principle,
however, this state will sit on top of a filled Fermi sea of occupied energy levels. Theenergy of this state1 is referred to as the Fermi energy EF , and sets the energy scale formany properties of the system [7, 69]. We frequently work with harmonically trapped gases,
for which the energies are quantized at the harmonic oscillator spacing. The Fermi energy
for N non-interacting fermions2 in a 3D harmonic oscillator is
EF = hν̄ (6N )1/3

(2.7)

where ν̄ = (νx νy νz )1/3 is the geometric mean trapping frequency [15]. At zero temperature,
the chemical potential µ is just the Fermi energy. Similarly, we can define the Fermi
temperature TF = EF /kB , which sets the relevant temperature scale of the system. A
Fermi gas becomes quantum degenerate for temperatures T /TF ≤ 1, which is to say that atsuch temperatures states with energy below EF begin to fill up, and the behavior deviates
significantly from that of a classical gas.

2.2.1

The local density approximation

We can gain a fair amount of insight into the properties of a trapped degenerate Fermi gas
by assuming that, locally, it behaves as a homogeneous gas. In other words, on a lengthscale short compared to the variation of the external potential, Vext acts only as an offset.
The local energy ε(n) is determined by the sum of the kinetic and interaction energies,
which depend only on the local density n(~r). The total energy is thenZE=d~r [ ε(n) + Vext (~r) n(~r) ]

1

(2.8)

or equivalently, for large numbers, the energy of the highest occupied state, at zero temperature.
The number N and density n in this thesis will always refer to the number of fermions in a single internal
state. Many solid state textbooks, however, will refer to the total number, making the assumption of an
unpolarized system of electrons, with two spin states. Be aware of this factor of 2.
222

This is known as the local density approximation (LDA), and is valid if the chemical potential is large compared to the quantization, that is, the harmonic oscillator spacing h̄ω inthe trap.
In a non-interacting Fermi gas, the local Fermi energy εF reflects only the local kinetic
energy, through the local density of k-states. In other words, this is just the Fermi energy
for a homogeneous gas of density n.
εF =h̄2 ³ 2 ´2/3
6π n
2m

(2.9)

In the same manner, we can define a local Fermi wavevector kF = (6π 2 n)1/3 .

2.2.2

Density Profile

The chemical potential, µ = εF (r) + V (r), is of course flat across the sample. If it werenot, the atoms would redistribute themselves. Wherever the potential is zero, which for aharmonic trap is at the trap center, we get εF = µ = EF . Inverting Equation 2.9 gives us
the peak density
n0 =

1
6π 2

The density profile is then

µ

Ã

q

2.3

¶3/2

r2
1− 2
RF

n(r) = n0
where the Fermi radius is RF =

2mµ
h̄2

.

(2.10)!3/2(2.11)2µ.
m ω2The Feshbach ResonanceOne of the developments which has fueled the recent progress in the field of degenerate Fermi
gases has been the identification and manipulation of Feshbach resonances [59]. Colloquially,a Feshbach resonance is a magic knob which allows us to tune the scattering length, andthus the interactions, of an atomic gas. More physically, it is a scattering resonance [99]which occurs when the state of two free colliding particles is degenerate with a bound state(see Figure 2-3). There is some coupling between states, from the hyperfine interaction, forexample. If the states have different magnetic moments, we can tune their energies relative
to one another with an externally applied magnetic field. The s-wave scattering length then23

B0
Scattering length a(B)∆E =∆µ (B −B0 )Energybound statea)free state

Interatomic distance

abg

0
∆B

b)

Magnetic field

Figure 2-3: The Feshbach resonance. a) Interatomic potentials for different spin configurations have a difference in magnetic moment ∆µ, and thus tune differently with magnetic
field. A free atomic state (red line) becomes degenerate with a bound molecular state (blueline) at a magnetic field B0 . b) Due to a coupling between these two states, two-bodyscattering is resonantly enhanced near this field. The s-wave scattering length a diverges
on resonance, and can be tuned in both magnitude and sign in the region ∆B aroundresonance. The general form for a(B) is found in Equation 2.12.
has the general magnetic field dependenceµa(B) = abg

1−∆B
B − B0¶(2.12)

where abg is the background scattering length, B0 is the location of the resonance, and ∆B
its width.
This remarkable degree of control is limited only by our technical ability to regulate
the magnetic field. Broad Feshbach resonances, such as in 6 Li [8], are more forgiving withrespect to field stability. On the other hand, narrow resonances, such as those in 40 K [77]and 23 Na [106], make it easy to quickly jump around the resonance.
A Feshbach resonance allows one to vary not only the strength of interactions, but thesign as well. Studies with bosons are usually performed with repulsive (a > 0) interactions.
A trapped BEC with attractive interactions is stable only at very small numbers. Above a
critical density, the attractive interaction overpowers the repulsion due to finite size kinetic
energy. Nevertheless, there are interesting aspects of such instabilities [43, 22]. Attractive
interactions, however, are more interesting in the case of fermions, as they give rise to BCS
physics, which we will discuss in Section 2.4.1.
24

Energy

EnergyB0a)Magnetic Fieldb)Interparticle separation

Figure 2-4: Feshbach Molecules. a) The atom-molecule coupling near a Feshbach resonancecreates an avoided crossing between the free atom continuum and a bound molecular state.
Atoms can be adiabatically converted to molecules by sweeping the magnetic field acrossthe resonance. b) Three body collisions can transfer weakly bound molecules into a moredeeply bound state, while a third body carries of the difference in binding energy as kinetic
energy.
While the s-wave resonances for 40 K and 6 Li, in particular, have been the workhorses ofthe field, there are other more exotic Feshbach resonances. Ordinarily, ultra-cold identicalfermions do interact, since s-wave collisions are disallowed by symmetry and other partial
waves are frozen out. However, p-wave resonances have been found [100, 94] which allow
one to circumvent this “rule”. Feshbach resonances between different atomic species havealso been identified [104, 60, 40]. And, while we usually associate Feshbach resonances withmagnetic tunability, the Innsbruck group has induced a Feshbach resonance optically [110].

2.3.1

Feshbach Molecules

As we mentioned, a Feshbach resonance involves the coupling of the free atomic state toa molecular bound state. This coupling is coherent, as evidenced by the atom-molecule
oscillations [36] it can produce. It also gives rise to an avoided crossing, which allows
for the adiabatic conversion of atoms into molecules, as depicted in Figure 2-4a. Basicscattering theory [99] tells us that a bound state exists for a > 0, and the binding energy is
Eb = −h̄2,
2mr a225(2.13)

where mr is the reduced mass of the system, which is just half the atomic mass, in the caseof a homonuclear molecule.
Our lab was one of the first to produce diatomic molecules from a BEC, by sweeping
across a Feshbach resonance. This technique has been implemented in the production of“bosonic” dimers3 of several species, including 133 Cs2 [54], 87 Rb2 [38] and 23 Na2 [116]. Thesebosonic dimers, however, have been of limited utility so far. While the binding energy of thismolecular state is very small, the overlap (Frank-Condon factor) with more deeply boundstates is significantly greater than that of two free atoms. As a consequence, three-bodycollisions, as illustrated in Figure 2-4b, occurs at a prohibitively high rate, limiting the
lifetimes of these samples to tens of milliseconds. Although Bragg spectroscopy has shownthat these dimers can be created with very high phase-space density [1], these samples do
not have enough time to thermalize, and thus can not be considered a BEC.
Fermions, however, do not suffer the same fate. The Pauli exclusion principle suppresses
three-body recombination, allowing “fermionic” dimers such as 40 K2 [95] and 6 Li2 [27, 63,
108] not only to Bose-Einstein condense [47, 64, 120], but to survive in this state for tensof seconds. These long lived molecular BECs are most exciting, however, in the context of
the BEC-BCS crossover, which we discuss in Section 2.4.1.

2.4

The Strongly Interacting Fermi Gas

In Section 2.2, we discussed the physics of non-interacting fermions. This was not as idealized as it may sound, since identical fermions at low temperatures simply do not interact.
It was, however, rather boring. Most of the exciting physics comes from the interactions.
In order to add interactions to our Fermi gas we incorporate more than one spin state. This
spin mixture, however, must be an incoherent mixture. Fermions in the same coherent superposition of spin states are still identical [122]. Fortunately, decoherence is easy to comeby.
Nevertheless, with two spin states, fermions can collide in s-wave. The two-particle wave
function must, of course, remain anti-symmetric under exchange. This just means that the
spin component of the wavefunction will be in the singlet configuration.
3bosonic here refers to the constituent atoms. All homonuclear dimers are bosons, whether they areformed from bosons or fermions.

26

2.4.1The BEC-BCS Crossover

As we discussed in Section 2.3, a Feshbach resonance allows one to control not only thestrength of interactions, but the sign as well. This has some profound ramifications fora two-component Fermi gas. Until quite recently, two distinct types of superfluidity haveexisted in separate, almost parallel worlds. On the one hand there are bosons, such as
4 He, which for repulsive interactions (a > 0) can form a BEC, which exhibits superfluid

properties. On the other hand, there are fermions, such as 3 He and electrons, which atvery low temperatures (T /TF ) and in the presence of attractive interactions (a < 0), form
Cooper pairs and can be described by the superfluid BCS state. The atomic Feshbachresonance acts as a bridge between these two regimes. The nature of the interactions canbe continuously tuned from repulsive, on the BEC-side of the resonance, to attractive, on
the BCS-side. The region in between is known as the BEC-BCS crossover, and has been
the subject of intense theoretical and experimental research lately [44].

2.4.2Unitarity

The divergence in Equation 2.12 seems to imply that interactions can be made arbitrarily
large in magnitude, but the s-wave scattering length does not tell the whole story. The
scattering cross section σ is given byσ=

4π 2 a2
1 + k 2 a2

(2.14)

where the wavevector k of the relative motion is typically given by the local Fermi wavevectorkF .
In the typical case for dilute atomic gases, a ¿ 1/kF , and we get the intuitive resultthat σ ' 4π 2 a2 . The atoms look like hard spheres of radius a. For strong interactionsa À 1/kF , however, the cross section has a different limiting behavior.
lim

a→±∞

σ=

4π 2
kF2

(2.15)

In other words, the effective scattering length is cut off at the mean interparticle spacing
n−1/3 , which, by no coincidence, is the inverse Fermi wavevector 1/kF .
The region for which |kF a| > 1 is usually referred to as the strongly interacting, or27

unitarity, regime. It is interesting to note that, in this limit, the collisional behavior does
not depend on either the magnitude or the sign of the scattering length.

2.4.3

Universality

It turns out we can incorporate interactions into the non-interacting equation of state (Section 2.2) without much difficulty. As a consequence of unitarity limited interactions, lengthscales other than 1/kF fall out of the picture. The interaction energy can only by givenby the local Fermi energy εF , times some constant β. So, we can write the local4 chemical
potential µloc asµloc = εF + Uint

(2.16)= (1 + β) εFh̄2 ³ 2 ´2/3
6π n.
= (1 + β)2m
The behavior becomes universal in the sense that specific details of the atomic potential
are no longer relevant [55]. The value of the universal parameter β has been of great
theoretical and experimental interest [44]. A number of measurements and calculations are
consistent with β ' −0.58. The fact that β is negative indicates that universal interactions
are intrinsically attractive.
Since the inclusion of unitarity limited interactions has not changed the functional formof equation of state (µloc ∝ n2/3 ), we expect the shape of a zero-temperature trapped Fermi
gas at unitarity to be the same as that of a non-interacting gas (Equation 2.11). The
overall size R and peak density np , however, will change in response to the interactions. Ina harmonic trap, R ∝ µ1/2 . Including interactions we find Rint ∝q

1/3

1 + β np , according toEquation 2.16. The product n × R3 remains invariant under the inclusion of interactions.
This simply reflects conservation of number. This allows us to express the peak density
np of a harmonically trapped Fermi gas with universal interactions in terms of that of a
non-interacting Fermi Gas n0 .
np = (1 + β)−3/4 no4(2.17)The global chemical potential is the sum of the local chemical potential and the external potentialµ = µloc (r) + Vext (r)28Equivalently, the size of the interacting gas becomes
Rint = (1 + β)1/4 RF .

(2.18)

Given the peak density, we can express the local Fermi energy εF at the trap center in termsof the non-interacting Fermi energy EF .
¯
¯

εF ¯

r=0

= (1 + β)−1/2 EF(2.19)

Effective Mass
There is another route to understanding the effect of universal interactions on the size anddensity of a trapped Fermi gas [49]. We can interpret the interactions as providing aneffective mass m∗ ,m
(2.20)
1+β√
which, in turn, scales the trap frequency to ωef f = 1 + β ω. The global chemical potential,m∗ =still given by the Fermi energy, becomesµ = h̄ ω̄ef f (6N )1/3 =p

1 + β EF ,(2.21)

from which Equations 2.17 and 2.18 follow.
Personally, I find this approach unappealing, as it is not immediately obvious where m∗
is applicable. For example, the dipolar oscillation frequencies are not rescaled bypm/m∗ ,

even though we have defined an effective trap frequency ωef f for the purpose of rescaling the
Fermi energy. The effective mass for an optical lattice (Equation 3.25), in contrast, is moreconsistent in its applicability, and does in fact modify the oscillation frequency [74, 39].

29

Chapter 3

Optical forces and lattices
3.1

The optical dipole force

In this section we will discuss the energy shift produced by an off-resonant light field, known
as the AC-Stark effect, and the optical dipole force which results when this shift has spatial
dependence. We will use a classical description in order to establish intuition, followed bya quantum mechanical treatment.

3.1.1

A classical dipole

An atom in an electric field will acquire an induced a dipole moment, as the electron cloudis pulled in one direction and the nucleus in another. The energy of a dipole in an external~ It follows that if the field is non-uniform, a dipole which iselectric field is E = −µ~e · E.
aligned with the field will get pulled into the region of stronger E field. In the case of an
A.C. electric field, such as a light wave, the atomic dipole responds as a driven harmonic
oscillator. The dipole has a natural oscillation frequency ω0 , and is damped, as an oscillating
dipole will radiate, scattering photons. We know that the phase of such an oscillator, withrespect to the drive field, varies with frequency [80]. Far below resonance, in the case of
red detuning, the dipole can follow the field oscillation. The atom will then be pulled into
regions of higher intensity. If the light is far blue detuned, the dipole will be π out ofphase, and thus antiparallel to the electric field. Atoms will then be repelled from intensity
maxima.

30

3.1.2The AC Stark shift

Quantum mechanically, of course, the internal states of an atom are discretized, and we canusually model the physics as a two-level problem. We consider a ground state |gi and an~ = E0 cos ωt ²̂ introduces
excited state |ei, separated by an energy h̄ω0 . The light field E(t)a coupling

D ¯
¯

¯ E
¯

~ ¯g =
Weg (t) = e ¯ −~µ · E(t)´h̄Ω ³ iωte + e−iωt2(3.1)

where the Rabi frequency is
Ω=−

e E0
he| ~r · ²̂ |gi
h̄

(3.2)

The polarization ²̂ of the light dictates which internal states can be coupled through selection
rules.
The time dependence can be eliminated by transforming to the appropriate rotating
frame, and the Hamiltonian becomesh̄  −δ Ω H̃ = 

2
Ω∗ δ

(3.3)

where we have introduced the detuning δ ≡ ω − ω0 . The result of the coupling is that real
eigenstates are some admixture of excited and ground state. The eigenenergies in this frame
areEe,g = ∓h̄ p 2
δ + Ω22(3.4)

For Ω ¿ |δ|, we can Taylor expand this, and we find that the coupling Ω shifts the energies.
∆Ee,g = ∓

h̄Ω2
4δ

(3.5)

This is what we refer to as the AC Stark shift.
An intuitive way of looking at this system is to consider the atom plus the light field.
If the atom is excited, it takes a photon of energy h̄ω out of the field (see Figure 3-1).
So the state |g; ni (i.e., atom in ground state and n photons in the laser field) is nearly
degenerate with |e; n − 1i. We know from perturbation theory that coupled states repel,
and this insight explains how the sign of the shift corresponds to the sign of δ.

31

|ei

∆Eg

|ei

|gi

∆Ee

δ = ω − ω0|gin photons

n-1 photons

Figure 3-1: The AC Stark Shift. Energy level diagram for a photon plus two-level atom
system. The bare atom has excited and ground states {|ei , |gi} separated by an energyh̄ω0 . A light field of frequency ω couples the two states. Because the transition from theground to excited state is accompanied by the removal of a photon with energy h̄ω, thetwo states are nearly degenerate. The coupling causes the states to repel, giving rise to the
AC stark shift ∆Ee,g of Equation 3.6. The digram is shown for a blue-detuned light field(δ > 0).

32

Perturbation Theory
In fact, we could have also used perturbation theory to solve for the AC Stark shift. The
perturbation to the ground state energy is second order.
µ∆E (2) =
=

1
1
h̄Ω2
+
4
ω − ω0 ω + ω0
h̄Ω2
h̄Ω2
+
4δ
4(2ω − δ)
|

{z

¶

(3.6)
(3.7)

}

Bloch Siegert shift

This approach, however, gives us an additional term which we did not find in Equation 3.5.
This term is referred to as the Bloch Siegert shift [9], and comes from the counter rotatingterm which we ignored in order to arrive at the simple Hamiltonian in Equation 3.3. This
term is indeed significant for many of our applications in which we use a far-detuned laser,in the infrared, for example.

3.2

The optical dipole trap (ODT)

Given what we know about the AC stark shift, its pretty straightforward to calculate thetrapping force produced by red detuned laser beam. A Gaussian laser beam of total power
P focused down to a spot size1 of w0 has a peak intensity I0 = 2P/πw02 . The intensity
profile is given by¡I(r, z) =

¢

2
r
2P
−2 w(z)eπw2 (z)(3.8)

qwhere the e−2 radius varies along the axis as w(z) = w0 1 + z 2 /z02 . The Rayleigh range isdefined as z0 = πw02 /λ, where λ is the wavelength of the light. In an harmonic approximation, the potential V (~r) becomesÃV (r, z) ' −V0

µ

r
1−2
w0

¶2

µ

−

z
z0

¶2 !

(3.9)

where, according to Equation 3.6, the trap depth V0 is given byh̄ γ02 I0
V0 =8 Isat1µ11+
ω − ω0 ω + ω0¶

(3.10)The beam waist is conventionally the e−1 radius of the E field, and thus the e−2 radius of the intensity.

33

λ

γ0 /2π

Isat

(nm)

(MHz)

(mW/cm2 )6 Li

670.96

5.92

2.56

23 Na

589.16

10.01

6.40

40 K

766.70

6.09

1.77

87 Rb

780.24

5.98

1.64

Table 3.1: Transition wavelength λ, naturallinewidth γ0 and saturation intensity Isat forsome popular alkali atoms, from [82].
which we have expressed in terms of the natural linewidth γ0 and the saturation intensity
Isat . We list these experimentally convenient parameters for some commonly used alkali
atoms in Table 3.1. The resulting trap frequencies are
s

ωr =

4 V0
, ωz =
m w02

s

2 V0
m z02

(3.11)

The weakly bound diatomic molecules and BCS-type pairs we work with essentially have
twice the polarizability of a single atom, and will experience twice the trap depth. They
will, however, experience the same trap frequencies as the unpaired atoms, since they also
have twice the mass. This is analogous to two identical pendulums which oscillate at thesame frequency whether or not they are attached to each other.
Scattering
While the AC Stark shift ∆E can become very strong near resonance, one must thencontend with the scattering of photons, which is generally counterproductive in a cold atom
experiment. From the photon scattering rate
Γs =I/Isatγ02 1 + I/Isat + (2δ/γ0 )2(3.12)

we see that at large detuning, Γs /∆E ∼ γ0 /δ, for a given laser intensity. Therefore, we usually stand to gain by using far-detuned light, and compensating with more power. Infraredlasers at 1064 nm are readily available at sufficient powers, and induce negligible scattering.

34

3.3

Optical Lattices

In the previous sections, we discussed the interaction of light with an atom int the context
of internal degrees of freedom. We neglected the atoms external degrees of freedom, thatis to say its spatial wavefunction ψ(~r). The absorption of a photon, for example, must be
accompanied by the absorption of that photon’s momentum h̄k. This is reflected in thethe fact that a spatially varying phase acts as an operator, connecting different momentum
states.
eikz =

X|pihp + h̄k|

(3.13)p

Much of our understanding of atoms in a periodic potential derives from the moremature field of condensed matter physics, which describes the behavior of electrons under
the influence of an ionic crystal lattice. However, as is often the case in quantum mechanics,
there exist a multitude of ways to understand a particular phenomena. In this Chapter we
discuss a number of frameworks, each which offers its own particular insight into opticallattice phenomena. The review paper, Ref. [85] can offer further illumination.

3.3.1

Two-Photon Transitions

The treatment of an atom interacting with two light fields is not any more complicatedthan that for a single light field, under certain conditions. Let us consider, for example,
two counter propagating beams. The beams are derived from the same laser, so they havethe same wavevector ~k1 = −~k2 = k ẑ, although we may introduce some small frequency
difference δ between them with, say, an AOM. Both beams couple the same internal states,|gi and |ei, but different momentum states, since they face different directions. For clarity,
we’ll say that beam 1 couples the state |g, pi to the state |e, p + h̄ki, while beam 2 couples|g, pi to |e, p − h̄ki. If we constrain our attention to the dynamics of an atom initially in asingle momentum state, say p = 0, we only need to consider a total of three states½¾|g, p = 0i , |g, p = 2h̄ki , |e, p = h̄ki
as depicted in Figure 3-2.
In fact, the situation is even more simple than that. What we have is a typical lambda
system, in which two states are coupled though an intermediate state of higher energy. The

35

dynamics are well described by a Raman process in which one photon is simultaneously
absorbed from one beam and another is stimulatedly emitted into the other beam. Ourcase differs slightly from the traditional Raman, in that we are changing momentum statesrather then internal states, but that is a semantic distinction. If the beams are far detuned
from the intermediate state (|e, h̄ki), it can be eliminated, and we are effectively left witha two-level system. In the appropriate rotating frame, the Hamiltonian becomes h̄δ/2

H=Ω∗1 Ω2h̄ 4∆h̄Ω1 Ω∗24∆2 2
k4 h̄2m− h̄δ/2



(3.14)

where Ω1,2 is the Rabi frequency for beams 1 and 2, and ∆ ∼ δ1 ∼ δ2 is the single photon
detuning. This looks just like the Hamiltonian from Equation 3.3, where Ω(2) ≡Ω1 Ω∗22∆ is thetwo photon Rabi frequency, and the effective detuning isδef f ≡ δ − 4h̄2 k 22m

(3.15)

This effective detuning reflects conservation of energy: the difference between the energiesof the photon absorbed and emitted accounts for the change in the atom’s kinetic energy.

3.3.2

Bragg Diffraction in the Two Photon picture

We can perform a coherent population transfer between two momentum states [72] by
abruptly turning on the two coupling beams. This process is known as Bragg diffraction, in
analogy with the scattering of x-rays off of crystals (upon which we will elaborate later inthis Chapter). It is a resonant process which can achieve complete transfer when δef f = 0.
This is known as the Bragg conditionh̄(ω1 − ω2 ) =q2+ ~v · ~q2m(3.16)

which we have generalized2 for beams of arbitrary pointing ~k1,2 which impart a momentum~q = h̄(~k1 − ~k2 ), as well as an initial atomic velocity ~v . Of course, one can always finda reference frame in which the atom is initially at rest, as in Figure 3-2a. Rather thanrepresenting kinetic energy, the ~v · ~q term in Equation 3.16 becomes the Doppler shift
2

For continuity, we will continue to treat the case of counter-propagating beams (|~q | = 2h̄k)

36

|ei

E

|e, p = h̄ki

h̄ω0

∆

Ω1

Ω2
δ ef f|gi
2 2

k
4 h̄2m|g, p = 2h̄ki

a)

c)

b)

p
0

2h̄k

0

2h̄k

d)

|g, p = 0i

0

2h̄k

Figure 3-2: The two photon process. a) Energy vs momentum for the excited and groundstates of an atom. A two photon process transfers momentum 2h̄k to the atom. This processconserves energy when the two photon detuning is equal to the kinetic energy imparted to2 2k
the atom. Diagram is not to scale (h̄ω0 À 4 h̄2m). b) The same process drawn as a lambda
system, showing the equivalence to a Raman transfer. c) Time of flight image of a Na BEC
for which a small fraction of the population has been transferred. d) A BEC which has
been divided equally into two momentum states by a π/2 Bragg pulse. A scattering halo
results from collisions between momentum states as they spatially separate.

37

incurred in going to that frame.
The population of the two momentum states evolve as a Rabi oscillation, and cantherefore be controlled by the duration of the Bragg pulse τ . We typically refer to the pulse
duration in terms of its product with the Rabi frequency Ω(2) τ . A π-pulse (Ω(2) τ = π),for example, transfers the population completely. We emphasize that this is a coherent
process. This language is also appealing in the Bloch sphere picture, where a π pulse
corresponds to a rotation of the Bloch vector by 180◦ . A π/2 pulse puts each atom into
an equal superposition of both momentum states. It is not until we make a measurement,such as taking the absorption image shown in Figure 3-2d, that this becomes a statistical
distribution.
Two-Photon Linewidth
In our treatment of the two level atom of Chapter 3.1.2, we were able to ignore kinetic
energy imparted to the atom from the photon. This recoil energy
Er =h̄2 k 22m

(3.17)

is typically on the order of 10 khz (depending on the photon wavelength and atomic mass,of course). The natural linewidth γ0 of an atomic transition is typically ∼10 MHz (seeTable 3.1), making the recoil rather insignificant. The linewidth of a two photon transition,
however, is smaller by a factor of ∼ Ω(2) /∆, since the occupation of the excited state isminuscule. Resolution on a scale small compared to the recoil energy is attainable. Whilethese parameters can vary greatly, for one experiment [1], values ofΩ(2) /2π ∼ khz∆/2π ∼ thz(δλ ∼ 1nm)

were typical. This high resolution allows, for example, the detection of small shifts in the
narrow velocity distribution of a BEC, which we will discuss further in Section 3.4.

38

∆E

a)

0

2h̄k

−4h̄k

−2h̄k

b)

0

0

2h̄k

2h̄k

4h̄k

4h̄k

c)
Figure 3-3: Kapitza-Dirac scattering. a) The two-photon Bragg process, which transfers
a quantity q = 2h̄k of momentum to an atom can only conserve energy for a particularvelocity class. This is the Bragg condition given by Equation 3.16. The finite duration ∆tof the Bragg pulse implies an energy uncertainty ∆E in the two-photon process, through theHeisenberg relation. This allows a spread of initial velocities to be out-coupled. b) Higher
order processes, such as this four-photon process, are possible. Because the dispersion
relation is quadratic, the Bragg condition for a four-photon process is met for a different
detuning (ω1 − ω2 ) than that for a two-photon process. c) An absorption image of a
BEC in expansion, which has been subjected to a Kapitza-Dirac pulse. For short enoughpulse duration, the energy uncertainty can cover multiple Bragg processes. Multiple s-wave
collision halos are visible.
39

Energy Uncertainty
The resolution of the Bragg process is effected by the finite duration of the Bragg pulse,
through the energy time uncertainty relation. In other words, a Bragg pulse contains a
spread of Fourier components. This has some obvious implications. The (single particle)
dispersion relation is quadratic, therefore the Bragg condition (Equation 3.16) is satisfied
for only one particular velocity class. This effect can be exploited as a way of measuring
the velocity distribution of an atomic sample spectroscopically. Even a BEC has a velocity
spread, due to its finite size, which can be resolved [107]. As Figure 3-3 illustrates, a shorter
pulse with greater energy uncertainty can accommodate a larger spread of velocities.
Higher Order Processes
The same approach we took in eliminating the intermediate state of a two photon process
can be built upon to describe higher order processes. Figure 3-3b shows, for example, afour photon process. The dynamics would evolve at a four photon Rabi frequency, and the
Bragg condition must reflect the kinetic energy of transferring a momentum of 4h̄k.

3.3.3

Kapitza-Dirac Scattering

For very short Bragg pulses, the energy uncertainty can become large enough that the
Bragg condition can be satisfied for multiple Bragg processes. This process is known as
Kapitza-Dirac scattering, and takes place for
τ¿

h̄
Er

(3.18)

Figure 3-3c illustrates the effect of a Kapitza-Dirac pulse on a BEC. For two beams of thesame frequency (δ = 0), a condensate initially at rest is distributed symmetrically amongst
momentum states of integer values of 2h̄k.

3.3.4

A light grating

In this section we offer a complimentary picture of the interaction of an atom with two laser
fields. Two intersecting laser beams will interfere to create a spatially modulated intensity
profile, as illustrated in Figure 3-4. This interference pattern can be treated as a spatially

40

~k1

~k2
Figure 3-4:
Laser beams withwavevectors ~k1 and ~k2 intersect
to produce an interference patternwith wavevector ~κ = ~k1 − ~k2 .

~κvarying AC Stark shift
V (~r) = 4 V0 cos2 12 (~κ · ~r − δt)(3.19)

where ~κ = ~k1 − ~k2 , and V0 is the AC Stark shift produced by a single beam (Equation 3.6).
A frequency difference δ between the beams sets this lattice in motion. For the time being,
we assume that the two beams have the same intensity, and neglect the spatial profile ofthat intensity. The spatial period of the lattice isλlatt =

λlaser2 sin θ/2

(3.20)

where θ is the angle between the lattice beams.
Diffraction from a grating
Bragg diffraction gets its name from the early experiments which involved scattering x-rays
off of crystals [12]. The diffraction of light from a periodic structure is ubiquitous in our
everyday lives (assuming you are in a laser lab everyday). One need not look any further
than the gratings used to stabilize our diode lasers and the acousto-optic modulators we
use to frequency shift our light.
We are of course interested in the Bragg diffraction of matter-waves. This, also, wasfirst accomplished quite some time ago, with electrons diffracted off of a crystal of nickel[30]. A little closer to home, Dave Pritchard’s lab diffracted a thermal beam of Na atoms
off of a periodic optical potential [79].
The principle of diffraction is that a wave which is scattered off multiple sources, likethe crystal lattice in Figure 3-5, will interfere constructively in some directions and decon-

41

θ1

θ2

d

Figure 3-5: X-ray diffraction. The angle of diffraction of a wave off a lattice of point
scatterers is given by the Bragg condition, Equation 3.21. This describes the condition for
constructive interference of the various scattered components.
structively in others. The Bragg condition is satisfied when different path lengths differ
by some integer number of wavelengths. For a crystal of lattice spacing d, interference isconstructive when the angles θ1,2 of incident and outgoing x-rays satisfyd (sin θ1 + sin θ2 ) = n λ(3.21)

or, in other words, the extra phase accrued by the longer path is an integer multiple of 2π.
Bragg diffraction : Atoms scattering off a light grating
Our “atomic physics version” of Bragg diffraction can equally be described in this grating
picture. The optical potential of a 1-D lattice acts as set of scattering planes, periodicallyspaced at d = h/q. An atom with incident velocity ~v has a de Broglie wavelength λ = h/mv.
We can rewrite Equation 3.21 as
2 d sin θ = λh ~v · ~q
h
−2
=
q vq
mv
q2
−~v · ~q =2m(3.22)

This is equivalent to the Bragg condition which we derived on energetic grounds in Section
3.3.2.

42

3.3.5

Band Structure

An obvious approach to understanding the physics of atoms in a periodic potential would
be to solve for the eigenstates of the system, in which the Hamiltonian includes the latticepotential. Fortunately the field of condensed matter physics has done most of the work for
us. The eigenstates are called Bloch states 3
The lattice potential is invariant under a translation of the lattice spacing d, i.e.,
Vlatt (z) = Vlatt (z + d). Bloch’s Theorem tells us that this implies that the wavefunction,
too, has translational symmetry, up to a phase. The Bloch wave functions areψQn (z) = eiQz uQn (z)(3.23)

where uQn (z) is some function with the periodicity of the lattice. n is the band index, andQ is the quasimomentum4 .
The eigenstates are not planes waves Φp of a certain momentum p, as they were for thefree particle case, but rather a linear combination of momentum states differing by h̄κ.
ψQn = . . . + c−1 ΦQ−h̄κ + c0 ΦQ + c1 ΦQ+h̄κ + c2 ΦQ+2h̄κ + . . .

(3.24)

where the c coefficients depend on n, Q and the lattice strength V0 . The quasimomentum
Q is essentially momentum modulo h̄κ, and we usually define Q to be in the range −h̄κ/2
to h̄κ/2, known as the first Brillouin zone.
In Figure 3-6 we shown the energies of the Bloch states for different lattice depth V0 .
In the limit of zero lattice, we recover the quadratic free particle dispersion relation. The
lattice has the effect of opening up gaps in the energy spectrum. For a very strong lattice,
the wave functions become localized on the lattice sites, which look like individual harmonic
oscillators. The bands flatten out, and the gap energy approaches the oscillator energy h̄ω.

3.3.6

Weak Lattice : Effective mass and interaction

Much of the physics of a shallow lattice (V0 ≤ 1Er ) can be captured in two phenomenologicalparameters. Or one, as we can see from Figure 3-6, the lattice changes the dispersion
3

named after Felix Bloch, not Immanuel Bloch.
Most literature uses “q” as the quasimomentum. I will use “Q” to avoid confusion with the previous
use of q as defined in Section 3.3.2
443

V0 = 0 Er

V0 = 5 Er

V0 = 10 Er

V0 = 15 Er

20Energy [Er ]

15

10

5

0

h̄κ
− 2

0

h̄κ
2

h̄κ
− 2

h̄κ
2

0

h̄κ
− 2

Q

Q

0

h̄κ
2

h̄κ
− 2

Q

0

h̄κ
2

Q

Figure 3-6: Band structure. A lattice potential modifies the free particle dispersion relation.
The translational symmetry allows us to describe a state by its quasi-momentum Q andband index n. In the limit of large lattice depth, the bands flatten and become the harmonic
oscillator energies for atoms on a single lattice site.
relation. For small changes, we can account for this by using an effective mass
Ã

Mef f

= h̄

2

!−1

∂2
E(Q)
∂Q2

(3.25)


√
√
2−4 2
√
32
+
s
³√
= M 1 − 16 2 √√ ´
32 + s2
32 + s2 + 4 2
where we have expressed the lattice depth s in units of the recoil energy Er .
The second effect of the lattice is to modify the the density, and therefore the mean-fieldenergy. We can account for this with an effective interaction.
Uef f

Z

=

4πh̄2 am=4πh̄2 a2α2 (4 + α2 )1+m
(1 + 2α2 )2"Vd~x |ψ0 (~x)|4

where D is the dimension of the lattice, andα≡s
4 (4 + s2 /8)
44(3.26)#D(3.27)

3.3.7

Bragg Scattering: Bloch State Interpretation

In Section 3.3.2 we introduced Bragg diffraction as the process of imparting momentum toatoms by redistributing photons between two laser beams. We can also describe this process
in the context of Bloch states. In the rest frame of the lattice (δ = 0), the Bragg process ish̄κ(Equation 3.16). It is no coincidence that
resonant for atoms moving with velocity v = 2m

this happens to be the Brillioun zone boundary. This (real) momentum state is an equalsuperposition of two eigenstates of the system
¯
¯
¯
E
E
E
¯
¯
¯
h̄κ
h̄κ
=
Q
=
;
n
=
0
+
Q
=
;
n
=
1
¯p = h̄κ¯¯222(3.28)¯¯

where n is the band index. Turning on the lattice projects the state ¯p = h̄κ2E

onto thisbasis, and the phases of the different components evolve at their eigenfrequencies. Whenthe relative phase becomes π, the population has been completely transferred into themomentum state
¯
¯
¯
E
E
E
¯
¯
¯
h̄κ
=
;
n
=
0
−
Q
=
¯p = − h̄κ¯¯Q = h̄κ222 ; n=1(3.29)

We know that the band gap is 2 V0 , so the probability of this transfer evolves as
P = sin2

V0 τh̄(3.30)

where it should be no surprise that the frequency of this oscillation is the two photon Rabi
frequencyΩ2
V0== 12 Ω(2)
h̄
4∆

3.3.8

(3.31)

Kapitza Dirac : a phase imprint

It is straightforward to to extend this description to include Kapitza-Dirac scattering, which
we discussed in Section 3.3.3. The abrupt turn on of the Kapitza-Dirac beams projects onto
the Bloch basis. The zero momentum state Φ0 becomes a superposition of Q = 0 states ineven bands (n = 0, 2, 4 . . .) due to symmetry. The Bloch states accrue a phase, and are thenprojected back onto the (real) momentum basis when the pulse ends, leaving the atoms
distributed over the discrete momentum states.

45

E
Figure 3-7: Bragg diffraction at the Brillouinzone boundary. The resonance condition for firstorder Bragg diffraction is satisfied for an atomwith momentum h̄κ
2 , with respect to the lattice, corresponding to the zone boundary. The
abrupt turn on of the lattice projects this plane
wave state onto the eigenbasis of Bloch states, as
an equal superposition of the lowest two bands.
The two eigenstates (filled and open circles) accrue a relative phase φ = 2 V0 t/h̄, corresponding
to the band gap. A relative phase of π corresponds to the real momentum state - h̄κ
2 .

ψ(t) =√1
2³|•i + |◦i eiφ(t)

0

´

2 V0

h̄κ/2

In fact, its quite easy to solve for what this distribution will be. Pulsing on the lattice
for a very short time can be treated as imprinting a phase on the wavefunction
exp [−i 2 V0 τ − i 2 V0 τ cos κz](3.32)

through the AC Stark shift. This sinusoidal phase can be written as a projection operator
over discrete momentum states∞Xeiθ cos κz =n=−∞Jn (θ) |e−inκz{z }(3.33)|p+nh̄κihp|

where the coefficients Jn are Bessel functions.
As for the validity of this approach : in order to treat the application of the lattice
as a phase imprint, we need to make sure all motion during the length of the pulse τ isnegligible compared to the lattice spacing d = h/q. Different momentum components movewith velocity v ∼ q/m, which means we want
vτ
τ

¿ d
hm
¿
q2

46

(3.34)

a)c)

Outcoupled fraction (a.u.)

b)

1

0

1

0

1

0
-100

-50

0

50

100

Detuning (khz)
Figure 3-8: Bragg spectroscopy of atoms and molecules. The spectra for a pure atomic
sample a) just above and b) below a magnetic Feshbach resonance at 907 G. c) Spectrumof a pure molecular sample just below the Feshbach resonance. Diatomic molecules recoilwith half the velocity of single atoms, due to their mass. Figure from [1].
which is equivalent to Equation 3.18.

3.4

Bragg spectroscopy

The need to conserve both energy and momentum makes Bragg diffraction a velocity selective process, as is evident in Equation 3.16. This opens the possibility for its use as aspectroscopic tool which can reveal velocity distribution of an atomic sample. The process
involves counting the number of outcoupled atoms as a function of the detuning δν between
the two Bragg beams. For a sample at rest, the signal (see, for example, Figure 3-8) is apeak at δν = 4Er /h, where the Bragg condition is satisfied. The width of the peak reflects
the velocity distribution, but may be broadened and shifted by a number of mechanisms.
The finite duration of the Bragg pulse introduces an energy uncertainty in the photons absorbed, as discussed in Section 3.3.2, and limits our resolution. The mean-field interaction

47

energy results in a shift of the resonance for a BEC. Distinguishable atoms experience twice
the mean-field interaction as do atoms in a condensate. This is reflected in the shift of the
Bogloliubov excitation spectrum (Equation 2.5), in the free particle limit. The mean-fieldenergy, however, is density dependent. This mean-field shift, then, must be integrated overthe density profile, resulting in an inhomogeneous mean-field broadening.
Bragg spectroscopy was first implemented with a BEC in Ref [107], and a more thoroughaccounting of the shifts and broadening can be found in Jamil Abo-Shaeer’s thesis [2]. We
used Bragg spectroscopy to study the velocity distribution of a cold sample of 23 Na2 dimers(See Appendix B). Starting with an atomic BEC, we converted atom pairs into diatomic
molecules by sweeping across a Feshbach resonance, as detailed in Section 2.3.1. The lifetimeof the molecules was too short to allow for thermalization, and could not necessarily beconsidered a BEC. However, we were able to show that the velocity distribution of themolecular cloud was consistent with a “temperature” that was well below the critical BEC
temperature.

3.5

Dynamical instability

In the previous sections we have discussed Bragg scattering in the context of single particles
and photons. Such a lattice can also influence two-body interactions, in a manner which
can redistribute momentum and result in dissipation. Ordinarily, two atoms with the same
momentum h̄k can not scatter off each other into different momentum states, say k + δk
and k − δk. There is no way to conserve both momentum and energy. However, an opticallattice can change the dispersion relation so as to make such a process possible (see Figure3-9). This process is analogous to the optical parametric generation of photons [16], inwhich the lattice provides for phase matching.
The energy spectrum of such excitations can be calculated for a BEC moving in an
optical lattice by considering small perturbations to the Gross Pitaevski equation [112, 78,113]. At a certain velocity, excitations with wavevector ±δk will have a complex energy andgrow exponentially. Dynamical, or modulational, instabilities do not exist below a velocityof 0.5 qB /m, where qB = h̄κ/2 is the Bragg momentum corresponding to the edge of theBrillouin zone. Above this threshold, however, these modes can be quite destructive [32].
In the photon picture, one can consider the two scattering atoms as sharing a Bragg

48

Energy−κ/20k0 −δkk0

k0 +δkQuasi-momentum

Figure 3-9: Dynamical instability. The scattering of two atoms with momentum k into stateswith momentum k ± δk does not conserve energy in the free particle dispersion relation. A
dynamical instability occurs when an optical lattice modifies the dispersion relation so as
to allow such a process.
excitation. At some momentum h̄k0 , a single atom can not satisfy the Bragg condition.
Although, together, two atoms can absorb the momentum kick −h̄κ. The two recoilingatoms scatter off each other with momentum ±h̄(κ − δk), in order to conserve energy. In
this description, both scattered states appear to the left of the initial state k0 . This isequivalent to our previous understanding, in which the state k0 + δk lies across the zone
boundary, and can be mapped back to the first Brillouin zone (see Figure 3-9).

49Chapter 4

Experimental Apparatus“Another flaw in the human character is that everybody wants to buildand nobody wants to do maintenance.”- Kurt Vonnegut, Hocus Pocus
When I first joined the lab, I was fairly intimidated by apparent complexity of the experiment: the jungle of BNC cables, racks of homemade electronics boxes and a forest of optics
on the laser table. Over time, I grew not only to understand this mess, but to contribute toit ... appreciably. In the summer of 2004, it was decided that the lab would change course,
directing our focus upon fermions in an optical lattice. Shortly after committing to this new
endeavor, at the ICAP poster session, we learned that the Zurich group [68] already had adegenerate Fermi gas in a 3-D lattice. Nevertheless, we felt we had a number of advantages.
In particular our fermion of choice, 6 Li rather than 40 K, was much longer lived. Indeed, by
May of 2006, we were the first (and still the only) group to have a superfluid Fermi gas ina 3D lattice. The question, of course, remains what to do with it.
This Chapter is intended to give an overview of the machine on which I have worked
for the past five years. It is far from a comprehensive owner’s manual. The Ph.D. thesis
of Dallin Durfee [37] chronicles the original building of this machine, as a 23 Na experiment.
The retrofitting of the oven as a double species model was done under the guidance andaccording to the precise specifications of Claudiu Stan [105], the many details of which canbe found in his thesis. A number of important modifications to the machine are recordedin the thesis Jit Kee Chin [23], as well. To this extensive body of work, I will add a largely
anecdotal account of things to do and things not to do.

50

4.1

Overview

While there are some variations on the theme, the preparation of a quantum degenerate
atomic gas has become fairly standardized. Our experiment begins with a sample of alkali
metal, heated in an oven, to produce a gas. A thermal atomic beam escapes through a nozel
and traverses the Zeeman slower. Here the atoms are slowed by a counter propagating laser
beam which imparts momentum as photons are scattered. As the atoms slow down and fallout of resonance with the light, due to the doppler shift, a spatially varying magnetic fieldcompensates by shifting the energy levels. The slowed beam makes its way into the main
vacuum chamber, where the background gas pressure is ∼ 10−11 Torr. Here, a magnetic
quadrupole potential and six intersecting laser beams create a Magneto Optical Trap (MOT)
which provides a confining as well as a viscous force.
After collecting atoms in a MOT, the light is shut off, and a Ioffe-Pritchard magnetic
trap confines weak field seeking atoms at a magnetic minimum. The sample is cooled further
by forced evaporation, in which a radio-frequency (r.f.) field flips the spin of the hottest
atoms, expelling them from the trap. The sample rethermalizes, and the r.f. “knife” isbrought to a lower temperature over thirty seconds. For 23 Na atoms, this results in a BEC.
The preparation of a 6 Li gas involves a few more steps.
Identical fermions can not be evaporated, as the Pauli exclusion principle prohibits thethermalizing collisions. As a first stage we use sodium as a refrigerant to cool the lithium
sympathetically. The cold lithium is then transferred to an optical trap, and the magnetic
field is ramped to 820 gauss. At this field we take advantage of a Feshbach resonancebetween the lowest two hyperfine states, which provides strong interactions and a coupling
to a weakly bound molecular state. We make a 50/50 spin mixture of these two spin statesand induce evaporation by lowering the optical trapping potential. The atoms pair up andform a BEC of molecules.
In the following Sections we will discuss the components of the experiment in greater
detail.

51

F=1 Fiber

RepumpBeam
Flipper
Waveplate

RepumpShutter

F=2 Fiber

D
F=2 Shutter

F=1 Table ShutterJ
MOT Fiber

SlowerShutter

Trap
Shutter

Slower FiberI

SlowerSidebands
1.71 GHz

G

Molassas
Sidebands
1.71 GHz

200 Mhz +Probe Scan
200 Mhz - e

HF

RepumpTTL
90 Mhz+1
0

Locking Set UpC+1
73 MhzTrap TTL

E
90 MhzSlower TTL+10
65 Mhz

125 Mhz +/--1

0 Slower AOM
325 Mhz
855 Mhz #2

to Wavemeter

B
Power Meter0+1

95 MHz red detuned from F=2 -> F'=3855 Mhz

A

899 dye laser

Figure 4-1: The sodium laser table. Letters denote the positions of power measurements aslisted in Table 4.1.

52

4.2

The Sodium laser system

4.2.1

The 899 dye laser

We use a Coherent model 899 dye laser to generate light at 589.160 nm (D2 line). Pumped
by 9.5 watts of 532 nm green light from a Spectra Physics Millenia solid state laser, we
get 1.6 watts of yellow light. The gain medium is Rhodamine 590G dye, dissolved in
ethylene glycol. The dye is prepared in ∼ 1.4 liter batches, which typically last 500 working
hours. The Rhodamine dye, dissolved in methanol, is added to the ethylene glycol until
the absorption of pump light by the dye jet is 90% (as measured at full power). The dye,
which is pumped through a high pressure nozzle at 12 bar, must be cooled to 6◦ C. This
maintains a high viscosity which results in the proper profile of the dye jet. The dye jetshould be flat at the center. Streaks in the jet may indicate a clogged nozzle, which can bedisassembled and cleaned with methanol in the ultrasound bath. Great care must be taken
not to nick the edges which form the nozzle opening.
It is not uncommon to find the laser power to have dropped precipitously since thelast use. Since the laser is a finely tuned machine, it pays to be prudent in your attempts
to get the power back, as not to cause greater harm. Adjustments to the birefringent
filter and the pump mirror alignment are the first course of action, and performed several
times a day. Additional fiddling should not be necessary more than once a month. Before
turning too many knobs, inspect the cavity mirrors for any obvious dust particles. Tweakthe output coupler next. This will require that the laser table be realigned. A full cavity
tune up, if necessary, should always be followed with a realignment of the reference cavity.
There are some less obvious surfaces which can accumulate dust, such as the underside of
the birefringent filter, and the fragile polarizer in the optical diode assembly. Wiping themirrors with lens paper and methanol is a necessary part of maintenance, but should be
kept to a minimum. As a Coherent tech once told me “the two worst things you can do toa laser cavity are, one, to get it dirty and, two, to clean it.”A less frequent source of trouble is the Millenia pump laser. A perfect Gaussian modeis essential to the efficient pumping of dye jet. The laser can begin to fail in such a way as
to put out a slightly distorted mode, usually at high power. I have seen two different units
exhibit this behavior. The signature of this failure mode is most evident when ramping
the laser power up from zero. The output of the dye laser will climb with increasing pump

53

Measurement Location

Power

A

Total Power

28 µA

1.55 WB

855 mhz #1

1.75 µA

C

855mhz #2

0.9 µAD

Repump TTL

0.7 µAE

Slower AOM

3.75 µAF

Slower TTL

3 µA

G

Slower Fiber

600 mV†H

Trap TTL

9 µA

I

MOT Fiber

500 mV‡

150 mW

J

Optical Pump

1.2 V

6 mW

33 mW

95 mW

Table 4.1: Typical laser powers. Measurement positions areindicated in Figure 4.2. Measurements in µA are taken withthe blue Hickok power meter. Measurements in mV are themonitor readings of photodiodes at the fiber output. Actual
powers in mW are measured with the Coherent optical power
meter.
† The slower is run at ∼ 350 mV.
‡ The MOT is stabilized to 425 mV.
power, and then drop abruptly at some threshold. It is possible to run the Millenia justbelow this threshold, if it results in sufficient power, but one needs to approach this value
from below, as there is hysteresis in the system. Obviously the best solution is to put in a
service call to Spectra.
There are two dirty tricks which can temporarily boost power, but can not substitute fora proper alignment. One is to add some dye concentrate (Rhodamine in methanol). This
only works if the current batch is getting old, and can quickly backfire if one adds too much.
A second, and cheaper, trick is to tighten the focus of the pump laser. Over-tightening thepump focus has two drawbacks. The output mode of the laser can change, taking on a “D”
shape, resulting in poorer fiber coupling. It will also make the laser less stable, falling outof lock much more frequently. When used in a pinch, however, these techniques can keep
things going long enough to get some data.

4.2.2

The laser table

For the most part, the laser table today remains exactly as it was when I first arrived.
An absorption cell is used for the locking, based on saturation-absorption spectroscopy. At
54

times, the lock signal may flicker between the familiar trace and sharp spikey noise. This justmeans that the thin etalon needs to be adjusted, as the gain profile is not centered on thedesired mode. We shift the light by a number of different frequencies using acousto-opticalmodulators. The basic principle is that an actuator, driven at some radio frequency, sends
acoustic waves down a crystal. This periodic density modulation acts like a moving grating,which diffracts the light into multiple orders, which are conveniently spatially separated.
As illustrated in Figure 4.2, we will pick off light from the main beam for a particularpurpose using an AOM. The zero order usually continues on to serve a different function.
It is noteworthy that the AOMs tend to pick out the best part of the mode, diffracting anearly perfect gaussian beam into the ±1 order, and leaving something less than ideal in
the zero order. As a consequence, redistributing power between paths is not necessarily aconservative process, when fibers are involved. On our table, light from the MOT path has
syphoned off for the Slower and Repump paths, and the fiber coupling is optimized for thisdepleted mode. If we were to take so many milliwatts out of the Slower by turning down
the Slower AOM, a disappointing fraction of that would end up coupled through the MOT
fiber.
Of all the optical components in the lab, one stands out as being the most valuable:
the 855 mhz IntraAction AOM. We purchase our AOMs from IntraAction Corp., wheneverpossible, since their products have the largest aperture size. Unfortunately, high frequency
AOMs can be challenging, and they claim not to be able to reproduce this model anymore.
And it would not be without precedent for it to fail. We used to have two in series, until
one died. To replace it with with the Brimrose would require that the beam be focuseddown to accommodate the smaller aperture, thus affecting everything downstream, whichis to say everything. Unfortunately, a more tightly focused beam, at 2 watts (on a goodday) would most likely exceed the damage threshold of the crystal.

4.2.3

The Sodium MOT

For Sodium, we use a dark spot magneto-optical trap [67]. The six intersecting MOT beams
have σ + polarization, and are tuned 22 mhz below the |F = 2i → |F 0 = 3i transition. A
single “repump” beam, tuned to the |F = 1i → |F 0 = 2i transition, pumps atoms out of the
F=1 state, which is dark to the MOT light. Only the outer edge of the MOT is illuminatedby the repump light. This is accomplished by imaging a dark spot, a blob of black nail55

polish on glass, onto the MOT. The glass cell is what most differentiates the New Lab, fromthe other three BEC machines on the hallway. It affords a superior optical access which ismost beneficial when the MOT needs realignment. Our well tuned Na MOT has produced
the largest BECs in the world (120 million atoms).

56

Measurement Location

Power

A

Total Power

40 µA

400 mW

B

Slower AOM

12 µA

110 mWC

Repump 228

15 µA

110 mW

D

Repump TTL

10.5 µA

80 mW

E

MOT TTL

11.5 µA

80 mW

F

Slower TTL

10 µA

75 mW

G

Slower Fiber

1.08 V

37 mW

H

MOT Fiber

1.65 V

55 mW

I

Repump Fiber

1.48 V

55 mW

Table 4.2: Typical laser powers. Measurement positions
are indicated in Figure 4.3. Measurements in µA are takenwith the blue Hickok power meter. Measurements in V are
the monitor readings of photodiodes at the fiber output.
Actual powers in mW are measured with the Coherent
optical power meter.

4.3

The Lithium laser system

We use a Toptica TA100 tapered amplifier laser system to generate the necessary light for
manipulating 6 Li. Many of the details of this system may be found in Ref. [101]. Priorto obtaining this relatively care free laser, we were able to work with a Coherent 699 dyelaser. While the power was more than sufficient, this laser required a significant amountof maintenance. In Figure 4.3, we show the arrangement used for obtaining the numerous
different beams required for the experiment.

57

Off Axis

On AxisSlower Sidebands
229 MHz0+1

From Laser
75

0 +1
750

250

0
+1

250

Repump239+ 58 mhz

250

C

λ/2

200

A

200
300

λ/4

λ/4

MOT TTL
81.1

Wavemeter

λ/4

0 -1

H+V

200

-1 0

75

-1 0

0

V

V

300

H

V

Slower TTL
79.9

V

H

0

E
H

300

D
175

Repump

H+V

300

G

f=150
500

V

-1

λ/2

λ/2

λ/2

-1

200

Slower

H

-1
0

I

MOT

0 +1

H

V

H

H

400

Slower
500

H

H

-1

-1 0

400
1/2 Img80+-20 (C)HF Img500±70 (A)

B

λ/2

0 +1

3/2 Img80±20 (B)100

RPMP TTL
80.1

F

0

Figure 4-2: The lithium laser table. Letters denote the positions of power measurements aslisted in Table 4.2.

58

4.4

The Infrared laser system

We use a 20 watt IPG Photonics fiber laser to generate the light for optical trapping andlattices. Figure 4-3 offers a schematic of the numerous IR beams which are used in theexperiment. The laser light is split up first on an optical table, the arrangement of which
details their chronology, before being sent to the experiment via optical fiber. In this Section
we describe how different beams are implemented.
Working with multiple beams
A single beam ODT will naturally produce a long cylindrical trap, since the Rayleigh range
is much longer than the beam waist. This geometry is not always desirable. We can,however, use two intersecting beams to produce a cross-ODT, which has a more sphericalshape. In this case, the trap frequencies produced by the two beams add in quadrature.
This is of course saying nothing more profound than that we add the potentials of the two
beams.
V (x) =

1
1
1
m ω12 x2 + m ω22 x2 = m (ω12 + ω22 ) x2
2
2
2

(4.1)

If this is the desired effect, we need to make certain that the two beams do not interferewith each other, resulting in a spatial modulation of the trap. In principle, one can set
the polarizations of the two beams to be orthogonal. However, this never works perfectly,
in practice. Additional security is gained by using different frequencies. If the beams arederived from AOMs which differ by tens on megahertz, any interference pattern will move
fast enough as to be time averaged away.
This, of course, assumes that your laser is single frequency. If it is not, you may wind
up with frequency components that differ just so as to satisfy a Bragg resonance (which
we discuss in the following Chapter). This was found to be the case when we used the
Versadisk laser by ELS, and resulted in significant heating even when the two polarizations
were made to be as orthogonal as possible.
Sometimes, even a single beam can be troublesome, as Figure 4-4 illustrates. There was atime when we noticed that a Na BEC released from our single beam ODT exhibited discrete
momentum components, characteristic of having been released from a lattice potential.
Except we weren’t applying a lattice. Or so we thought. The ODT was aligned along
our imaging axis, which used a dichroic mirror. While visible light was reflected onto the59TightHorizontal

Left
a)

TOP VIEWb)

Right
TOP VIEWTop

Beam waists (µm)
Horizontal

55

Vertical

60

Left

81

Right

89

Top

101

Tight

20

Verticalc)

SIDE VIEWd)Figure 4-3: Infrared laser system. Beam lines are shown with respect to the glass cell(box). a) The Horizontal and Tight beams are overlapped on a PBS cube before being
directed onto the imaging path by a SWP dichroic mirror (not shown). b) The Left andRight lattice beams are oriented in the horizontal plane at 45◦ to the slowing axis. c) The
Vertical beam is overlapped onto the MOT path using a SWP dichroic mirror. The Top
lattice beam makes an angle of 20◦ with the horizontal plane. d) Beam waists (e−2 radius).
60

Dichroic
MirrorBeamblock
CameraGlass Cell

Figure 4-4: An accidental lattice. An optical dipole trap (ODT) is aligned on the imaging
axis. A long wave pass dichroic transmits the ODT beam, which impinges on a beamblock.
When the beamblock is located at the focus of the imaging system, much of the scattered
light is imaged back onto the atomic sample, interfering with the incident ODT beam toproduce a lattice.
camera, infrared was transmitted, and so a beam block was positioned in order to preventthat Watt of IR from wandering aimlessly. This beamblock had been positioned, without
much thought, very close to the focus of the imaging system. So, while the light was being
scattered uniformly, much of it was simply re-imaged back onto the condensate, interferingwith the incident ODT.
Once the cause of this accidental lattice was identified, we simply moved the beamblock,alleviating the problem.

4.4.1

The different infrared potentials

The Tight Trap
After sympathetically cooling our Li sample, we transfer to an optical trap in order towork at high magnetic fields. At this stage, we need a large volume trap, so a single beam
trap is preferable to a cross ODT. Other experiments [121] use a hybrid trap, in which
the IR provides radial confinement, whereas the axial trapping is done magnetically. Thisconfiguration requires some finesse, as the magnetic moment is flipped when the atoms aretransferred from the |F = 3/2i to the |F = 1/2i state. Instead, we opted to use tightly focused beam which has a Rayleigh range sufficiently short so as to provide axial confinement.

61

The Tight beam is focuses up to 6 watts of power down to 20 µm. It is overlapped with
the Horizontal IR beam with a polarizing cube, and then onto the imaging path with ashort wave pass (SWP) dichroic mirror (not pictured). This beam uses enough of the totallaser power so as to limit other applications. For that reason, we are able to dynamicallyallocate power on the laser table. A λ/2-plate controls the distribution of power between
the Tight beam and other branches, splitting the two paths on a cube. The λ/2-plate ismounted on a rotation mount which is controlled by a servo motor. During the relatively
slow (1 s) evaporation stage we rotate the plate, putting the light to work elsewhere.
The cross ODT
We form a cross optical dipole trap at the intersection of the Horizontal and Vertical beams,
which have beam waists of 55 µm and 60 µm, respectively. This provides a relatively
spherical trap for our lattice work. This is preferable from primarily a technical standpoint.
An elongated trap presents too large of a surface area to the lattice beams. The beams are
detuned by 60 mhz with respect to each other, to prevent interference effects.
The optical lattice
The optical lattice featured in Ref [24], which we discuss in Chapter 6, is comprised of threedifferent beams, retro-reflected upon themselves, and detuned by tens of mhz with respect
to each other. The Left and Right beams are in the horizontal plane, intersecting the slower
axis at 45◦ . The Top beam makes an angle of 20◦ with the horizontal plane, resulting in a
lattice which is sheared rather than square.
The moving lattice
In the critical velocity experiment (Chapter 7) we use a moving 1D lattice to probe the Li
sample, in one of two different configurations (see Figure 7-3). A moving lattice usually
involves two different beams1 . A frequency difference between the beams δν sets the latticein motion with velocity v = δν λL . The beams, which intersect at 90◦ , produce a latticespacing of λL = 0.75µm, corresponding to a recoil energy Er = h × 7.3 khz for a Li pair, andtwice that for a single atom (see Equation 3.17). The frequencies of the beams are shifted
1

Alternatively, a DC EOM can be placed in the retro-reflecting path of a single beam. The lattice is thentranslated by changing the voltage across the nonlinear crystal.

62

by AOMs on the laser table. For such small shifts (∼ 10 khz), the AOM deflection is not
significantly altered, and the coupling into the fibers is preserved. This velocity can be becomputer controlled for experimental convenience. An Agilent function generator provides
the signal to the AOM driver, and can be GPIB controlled using a fine piece of software
written by Widagdo Setiawan [101].
The first lattice geometry we use is similar to that used in the 3D lattice experiment [24],in which the lattice is relatively homogenous over the size of the trapped cloud. The sample
is held in a cross ODT, formed by the Horizontal and Vertical beams. The 1D lattice isformed by the interference of the Left and Right lattice beams (the retro-reflections areblocked).
The second configuration we use aims to localize the lattice within the trapped cloud.
Here we use the Left and Right lattice beams as a cross ODT, which has a larger volume
then that formed by the Horizontal and Vertical beams. The lattice is created at theintersection of the Vertical and Tight beams.

4.4.2

ODT Calibration

While the trap frequencies are simple to calculate, we never take such calculations at facevalue. The most reliable method of calibrating our trap is to watch a dipole oscillation.
A small pulse of current through one of the magnetic coils will kick a condensate, and we
can measure the oscillation frequency by taking shots at successive hold times. For our two
species experiment, it is often easier to characterize the trap with a 23 Na condensate, andscale the trap frequencies for 6 Li. The scaling goes as the square root of the polarizability
over the mass, and for 1064 nm light is
ωLi
= 2.1
ωN a

(4.2)

For a shallow trap, one must be careful, as the two species experience a different gravitationalsag. Usually this gradient can be compensated by magnetic levitation.
There are some situations in which applying a magnetic kick is not possible, at least notalong all three principle axes. For example, at very high bias fields it is difficult to apply
a radial gradient, since fields would have to add in quadrature to the strong bias. In thiscase, it may be best to use parametric heating. A modulation of the trap power at twice

63

1.0
0.8

E0,2 − E0,0 [Er ]Band Populations

14

0.6
0.4
0.2

12
10
8
6
4
2

0.0

0
0

5

10

15

20

0

Lattice Depth s

a)

b)

5

10

15

20

Lattice Depth s

Figure 4-5: Band populations and energy gap. a) At a lattice depth s, the zero momentum
state is a superposition of the eigenstates with zero quasimomentum in the n = 0 andn = 2 bands. The weightings of ψ0,0 (solid line) and ψ0,2 (dashed line) are shown (see also
Equation 4.3). b) The energy difference between the n = 0 and n = 2 bands at Q = 0 sets
the oscillation frequency of Kapitza-Dirac scattering, and can be approximated by Equation
4.4.
the trap frequency will induce resonant heating.

4.4.3

Lattice Calibration

There are a number of ways to calibrate an optical lattice [117]. On can, for example, lookfor resonant heating. Modulation of the lattice depth will drive a transition between the
n = 0 and n = 2 bands at the correct frequency. Similarly, shaking the lattice back andforth will excite transitions between the n = 0 and n = 1 bands, although this is not easily
accomplished in a simple retro-reflected configuration.
We usually calibrate our lattice using Kapitza-Dirac scattering (Section 3.3.3). Abruptly
turning on the lattice potential projects the zero momentum state Φ0 onto primarily thelowest two even bands. The band populations as a function of lattice depth s = V0 /Er are
shown in Figure 4-5a. The overlap with the second band is approximately|hΦ0 |ψ0,2 i|2 ≈0.4 s2.
73 − 2.86 s + s2

(4.3)

The population of the zero and first order diffraction peaks, in time of flight, oscillate as afunction of the pulse duration at a frequency given by the energy difference between bands.

64

This energy difference, shown in Figure 4-5b, is approximatelyrE0,2 − E0,0 ≈

16 +

12 2
s Er .
25(4.4)

In the limit of vanishing lattice depth we recover the free particle relation. The kinetic
energy associated with absorbing two photons is 4 Er . At low lattice depth, this curve isfairly flat, and the Kapitza-Dirac oscillation is not a very sensitive measurement of V0 . It
is best to work with a potential V0 > 10 Er .

65

Figure 4-6: The broad Feshbach resonance between the two lowest hyperfine states in 6 Li,
as given by Equation 4.5, from [8]. A narrow resonance also exists at at 543 G, near the
528 G zero crossing, and is not resolved on this scale.

4.5

The Lithium Feshbach resonance

While not exactly a piece of equipment, the Feshbach resonance between the two lowest
hyperfine states in 6 Li does provide a “knob” which we quite frequently turn. A good dealof effort has been put into precisely characterizing this resonance [8]. Here I will simply
state the result, which we show in Figure 4-6.
µa(B) = abg

1−∆B
B − B0¶(1 + α(B − B0 ))(4.5)

with abg = −1405 a0 , B0 = 834.15 G, ∆B = 300 G, and α = 0.040 kG− 1; here a0 = 0.529177
nm is Bohrs radius.

66

z( N2 + 14 ) λ
}|

{

75 Ω

Figure 4-7: The Q-section antenna, built from a 75 Ω BNC cable.

4.6

The Q-section antenna

Efficient microwave evaporation is an important aspect of the preparation of a degenerategas. High power amplifiers are expensive. Not only is it a shame to waste power throughpoor coupling, but it can be counter productive. Before we made any attempt to better
impedance match our antennas to the 50 Ωtransmission line, many antennas mysteriously
failed after a few days of use. Fortunately there is a simple way to achieve decent impedance
matching.
A full-wave rectangular loop antenna, i.e. an antenna that is one wavelength in perimeter, has an impedance of about 100 Ω, as a rule of thumb. If we use a section of 75 Ω
cable in between the antenna and the 50 Ω cable which runs to the amplifier, we can cancelmuch of the loss. Specifically, the 75 Ω Q-section needs to be λ/4 in length (plus an integer
multiple of λ/2). The result is that the reflection off the 50 Ω to 75 Ω interface will interfere
deconstructively with that off the 75 Ω to 100 Ω interface.
We implemented this by making the antenna out of 75 Ω BNC cable (RG59). A lengthof the cable is left intact for the Q-section, and the rest is stripped. The core was cut tothe appropriate length for one wavelength, which for 1.7 Ghz, is about 7 inches. The end
is then soldered to the shielding to complete the loop. As a finishing touch, the bare wire
was insulated by wrapping Teflon pipe seal tape around it.
The antenna which we have made in this fashion has worked reliably for two years,
and is many times more efficient than unmatched antennas, as measured by the minimum
microwave power required to achieve Na BEC.

67

Chapter 5

Matter-wave Interference in a
Thermal Cloud
Wave particle duality is fundamental to the quantum mechanical description of matter.
Since the first observation of matter wave interference [30], matter wave interferometry
has been discussed as superior to optical interferometry, in certain applications, owing to
the accessibility of short wavelengths and long interaction times. Atom interferometers havesince emerged as the leading instrument for certain precision measurements [51, 75, 90, 50].
While a BEC exhibits a spatial coherence that is equal to its size [87, 52, 10], a trapped thermal cloud can exhibit strong interference effects. This motivates a better understanding of
the coherence properties of non-condensed atoms.

This chapter supplements work reported in the following publication:
D. E. Miller, J. R. Anglin, J. R. Abo-Shaeer, K. Xu, J. K. Chin, and W.
Ketterle “High-contrast interference in a thermal cloud of atoms,”
Phys. Rev. A 71, 043615 (2005).
(Included in Appendix A) Ref [83]

68

5.1

A Young’s double slit for cold atoms

In the classic Young’s double slit experiment, light is incident on a plate which has two slits
cut into it, separated by a distance d. Further down the line, the intensity of the transmitted
light exhibits fringes, resulting from the interference between the two “coherent” sources.
The fringe spacing is inversely proportional to the slit spacing d.
A similar experiment was able to clearly demonstrate the wave nature of matter [30].
Electrons passing through a double slit would impinge upon a detector, registering as anindividual spot. The accumulation of many spots, however, would add up in the form of aninterference pattern. While the measurement of each electron collapsed the wavefunction
to a single position, the ensemble average revealed a probability distribution |ψ|2 which had
fringes.

5.1.1

Matter wave interference of BEC

The first matter wave interference with BEC used a geometry similar to the Young’s double
slit experiment [6], which we illustrate schematically in Figure 5-1. A blue detuned laser
beam was used to divide a trapped BEC into two. When released from the trap, the timeof flight image revealed interference fringes.
During ballistic expansion, the wavefunction acquires a quadratic phase profile. In otherwords, an atom which has traveled a distance x during an expansion time t must have had
a velocity v = x/t, and accrued a phase proportional to this kinetic energy. We can write
the wavefunction as the combination of two wave packets, separated by a distance d.
½qψ± (x, t) =n± (x, t) exp

im(x ± d/2)2
2h̄t

¾

(5.1)

where n(x, t) is the density. The resulting matter wave interference pattern is"n(x, t) = n0 (x, t)

Ã1
1
2 + 2 cos

x
2π
λf

!#

(5.2)

where n0 (x, t) is the density envelope, and the fringe spacing is given by
λf =

ht
md

69

(5.3)

d
Figure 5-1: Young’s double slit type experimentwith matter waves. A wavefunction consistingof two wavepackets separated by a distance d reveals an interference pattern with fringe spacing
λf = ht/md, after a time t of ballistic expansion. This reflects the different phases accruedby the different path lengths.

0
5.1.2

x

Trapped atom interferometry

An interesting application of this geometry would be to create the analogue of Mach-Zenderinterferometer with atoms. For example, we could have included a fixed phase term in Equation 5.1. Applying a phase shift to one wave packet shifts the position of the interference
fringes. A number of experiments have demonstrated ... double-well

5.2

Interference of Thermal Atoms

The interference pattern we described in Equation 5.2 is just as valid for a single atom
wavefunction as it is for the macroscopic wavefunction of a BEC. However, an ensembleof thermal atoms will add up incoherently, reducing the fringe contrast. In addition, forthermal atoms, the double-well potential does not in general result in the wavefunction we
desire: that is, a superposition of two spatially separated wavepackets. The thermal de
Broglie wavelength λT is usually shorter than the well separation d which can be achieved,
and one ends up with two incoherent thermal clouds. We can, however, create this wavefunction using Bragg diffraction.

5.2.1

Autocorrelation pulse sequence

In our experiment, a sample of 23 Na atoms was cooled in a magnetic trap to a temperature
T above the critical Bose-Einstein condensation temperature Tc , using rf-evaporation. We
used a sequence of π/2 Bragg pulses to prepare each atom in a superposition of spatiallyseparated wavepackets. The details of the sequence are given in Figure 5-2. The two Bragg
beams, which intersect at ∼90◦ , were large enough to be considered uniform over the size
RT of the trapped thermal cloud. In addition, the duration τ = 10 µs of the Bragg pulse

70xFigure 5-2: Autocorrelation pulse sequence.
The first π/2 Bragg pulse creates an equal superposition of states with momentum 0 and h̄kr .
After a time τw , the two states have separated
spatially by a distance d. A second π/2 pulse
converts each wavepacket into a superpositionof the two momentum states. The result is one
pair of wave packets at rest, separated by d, anda second pair traveling with momentum h̄kr .

t=0

t = τwdh̄krt
was chosen to be short enough so as to couple the entire range of velocites in the thermal
cloud, as in in Figure 3-3a.

5.2.2

Interference fringe contrast

Figure 5-3a shows an absorption image of a thermal cloud which has expanded after thepulse sequence. We fit the integrated optical density to the following function
"

!#

Ã

2π
x+φ
n(x) = f (x) 1 + C sin
λf

.

(5.4)

to obtain a value for the fringe contrast C. The envelope function f (x) is that of two
gaussians, accounting for both the stationary and recoiling (h̄k) clouds. The contrast is
shown to be a decreasing function of cloud temperature, as shown in Figure 5-3c.
The functional form of the contrast can be easily derived. As we discussed in Section
5.1.1, a single particle wavefunction gives perfect contrast, with a fringe spacing λf = ht/md.
We can model our thermal cloud as an ensemble of gaussian wavepackets of a width givenby the thermal de Broglie wavelength1h
λT = √
2 m kB Tq
1

The de Broglie wavelength is sometimes defined as λT =the partition function as ζ ≡P −²/kT
e

(5.5)
2πh̄2
.
mkB T

This choice is made so as to write= V /λT , where V is the volume, as in [96, 91]

71

a)

c)
Fringe Contrast

1.0

b)

0.8

0.6

0.4

0.2

0.0
0

1

2

3

4

5

Temperature [µK]
Figure 5-3: High contrast interference of a thermal cloud of atoms. a) Absorption image
taken after 48 ms expansion. b) The integrated optical density is fit to Equation 5.4, resulting in a value for the fringe contrast C. c) Measured fringe contrast C as a function oftemperature, for fringe spacing λF = 340 µm, 230 µm, 170 µm (circles, triangles, squares).
The theoretical contrast of Equation 5.8 (dashed lines) describes the data well at low temperature.
These wavepackets are distributed over the thermal size of the cloud RT in the trap
s

kB T
m ω2

RT =

(5.6)

where ω is the trap frequency along the axis we are probing. Integrating over the size ofthe cloudZ∞22

dx0 √ 1 2 e−x0 /2RTn(x) =−∞

1
2 f (x − x0 )

2πRT

h

i

1 + cos λ2πf (x − x0 )(5.7)

we find the reduced fringe contrastÃ

2π 2 RT2
C = exp −
λ2f

!

(5.8)

More simply stated, we loose contrast when we smear out the signal over a size equal to
the fringe spacing. In Figure 5-3c we see that this result, without any free parameters, is inexcellent agreement with the measured values of contrast, for several values of λf . At high72

temperatures , however, the data deviates from Equation 5.8. We will discuss the reason
for this in Section 5.3.
Density Matrix
The model we have suggested, gaussian wavepackets of width λT distributed across RT (asillustrated in Figure 5-4), may appear to have been derived haphazardly. However, we notethat this is simple model is exactly what comes from a more rigorous approach. The k-space
distribution of noninteracting bosons in a harmonic trap, in the high temperature limit inwhich Maxwell-Boltzmann statistics apply, is given by the single particle density matrix fora harmonic oscillator. This may be written exactly as an ensemble of Gaussian pure states,
incoherently averaged over their position x0 :q

1+tanh2 2kh̄ωT Z
B

0hk |ρ̂HO (T )|ki =

2π0×ei(k−k )x0 e−→

kB T Àh̄ωwhere RT =

1
2π

Z

dx0 e

dx0 e

ω 2
−M
x coth kh̄ωT
2h̄ 0
B

h̄
(k2 +k02 ) tanh 2kh̄ωT
− 2M
ω

x2
− 02
2R
T

B

0

1

2

2

02

ei(k−k )x0 e− 2 λT (k +k ) .

p

kB T /M ω 2 is the classical thermal size of a Maxwell-Boltzmann cloud ina harmonic trap of frequency ω. It is therefore appropriate to regard the thermal cloud
as a collection of wave packets of gaussian width λT , distributed in space according to
Maxwell-Boltzmann statistics2 .

5.2.3

Coherence length

It is also convenient to rephrase Equation 5.8 in the following wayÃd2
C = exp − 2
2 `c

!

(5.9)

where we have defined a coherence length `c as`c =2

λT size in TOF
2π size in trap

I thank James Anglin for validating my balls and springs model.

73(5.10)

a)

b)

RT

λT

Pulse Sequence
andBallistic Expansion

Figure 5-4: Reduced fringe contrast of a thermal cloud. a) The wavefunction of a single atom
subjected to the autocorrelation Bragg pulse sequence is modulated with perfect contrast
after expansion. b) The probability density of multiple thermal atoms add incoherently.
An ensemble of single atom wavefunctions, of width λT , distributed over a trap with size
RT result in a fringe contrast less than unity (Equation 5.8).

74Figure 5-5: Contrast emerges in time of flight.
The coherence length `c grows during expansion,and eventually exceeds the initial separation d =2 µm. Expansion time is a) 14 ms, b) 20 ms, andc) 25.

Expansion time

a)

b)

c)

That is, the coherence length of an atom in the trap is λT , and contrast is lost when theseparation d is larger than `c . However, during expansion, the local spread of momentum
states decreases, and `c grows in proportion to the size of the cloud ht/mλT .
The implication is that we can always see interference from a thermal cloud, provided
we give it enough expansion. In Figure 5-5 we show that interference which is imperceptible
shortly after the cloud is released emerges in longer time of flight.

5.2.4

Ramsey Fringes

There is another framework for describing this experiment which I like, and that is to invoke
the paradigm of Ramsey spectroscopy. In a Ramsey’s method of separated oscillatory fields,
two π/2 pulses are administered, separated by some wait time τw . The probability of being
in the excited or ground state oscillates as a function of the wait time (or, equivalently,
the phase of the field which induces the π/2 pulse). In the Bloch sphere picture, the first
pulse rotates the Bloch vector up to the equator. During the wait time, the Bloch vector
precesses, and a second pulse rotates the Bloch vector again. The projection onto the zaxis, and thus probability of being in one particular state, depends on how far it precesses
azimuthally.
In this experiment, rather than varying τw or the relative phase of the pulses, we use aspread of momentum states. A momentum state |h̄ki is coupled to |h̄(k + kr )i, where kr is

75

a)

0.9

c)

0.6

Fringe Contrast

0.7
0.5

0.5

0.3
1

0.4

2

3

4

5

0.3

0.2

b)
0.1

1

2

3

4

5

Temperature [µK]
Figure 5-6: Fringe contrast for a filtered thermal cloud. A τw = 30 µs long Bragg pulse
is used to out-couple a narrow slice of the available momentum states. a) An absorption
image shows high contrast interference for this velocity subset with λf = 170 µm. b)
The integrated optical density is fit to extract a numerical value for the contrast C. c)
Measured fringe contrast is found to exceed that predicted by Equation 5.8 (solid line). A
model which introduces an effective de Broglie wavelength λ0T (Equation 5.12) above an
onset temperature T0 follows the data (dashed line). This model is also applied to theτw = 10 µs pulse data from Figure 5-3: shown for λf = 340 µm (inset).
the recoil momentum. The relative phase which accrues during τw isδφ =h̄(k)2h̄kr τwh̄k 2 τwh̄(k + kr )2
τw −
τw =
k+ r
2m
2m
m
2m

(5.11)

where the last term is just a constant. In other words, the phase is linear in the initial momentum h̄k. And since the cloud in time of flight reveals the in trap momentum distribution,
the image is like a Ramsey fringe taken in one shot.

5.3

Momentum “Filtered” Interference

In Section 5.2.2 we discussed a simple model which could quantitatively describe the contrastof an interfering thermal cloud. At low temperature our model provides a very accurate
description of the observed fringe visibility. At higher temperatures, however, the observed
contrast is consistently greater than expected. One possible explanation for this behavior76

is that our model fails when the Bragg pulses becomes velocity selective. At temperatures
greater than some characteristic temperature T0 , the momentum spread h̄/λT will exceed
the Fourier width of the Bragg pulses. Some momentum components are then excluded
from the process. The Bragg coupling spans a range of δp = πm/kr τp . This implies anonset temperature T0 = δp2 /2mkB for velocity selectivity. Indeed, for a 10µs pulse, we find
T0 = 2.7µK, which agrees with where the data begins to deviate from the theory curve.
In order to further investigate this possibility, we repeat the experiment at a longer pulse
duration τp = 30 µs, which is more velocity selective. In Figure 5-6a, it is clear that we
address a narrow range of momentum space. We must now account for the large backgroundwhen fitting the cross section. While the fraction of atoms out-coupled falls with increasing
temperature, the interference is still clearly visible. In fact, we find that the contrast (Figure5-6c) is greatly enhanced over our simple theory (solid line).
The reason for this finding is not clear. Generally, one would not be surprised to findless coherence than expected, but to find more is striking. Perhaps this reflects a fault withthe distribution in our single particle model, perhaps interactions play a role and a single
particle model is insufficient. There is, however, no obvious weakness in our treatment. In
the following Section, we describe a phenomenological model which does a pretty good job
of following the data, but should be taken with a very large grain of salt.

5.3.1

Phenomenological Model for Enhanced Contrast

When data is found to follow some completely unexpected trend, the inclination is to finda model which describes this trend. This Section is the result of such an effort.
We found that the fringe contrast of an interfering thermal cloud was significantly enhanced when we filtered the sample, by using longer Bragg pulses. Since this implies anarrowed region of momentum space, correspondingly our atoms may be considered tohave a larger effective de Broglie wavelength, and thus a larger coherence length. We canqualitatively introduce the effective de Broglie wavelengthh̄
λ0T = √
(T T0 )−1/4
2M kB(5.12)

If we use λ0T in our description of the coherence length (Equation 5.10), we predict a contrast
which agrees remarkably well with the data. In Figure 5-6c, we show this prediction (dashed77

line) for both the long Bragg pulses, as well as the short pulses (inset) discussed earlier.
When we allow the power of T and T0 to vary as a free parameter in the fitting (i.e.

³

T α T01−α

5.4

´1/2

) we find that Equation 5.12 is a good guess (α ' 0.53).

Conclusion and Outlook

In this chapter we have explored the coherence properties of a thermal cloud of atoms.
Under the appropriate conditions, a thermal sample can exhibit high contrast interference,
analogous to a white light interferometer. Bose-Einstein condensation is not a precondition
for obtaining an interference signal which may be applicable for a number of measurements.
This, however, is a single particle effect. Unlike two BECs, two independent thermal clouds
will not interfere with each other.

78

Chapter 6

Degenerate Fermions in an Optical
Lattice
Strongly interacting Fermi gases have emerged as a new frontier where atomic physics andcondensed matter physics overlap. The study of these quantum gases in the presence of an
optical lattice may yield important insight into the nature of superconductivity in crystalline
materials. The tools of atomic physics present a degree of control unavailable in solid statesystems, offering a promising test bed for theoretical models which have sought greater verification for decades, as well as the opportunity to explore exciting new phenomena which
are uniquely atomic.

This chapter supplements work reported in the following publication:
J.K. Chin, D.E. Miller, Y. Liu, C. Stan, W. Setiawan, C. Sanner, K. Xu and
W. Ketterle “Evidence for Superfluidity of Ultracold Fermions in an opticallattice,”Nature 443, 961 (2006).
(Included in Appendix C) Ref [24]79

I was in a bar, in Knoxville, when I received an ecstatic phone call from the lab. From
what I could make out, my labmates had just seen the first interference pattern of a fermion
pair condensate released from a 3D optical lattice, conveniently, while I was away for the
DAMOP conference. Over the following weeks we would complete what was to be the first
paper of the newly refurbished machine, a triumph which was celebrated with a champagne
toast at a 10 A.M. group meeting, following a sleepless night in the lab. In this Chapter,
I will discuss these results, as well as some which were not featured in this publication.
The Ph.D. thesis of Jit Kee Chin will provide a far more comprehensive account of thisexperiment.

6.1

The signature of superfluidity

There is almost no limit to the phenomena which can be studied in an optical lattice.
To name just a few, experiments have demonstrated Bloch oscillations [97] and fermionic
antibunching [98], whereas the phenomena anti-ferromagnetic order [111] and d-wave superfluidity [56] remain on the horizon. It was our goal to achieve superfluidity in a 3-D
optical lattice. The signature for superfluidity, with some technical qualifications, is theappearance of sharp interference peaks at the reciprocal lattice vector, after expansion.
Despite our lab’s extensive experience with loading Na into an optical lattice [114, 115],
and a healthy Li pair condensate1 , the first signal proved elusive. When released from thelattice, the cloud appeared heated. However, if the lattice was adiabatically ramped down
before the trap was switched off we regained a condensed sample. It soon became apparent
that collisions between the discrete momentum components were the culprit. In an atomic
BEC this effect manifests as the s-wave collision halo, visible in Figures 3-2 and 3-3. Fora strongly interacting Fermi gas, however, the collisional cross section is much larger. Secondary and higher order collisions become appreciable, and the sample looks isotropicallyheated. The solution was to cut the magnetic field as quickly as possible, reducing interactions, before turning off the optical lattice. Figure 6-1 illustrates the effects of collisions at
different magnetic fields. We found that keeping the lattice on for an additional 150 µs as
the field is ramped was sufficient to ameliorate the effect of collisions.
In Figure 6-2, we show a schematic of of the imaging geometry which details the pro1

the preparation of which is detailed in Chapter 480

Interaction Parameter kFa
10a

Magnetic Field (G)

0.3

1
b

c

822

x

730

470

//
//

0

0.15

2

4

6

8

Time (ms)Figure 6-1: Dissipative collisions between momentum components. A Li pair condensate
is prepared at 822 G. The magnetic field is cut abruptly when the cloud is released fromthe trap. As a probe of the scattering cross section σ, a short Kapitza-Dirac pulse isadministered at different times during the field ramp, corresponding to magnetic fields (a)822 G, (b) 749 G and (c) 665 G. The absorption images reveal different degrees of heating
due to collisions between the discrete momentum components. For a sufficient reductionof the interaction parameter kF a, and therefore the scattering cross section, the distinct
momentum peaks are visible.

81

a)

b)

c)

d)

e)

f)

g)

h)

i)

Figure 6-2: Interference of fermion pairs released from an optical lattice. a) Schematic of
the imaging geometry, detailing the projection of the reciprocal lattice onto the imaging
plane. (b-i) Absorption images of fermion pairs at a field of 822 G, released from varying
lattice depths V0 . Values of V0 are b) 0 Er , c) 2.5 Er , d) 4 Er , e) 5 Er , f) 6 Er , g) 7 Erand h) 9 Er . i) The interference pattern is resurrected for a cloud which is ramped up to
10 Er , and then back down to 2.5 Er before being released.
jection of reciprocal lattice onto the imaging plane. We also show time of flight images fora pair condensate released from an optical lattice of varying depth. The sharpness of theinterference peaks, with respect to their separation, provides an upper bound for the coherence length of ten lattice sites [42]. This neglects broadening mechanisms, such as the mean
field energy. For deeper lattices, the interference pattern is replaced by a broad gaussian, apossible indication of an insulating state, which we discuss in the following Section.

6.2

The insulator state

Let us consider a sample of bosonic atoms in an optical lattice. In the superfluid state, theatomic wavefunction is delocalized across lattice sites. In expansion, the wavefunction ateach lattice site acts as a coherent source, which adds up to produce the familiar interference
82

h̄ω

Figure 6-3: Individual lattice sites can beapproximated as harmonic oscillators withenergy spacing h̄ω. For repulsive interactions, there is an energy cost U for two
atoms to occupy the same lattice site. The
application of a potential gradient results
in an energy difference ∆E between sites.
For a conducting state, this will result incurrent. An insulating state, however, will
not respond.

U∆E

pattern. This delocalization implies an uncertainty in where an atom will be found, andtherefore an uncertainty in the number of atoms which will be found on a particular latticesite. The occupation number of each lattice site is given by a Possionian distribution. Thereis an energy “J” associated with this delocalization, referred to as the tunneling energy.
For an interacting gas, this number uncertainty has a cost. A state in which two (ormore) atoms can be found at the same lattice site must pay the interaction energy U , whichis positive for repulsive interactions. If the cost of interaction U exceeds the benefit ofdelocalization J, it becomes energetically favorable for each atom to find its own lattice siteand stick to it. This is known as the Mott insulator state [46], in which the wavefunction
at each lattice site is described by a Fock state: a fixed atom number at each site. Thiscertainty in number, however, corresponds to an uncertainty in the phase between adjacentsites, which washes out the interference pattern, leaving only a blurred gaussian cloud inexpansion.
The ratio U/J can be increased with the lattice depth V0 . It is a characteristic featureof the Superfluid-Mott insulator transition [45] (SF-MI) that the interference pattern is lostas the lattice depth is increased and U/J exceeds some critical value. Because the Mott
insulator is the ground state, lowering the lattice can adiabatically convert the system backinto a superlfuid state, restoring coherence. There are some subtleties in the interpretation of the interference pattern [42, 35]. There do exist, however, other methods to moreaccurately identify the Mott insulator transition [3, 86].
Fermions
We turn now to the system at hand: a spin mixture of strongly interacting fermions in an
optical lattice. In Figure 6-2, we observe the disappearance of the interference pattern with83Central peak width (a.u.)

a)

16

14

12

10

8

1

2

3

4

5

6

7

8

Probe lattice depth Vprobe
b)

∆T

Time

Lattice depth

VprobeVprobeGradient B 0

Gradient B 0

Lattice depth

c)

∆T

Time

Figure 6-4: Excitation of a superfluid Fermi gas in a lattice. Starting with a pair condensate
at 822 G, we ramp a 3-D optical lattice up to V0 = 8 Er , then back down to 2 Er , where itis held for 150 µs before release. The ramp up is interrupted at a particular value Vprobe fora time ∆T , during which magnetic gradient of B 0 = 4 G/cm may be applied. A schematicof the ramp sequence for different values of Vprobe is given in (b) and (c). After expansion,
the gaussian width of the central momentum peak is used as a measure of coherence. (a)The measured width is shown as a function of the lattice depth Vprobe at which the gradientis applied (filled markers). For comparison, the ramp sequence is repeated without theapplied gradient (open markers). As Vprobe is increased, the excitation effect of B 0 is reduced,
perhaps indicating an insulating state. The depth Vprobe at which this insulating behavior isobserved varies with the hold time ∆T (10 ms and 30 ms; circles and triangles, respectively).

84

increasing lattice depth, as well as its restoration when the lattice is subsequently lowered.
This behavior is qualitatively consistent with the superfluid-insulator transition described in
the previous Section. However, one must be careful in our treatment of this pair condensate.
While the pairs do in some respect behave as composite bosons, at some level the natureof the constituent fermions must play a role. In particular, the interaction energy U is not
likely to be valid, insofar as we have defined it. The Pauli exclusion principle requires thatpairs on the same lattice site be in orthogonal states, and the relevant “interaction” energy
may be the harmonic oscillator spacing h̄ω. In this case, the single band model is no longer
sufficient.

6.2.1

Insulating Behavior

While our description of the Mott-insulator is not strictly applicable to a unitary Fermi
gas, the concepts are still relevant. Just as BEC superfluidity evolves continuously into
BCS superfluidity in the BEC-BCS crossover, so the Mott insulator is expected to evolve
into its counterpart, the band insulator [119]. While a comprehensive study of insulating
behavior throughout the BEC-BCS crossover has yet to be performed, we have made somepreliminary measurements.
The application of a potential gradient results in an energy difference ∆E between adjacent lattice sites. In a conducting state, neighboring sites accrue a phase difference, resulting
in a current. Nonlinear effects can randomize this phase across the sample, broadening theinterference pattern in time of flight. On the other hand, the phase between adjacent lattice sites in a Mott insulator state is completely uncertain, rendering any accrual of relative
phase meaningless. An insulator is not excited by a gradient, and can therefore be converted
back into a superfluid, producing a sharp interference pattern in expansion.
The excitation spectrum of the Mott insulator does, however, exhibit resonances. An
excitation gap opens up for ∆E = U , due to the creation of particle-hole pairs. This
excitation gap has been measured for a BEC in the Mott insulator state [45]. The widthof the central peak in time of flight was used as a measure of excitation. At low V0 , thesuperfluid is uniformly excited as a function of ∆E. At deeper V0 , however, excitation is
only observed at distinct values of ∆E, indicative of a Mott insulator.
Technical limitations precluded the replication of this experiment with our Li gas. Our
pinch coils produce a gradient of 0.4 G/cm, per 1 A of current. This corresponds to a85∆E = h × 20 hz, across lattice sites2 . (For comparison, the pull of gravity is equivalent toa magnetic gradient of 2 G/cm.) For our strongly interacting system, we do not expect toopen an excitation gap until the gradient is on the order of the harmonic oscillator spacingh̄ω of the lattice, which is tens of khz.
We did, however, study the excitation of our system in response to a fixed gradient,
as we varied the lattice depth. Figure 6-4 shows the response to an applied gradient, asa function of V0 . At low lattice depths, the central peak width is significantly broadened,
indicating excitation, whereas for deeper lattices the effect of the gradient is negligible. This
would seem to indicate insulating behavior. However, the depth V0 above which the widthis not affected appears to be dependent on the duration ∆T of the applied gradient. Thismay be an effect of inhomogeneous density, and requires further study.

6.3

Recoherence

The field ramp technique which we employ (Figure 6-1), in order to reduce the collisioncross section, raises a question: Are we actually probing the physics at the initial magnetic
field? We may wish to infer from the sharp interference peaks in the time of flight images
that the sample was coherent. However, is it possible that this coherence is establishedduring the 150 µs field ramp? After all, the interactions are very strong, which may imply
a short time scale for such an effect.
In order to address this question, we study the timescale on which coherence can bereestablished. A lattice is adiabatically ramped up to 8 Er ; a depth at which the interference
pattern is essentially absent. The lattice depth is subsequently ramped down to 2.5 Er overa variable ramp-down time τ , and held for 150 µs during the magnetic field ramp, before
release. Again the width of the central peak is used as a measure of coherence. In Figure
6-5, the narrowing of the central peak shows that coherence is restored on a relatively shorttime scale of ∼ 500 µs, indicating that the sample remains in the ground state during theapplication of the lattice. This is on the order of the single-particle tunneling time h/J for aV0 = 2.5Er lattice. While the 150 µs duration of the field ramp is not completely negligible
in comparison, we can conclude that the interference pattern we observe is indicative of thecoherence of the sample at the original field.
2G
0.5
1.4 mhzµm, where the× 0.4 cm×√G
2
pinch coil axis.

√

2 reflects the 45◦ orientation of the lattice with respect to the

86

a)

b)

8

2.5

150 µsCentral peak width (µm)

Lattice depth ( Er )

160

140

120

100

80

60

τ

0

Time

200

400

600

800

1000

Ramp-down time τ

Figure 6-5: Restoring coherence from a deep lattice. a) The lattice is adiabatically ramped
up to V0 = 8 Er . A sample released from a lattice of this depth does not exhibit interference.
In order to study the time scale on which coherence can be reestablished, we ramp V0 down
to 2.5 Er over a variable ramp time τ . The sample is held here for 150 µs, while themagnetic field is ramped down, before release. b) As a measure of coherence, the widths ofthe central peak after 6.5 ms expansion, obtained from a gaussian (bimodal) fit, are plotted
as solid circles (diamonds), as a function of τ . The fast time scale over which coherence isrestored indicates that the sample stays in the ground state during the application of thelattice. A sample which has been decohered (open circles), through the application of amagnetic gradient during the lattice ramp-up, does not regain coherence under the same
conditions.

87

Additionally, we can study a system which is not in the ground state. If we apply amagnetic gradient during the ramp up of the lattice, we can excite the sample. Such adephased cloud will not recohere for any experimentally accessible ramp times. This illustrates the difference between a dephased sample and a ground state with phase uncertainty,
and demonstrates that any recoherence, due to evaporative cooling for example, is negligible
during the field ramp.

6.4

Conclusion and Outlook

In this chapter we discussed some of the first findings resulting from the loading of asuperfluid Fermi gas into a 3D optical lattice. The future has much in store for this system.
The search for anti-ferromagnetic order and d-wave superfluidity will occupy much of thecoming research agenda. Degenerate Fermi gases are likely to produce a tremendous amountof insight into the workings of strongly correlated many-body quantum systems.

88Chapter 7

Critical Velocity in the BEC-BCS
Crossover
The flow of a current without dissipation has intrigued physicists since Kamerlingh-Onnes’
discovery of superconductivity in 1911. Since then, the realm of superfluidity has expanded
to include liquid helium, and more recently cold atomic gases, and comes in both bosonicand fermionic flavors. The nature of superfluidity can be understood as a consequence ofcollective excitations, which modify the free particle dispersion relation so as to render flow
below a critical velocity dissipationless. Above the critical velocity, the current suffers anenergetic (Landau) instability, resulting in excitations. Superfluid atomic Fermi gases offer
the unique opportunity to study the transformation between bosonic superfluidity, such as
in 4 He, to fermionic superfluidity, as in 3 He.

This chapter supplements work reported in the following publication:
D. E. Miller, J. K. Chin, C. A. Stan, Y. Liu, W. Setiawan, C. Sanner and W.
Ketterle “Critical velocity for superfluid flow across the BEC-BCS
crossover,”preprint condmat/0707.2354 (2007).
(Included in Appendix D) Ref [28]

89

7.1

The Landau criterion

While superfluidity is, in essence, a quantum mechanical phenomena, we need nothing morethan high-school mechanics to arrive at a fairly satisfying explanation of its origin. Consider
an object of mass m moving with velocity ~v0 through a stationary fluid at zero temperature.
The object may create an excitation in the fluid with momentum p~ and energy E(p). Thishas the effect of damping the object’s motion, reducing its velocity to ~v . Conservation ofenergy gives us
2
2
1
1
2 mv0 = 2 mv + E(p)(7.1)

whereas conservation of momentum gives
m~v0 = m~v + p~

(7.2)

From these conditions we findp~ · ~v0 =p2+ E(p) .
2m(7.3)

This is just another way of saying that, in the frame moving at v0 , the work done by thefluid is equal to the energy of the fluid excitation plus the kinetic energy transferred to theobject. If we recognize that p~ · ~v0 ≤ p v0 , and consider an object of very large mass, we seethat excitations are only possible for
v0 ≥

E(p)
.
p

(7.4)

This is the Landau criterion, which establishes a critical velocity for dissipationvc = min E(p)/p .

(7.5)

For motion below this velocity, viscosity will be absent, and the flow is superfluid.
The complicated part, of course, is in the determination of the dispersion relation E(p),which is where quantum mechanics enters the picture. The quadratic free particle dispersion
E(p) = p2 /2m gives a critical velocity of zero, i.e., no superfluidity. Collective excitations,however, can produce a more interesting dispersion relation, such as the one pictured in
Figure 7-1. The Landau criterion may be depicted graphically as finding the line of theshallowest slope which passes through the origin and is tangent to E(p). The slope of this
90

a)

b)

c)

~v0

~v

E(p)

p~

p
Figure 7-1: The Landau criterion for superfluidity. (a) An object passing through a fluidwith velocity v0 can (b) create an excitation in the fluid with momentum p. As a result, themotion is damped, reducing the velocity to v. Conservation of energy and momentum placea lower bound on the velocity for which excitations can occur, known as the Landau criterion(Equation 7.4). (c) This critical velocity, vc = min E(p)/p, can be depicted graphically as
the slope of a (dashed) line which is tangent to the dispersion relation E(p) (solid line).
line is the critical velocity.
Energetic instability
In general, we can calculate the energy spectrum of excitations by considering small perturbations to the ground state. A Landau instability develops when the energy of an excitation
with some wavevector k becomes negative. The system can lower its energy by incorporating
elementary excitations. In this way, we refer to dissipation above the Landau threshold as
an energetic instability. For a superfluid, the energy of the system is a local minimum, and
it is stable against small perturbations. Above the critical velocity, however, the superfluid
flow lies at an energetic saddle point and is unstable [113].

7.1.1

Phonon excitation spectrum

In Chapter 2.1.6 we introduced the Bogliubov excitation spectrum, which describes theelementary excitations for a BEC. From Figure 2-2, it is clear that, according to the Landaucriterion, the critical velocity is given by the speed of sound. The sound velocity was found
to be

s

c=

4πh̄2 a
n .
m2

(BEC)

(7.6)

This is, of course, a specific case of a more general thermodynamic relation. The hydro91

dynamic description [57, 109, 81] is based on the continuity equationand the Euler equationm∂n + ∇(n~v ) = 0
∂t

(7.7)

h
i
∂
~v + ∇ µ(n) + 12 mv 2 = 0 ,
∂t

(7.8)

where ~v is the velocity field. Together, they produce a wave equation for small density
perturbations, which travel at the speed of sound.
s

c=

n ∂µ
m ∂n

(7.9)

In the case of a two-component Fermi gas in the deep BCS limit (1/kF a → −∞) thesound mode equivalent is known as the Bogoliubov-Anderson mode [53, 44]. Here, the gas
behaves essentially as a noninteracting Fermi gas. Equation 2.9 tells us that µ ∝ n2/3 , fromwhich it follows that
vF
c= √
3

(BCS)

(7.10)

Figure 7-2 illustrates the evolution of the speed of sound throughout the BEC-BCS
crossover [26, 53]. On the BEC side, c increases gradually with increasing interactions. This√
trend continues monotonically until c reaches the noninteracting Fermi gas limit vF / 3.
We can solve for the speed of sound on resonance using the chemical potential for a zerotemperature universal Fermi gas (UFG) which we derived in Equation 2.16.
vF
c = √ (1 + β)1/2
3

(UFG)

(7.11)A recent experiment at Duke [65] has measured the propagation of sound in an elongatedsuperfluid Fermi gas across the BEC-BCS crossover, using a technique first implemented
here at MIT [5] in which a far detuned laser creates a density perturbation which thentravels down the sample. From the measurements on resonance, the universal parameterβ was determined to be -0.570, in good agreement with other experimental measurements
and theory [44].

927.1.2Pair-breaking excitation spectrum

For a gas of superfluid Fermion pairs, phonons are not the only type of excitation which
contribute to the Landau instability. Pair-breaking excitations may also contribute to dissipation. The solution of the Bogoliubov-de Gennes equation [31] gives the excitation spectrum
E(p) =

s
µ 2
p

2m

¶2

−µ

+ ∆2

(7.12)The pairing gap ∆ represents the minimum energy required to create a single particle
excitation, and thus break a pair. At unitarity, it is on the order of the Fermi energy. In
the BCS limit, the gap falls off exponentially with (kF a)−1 , indicating the fragile nature of
the BCS superfluid.
The critical velocity for these single particle excitations is, according to the Landaucriterion,q

vsp = 

7.1.3

1/2

∆2 + µ2 − µm.

(7.13)The Landau criterion in the BEC-BCS crossover

A simple estimate of the critical velocity in the crossover is given by the minimum velocityof these two loss mechanisms.
vc = min (c, vsp )(7.14)In Figure 7-2, we see that this intersection yields a pronounced maximum near resonance.
The critical velocity also defines a generalized healing length
ξ=

h̄
.
m vc

(7.15)In the BEC limit this was given by Equation 2.4, and related to the speed of sound. In the
BCS regime, it corresponds to the size of Cooper pairs.

93

c /vF , vsp /vF

0.6

0.4

BEC

BCS

0.2

0.0
10

8

6

4

2

0

-2

1/kF aFigure 7-2: Speed of sound and pair-breaking threshold in a twocomponent Fermi gas. The speed of sound c (solid line) increases monotonically across the BEC-BCS crossover. The threshold velocity vsp for
pair-breaking falls exponentially in the BCS limit. The Landau criticalvelocity is given by the minimum velocity of these two, and is sharply
peaked near resonance. Adapted from [26].

94

7.2

Measuring dissipation in a quantum gas

In this section we will review some of the previous experiments which have measured thecritical velocity in an atomic BEC, as well as introduce the techniques we employed in ourstudies of a superfluid Fermi gas.

7.2.1

Stirring a BEC

Appropriately enough, the first measurements of critical velocity in a BEC were performedin the New Lab, by Chandra Raman and Roberto Onofrio [92, 89, 93]. In these experiments,
a blue detuned beam (split off from the Argon ion laser which used to pump the dye laser)
was used to stir a magnetically trapped BEC, as illustrated in Figure 7-3a. The beam waspassed through an AOM, and the pointing could be controlled dynamically by changing
the driving frequency. The stirring beam was scanned back and forth, along the long axisof the condensate, heating the sample. A bimodal fit to the expanded cloud produced acalorimetric measure of the heating. As a function of the stirring velocity, the heating
rate showed an abrupt onset at some critical velocity vc ' 0.1 c, where c was the speedof sound calculated at the peak density of the condensate. This value of vc was obtainedby considering a drag force F = κ (v − vc ), where κ is the drag coefficient [61, 41], whichtransfers energy to the system at a ratedE
= F · v = κ v (v − vc ) .
dt(7.16)Such a small value of vc /c may come as a surprise, based on our previous discussionof the Landau criterion. There are, however, a number of effects in this experiment with
act to reduce vc . The first is a purely geometrical concern. In the case of an impenetrable
stirring object, the fluid must flow around the perimeter. Therefore, the actual velocityof the fluid is faster than that of the stirrer [41]. The second correction we must make isto account for the inhomogeneous density of the sample [62]. One may expect that the
local speed of sound depends on the local density, which decreases towards the edge ofthe cloud. In such a case, there should be no sharp onset of dissipation, since the stirring
beam pierces through a region were the density, and therefore, the critical velocity goes tozero. The MIT experiment did, however, observe an abrupt onset, and it turns out that theproper treatment of the density inhomogeneity is more complicated than we have let on.
95

a)

b)

c)

d)

Figure 7-3: Various configurations for perturbing a condensate. a) A single far-detuned
laser beam is focused at the center of an elongated cloud. The beam is scanned back andforth. b) Uniform lattice: Two intersecting laser beams with some frequency difference
produce a moving optical lattice. An atomic sample is confined in the center, experiencing
a relatively homogeneous lattice depth. c) Centered lattice: Two intersecting laser beams
are focused to a size smaller than the trapped cloud, producing a moving optical latticewhich probes a relatively homogeneous region of density. Trapping potentials (optical ormagnetic) are not shown. The arrows indicate the direction of motion. d) Absorption
images of a pair condensate subject to a lattice moving with increasing velocity (left toright) show an abrupt threshold for dissipation.
We will find that accounting for density inhomogeneity will be a recurring theme in such
experiments.

7.2.2

BEC in a moving lattice

Similar experiments were conducted in Florence, in which a 1D optical lattice was used toperturb the system, rather than a stirring beam. The conclusions reached, however, wererather different. In the initial experiment [14], the BEC was displaced in the harmonic trap
and set to oscillate. A more controlled experiment was performed later, in which the lattice
was moved at a constant velocity [32]. The Florence group observed a gradual increase indissipation with velocity, which saturated at a particular value. In their interpretation, thecritical velocity is given by the local speed of sound. The lattice velocity v then sets somethreshold density nth , for which the speed of sound is c(nth ) = v. For a given velocity, the96fraction of the cloud which is below this threshold density experiences loss, while the rest isimmune. When the lattice velocity exceeds c(np ), at the peak density np , the entire sample
will incur dissipation, and the loss is flat with respect to v.
The behavior at low velocities, however, was not studied in detail. It is difficult to
exclude the possibility of an abrupt onset of dissipation, such as that which was found in
the MIT experiment. Additionally, the suggestion that the loss reaches a plateau seems
difficult to reconcile with most models of friction, which are velocity dependent, as inEquation 7.16. The presence of a dynamical instability, which we introduced in Chapter
3.5, greatly limits the range of velocity over which the loss appears constant. In addition,
lifetime data in this same paper does not show the same plateau as the condensate number.
The Florence group has also shown that thermal atoms participated in the loss process.
For these experiments, the atomic sample was held in a magnetic trap. The application of
an “RF-shield” prevented the accumulation of a thermal component as the condensate washeated by the moving lattice. The lifetime of shielded cloud was shown to exceed that of theunshielded cloud by an order of magnitude, at velocities below the threshold for dynamical
instability. This strongly suggests that the loss they observe at low velocities is due, atleast in part, to the collision of thermal impurities with the condensate; a finding which
will guide the interpretation of our own data in the following Sections.

7.2.3

Superfluid Fermi gas in a moving lattice

In our determination of the critical velocity in a superfluid Fermi gas, we also study thedissipation induced by a moving 1D optical lattice. There are two lattice geometries thatare used, which are shown in Figure 7-3, the experimental details of which are given in
Chapter 4.4.1. In the first, the lattice beams extend to cover the entire trapped sample.
The lattice depth is then fairly uniform across the pair condensate. The density which we
probe, however, is not. In a second configuration, the lattice beams are focused down to asize smaller than the trapped cloud, sampling only the center of the cloud where the density
is relatively homogeneous. In both cases, we observed a threshold velocity for dissipation,
as shown in Figure 7-3d. In the following Sections we describe how we parameterized theloss signal, and how it was interpreted in the context of Landau instability.

97Density Profile (a.u.)

Atom number (×105 )

5

-0.5

0

4

3

2

1

0.5
0

Position (mm)
a)

0

b)

100

200

300

Hold time (ms)Figure 7-4: Dissipation in a moving lattice. a) A bimodal fit of the density profile, after
17 ms expansion, provides a measure of the heating incurred in the moving lattice. b) The
number of pairs in the thermal component Nth (open circles) and those remaining in thecondensate Nc (filled circles) as a function of the time held in the moving lattice. The
thermal component grows approximately linearly (dashed line), whereas Nc is fit to model(solid line) in which the loss is proportional to Nth (Equation 7.18).

7.3

Uniform lattice / inhomogeneous density

The first system we studied was that of the uniform lattice, depicted in Figure 7-3b. Afterthe preparation of a pair condensate, near resonance, the optical lattice is adiabaticallyramped up, moving with a fixed velocity. The lattice is kept on for a duration up to 2 s.
After the lattice is ramped off, we allow the cloud to expand, ramping the magnetic field
as usual to preserve the momentum distribution [121].
From a bimodal fit to the density distribution, we extract the number of condensed
pairs Nc , as well as the number in the thermal component Nth . Figure 7-4b shows the
temporal dynamics of the dissipation (which is characteristic of both lattice configurations
used). Atoms which are excited from the pair condensate either escape the optical trap or
contribute to the thermal fraction. In order to quantify the loss, we first use a linear loss
model.
Nc (t) = N0 (1 − γ t)(7.17)In Figure 7-5, we interpret the dissipation through the linear loss fit parameter γ(v). The
loss at low velocities is essentially zero, whereas above some critical velocity there is a kink,and the loss rate increases sharply.
Furthermore, we study the loss on both sides of the resonance. Before the lattice is98brought up, we adiabatically ramp the magnetic field to probe the sample at the desired
value of kF a. We find that the superfluid is most robust near resonance, in that the onset of
dissipation occurs at the highest velocity. This velocity is, however, quite small: about anorder of magnitude smaller than the speed of sound c ' 19 mm/s, as calculated by Equation
7.11. We attribute this discrepancy to the inhomogeneous density of the cloud. The speedof sound which we have quoted is that at the trap center, i.e. for the peak density. As thedensity falls off radially, so does the local speed of sound. It is interesting, then, that we
see a discontinuity in the loss rate at all. After all, the lattice probes the outer edges of thecloud, where the density approaches zero.
This observation, however, is not inconsistent with the M.I.T. stirring experiments [92,93] which probed regions of vanishing density, as well. The propagation of sound in an
elongated degenerate Fermi gas [18, 65] also discredits the simplistic portrait of a local
speed of sound. Axially propagating wavefronts do not distort in accordance with theradially decreasing density profile. Rather, they remain flat, in apparent defiance of thelocal sound velocity. In fact, the concept of a local speed of sound may not even be valid
in the outer wings of the cloud, at least insofar as we have defined it. The local density
approximation, which we have invoked, fails at the outer boundary of the cloud, where thedensity can not be considered slowly varying.
In any case, it is clear that accounting for the inhomogeneous density is a messy affair.
This is what motivated us to use a different lattice geometry, in which only the central
region of the cloud was probed. I will address these results in a later Section, but before
doing so, I would like discuss the temporal dynamics of the loss process.

7.3.1

Loss dynamics

Figure 7-4b offers a characteristic portrait of the loss dynamics. We chose to parameterize
this data in terms of a linear loss model (Equation 7.17), which was adequate for identifying
the sharp onset of loss. Yet, this was merely a convenient choice which lacked any physicalmotivation, and it is interesting to speculate upon the nature of the dissipation so as toconstruct a more robust model.
In Section 7.2.2 we discussed an experiment which provided evidence for the participation of thermal atoms in the dissipation of an atomic BEC in a moving lattice. We haveseen evidence that the thermal component plays a similar role in our fermionic system. We
99

8

BEC

4

0.5

Linear loss rate γ (a.u.)

0

0

2

4

6

8

1

4

kF a

0.1

0

0

2

4

6

8

BCS

4

-0.4

0

0

2

4

6

Lattice velocity (mm/s)

Figure 7-5: Abrupt onset of dissipation. The time series of loss is fit witha linear function (Equation 7.17), which characterizes the dissipation at agiven lattice velocity by a linear loss rate γ. We see a sharp kink in theloss behavior at some critical velocity. The loss is studied throughout the
BEC-BCS crossover, and indicates a critical velocity which is greatest nearresonance.

100can prevent the accumulation of a thermal component during exposure to the lattice by
gradually reducing the trap depth from U0 to some final value Umin , allowing the heatedpairs to evaporate. We find that the lifetime is enhanced by at least a factor of two overthat of a sample held in a fixed trap, at either U0 or Umin . An experiment with greater
control over the temperature would certainly shed more light. Unfortunately, the use of an
optical trap does not permit the variation of the trap depth independently of the trapping
frequencies, and therefore the density of the sample.
Nevertheless, our finding supports a picture of dissipation which is driven by the collisionof thermal impurities with the condensate. This may suggest a model of an exponential loss
Nc (t) = Nc (0) e−Γt ,(7.18)

in which the loss rate Γ = γ0 Nth is proportional to the number of thermal atoms Nth .
In Figure 7-4 we apply this fitting function, where we have approximated the thermal
component as growing linearly in time. This model satisfactorily describes a loss which isfaster than linear, consistent with our observation. Impurities in the form of unpaired atoms
would allow us to better test the validity of this model. Unfortunately, phase separation
effects [102] make controllably varying the population imbalance difficult for our geometry.
A dissipation which is driven by thermal impurities is also more consistent with thethreshold behavior we observe. The lattice potential itself should only provide a matrix
element between momentum states differing by h/λL . This would result in a loss which isresonant, centered at a lattice velocity v = 22 mm/s, corresponding to the Bragg resonance.
Rather, we observe a threshold behavior. If the loss is driven by thermal impurities which
are dragged along by the lattice [71], the matrix element is essentially a point source which
can couple to excitations at all wavevectors. This type of coupling can be used as a probe
for Landau instability.

7.4

Centered lattice / homogeneous density

The obvious shortcomings in a system of inhomogeneous density motivated us to adopt alattice configuration in which only the central region of the cloud is probed (Figure 7-3c).
This arrangement also yielded a threshold for loss. In the previous Section, we extracted
a loss rate from the temporal behavior, a process which is both data intensive and model101

Critical velocity vc (mm/s)

Condensate number Nc

4
3
2
1
0
0

a)

2

4

6

Lattice velocity (mm/s)

9
8
7
6

5 x 10 5

5

4

4

3

3

8

100

b)

2

3

4

5 6 7 89

1000

Condensate number Nc

5 x 10 5

2

Hold time (ms)Figure 7-6: Critical velocity at the cloud center. a) Number of pairs which remained in
the condensate Nc after being subjected to a V0 = 0.2 EF deep optical lattice for 500 ms,
moving with velocity vL , at a magnetic field of 822 G (1/kF a = 0.15)) An abrupt onset of
dissipation is observed above a critical velocity vc , which we identify from a fit to Equation
7.19. b) The critical velocity we obtain (filled circles) is weakly dependent on the time heldin the lattice (logarithmic scale). Some of this variation may in fact be attributed to the
reduction in numbers, and therefore speed of sound, due to the sample’s finite lifetime. We
show Nc as a function of hold time in a stationary lattice (crossed circles).
dependent. Here we identify the critical velocity from a trace taken for a single exposure
time in the lattice. In Figure 7-6a, we show the number of condensed pairs remaining as afunction of lattice velocity vL . We define the critical velocity vc from a fit to the intersectionof two lines.
Nc (v) = Nc (0) × [ 1 − max(0, α × (v − vc)) ](7.19)

As we show in Figure 7-6b, this approach gives a fairly consistent value over a large rangeof hold times. Longer hold times do consistently point to a somewhat smaller vc . Some ofthis effect, however, may be due to the reduced numbers at long hold times. In a stationary
lattice (or without a lattice at all) the total number will decay substantially over 2 s. Thishas the effect of reducing the density, and thus the speed of sound.

7.4.1

Critical velocity throughout the BEC-BCS crossover

We repeat this measurement of the critical velocity at different magnetic fields, as shownin Figure 7-7. We find that vc is maximum near resonance, falling off precipitously on the
BCS side and more gradually on the BEC side. This behavior is consistent with the modelfor Landau instability which we discussed in Section 7.1.3. On the BEC side, the critical102

Magnetic Field (Gauss)
770

790

810

830

850 870 890

6.0
0.15

5.0

0.125

4.0

vc / v F

Critical Velocity vc (mm/s)

750

0.1

1.5

1.0

0.5

0.0

-0.5

Interaction Parameter 1/kF a
Figure 7-7: Critical velocity throughout the BEC-BCS crossover. The value ofvc at each magnetic field is obtained from a fit to Equation 7.19, as shown in
Figure 7-6. The critical velocity falls off gradually on the BEC side along withthe speed of sound, and more sharply on the BCS side due to pair breaking.
Data is shown for a V0 = 0.2 EF deep lattice, held for t=500 ms.

103

velocity is determined by the speed of sound, whereas on the BCS side it is given by thepair breaking threshold.
While this data is in qualitative agreement with such an understanding, there are stillsome quantitative issues which need to be resolved. As most theory papers discuss homogenous systems, or equivalently work in the local density approximation, it is most convenient
to reference the local Fermi velocity (see Equation 2.19) at the trap centervF = (1 + β)−1/4

q

2EF /m

(7.20)The (non-interacting) Fermi energy for this system is EF = h × 7.6 khz. The corresponding
Fermi velocity is then vF = 39 mm/s.
In terms of the Fermi velocity, the speed of sound is cs = 0.37 vF , according to Equation
7.11. The pair breaking threshold, given by Equation 7.13, is vsp = 0.34 vF , where we haveused ∆ = 0.50 εF [19, 20]. These two values should provide a good estimate of the criticalvelocity on resonance, although our measurements appear to come up short. If we are to
make a quantitative comparison with theory, we need to address the effects of a finite latticepotential.

7.4.2

Critical velocity at different lattice depth

Numerical simulations for both a BEC [58] and a superfluid fermi gas [103] have shown that
the Landau criterion provides an upper bound for the critical velocity which is attained
only in the limit of vanishing perturbing potential. While the lattice does reduce the speedof sound [73], through the effective mass, this effect is insignificant for the lattice depths
we explore. Rather, nonlinear corrections to the Bogoliubov spectrum [84] are expected toplay a role. In Figure 7-8, we explore the critical velocity near resonance (1/kF a = 0.15)
over a large range of lattice depths. In the limit of low lattice depth, vc converges to
vcpeak ≈ 0.25 vF . Measurements at the smallest lattice depths became more uncertain as
the hold time required to observe an effect of the lattice approached the natural lifetimeof our sample. It is for this reason that we studied the field dependence (Figure 7-7) at
an intermediate lattice depth where vc was more well defined. Nevertheless, the value we
obtain for vcpeak near resonance is within 10% of the predictions. Extrapolation between thepair breaking and collective excitation branches may reduce the effective vc . Any additional

104

0.3

10

8

0.2

vc / v F

Critical Velocity vc(mm/s)

12

6

4

0.1

2

0

0
2

0.01

3

4 5 6 72

3

4 5 6 70.1

21

Lattice Depth V0 /EF
Figure 7-8: Critical velocity at different lattice depths. The results show vc
to be a decreasing function of lattice depth V0 . In the limit of low V0 , vc
converges to a maximum value of 0.25 vF . Data was taken near resonance, at
822 G (1/kF a = 0.15) for hold times t = 250 ms, 500 ms, 1000 ms, 2000 ms(squares, diamonds, circles, triangles).

105discrepancy which we observe may be attributed to the density variation across the lattice.

7.4.3

Dynamical instability

While we have framed our results in the context of an energetic instability, there exist otherloss mechanism which may occur in a moving lattice. A dynamical instability, which we
introduced in Chapter 3.5, occurs when two particles from the condensate collide and scatter
into states with opposite momentum ±δk. The lattice modifies the dispersion relation sothat this process becomes energetically possible when condensate has a certain momentum
in the lattice frame. Our results, however, are not consistent with the simple understandingof a dynamical instability, which can not set in below 0.5 qB , corresponding to v = 11mm/s, where qB = h/2λL is the Bragg momentum which defines the edge of the Brillouinzone. The highest critical velocity observed, in the low V0 limit, is just below this threshold(see Figure 7-8). However, vc falls rapidly for increasing V0 . Recently it has been shown[86] that strong interactions can lower the threshold for dynamical instability, reaching zeroat the Mott Insulator transition. However, it is very likely that the range of lattice depths
explored here (V0 ≤ 2 Er ) are too shallow to exhibit this effect. The loss of coherence which
typically accompanies the superfluid to Mott insulation transition was found to occur at
V0 ' 6 Er in a 3-D optical lattice [24]. We have observed that coherence persists in a 1-D
lattice up to V0 ' 25 Er . For these reasons, we rule out the role of dynamical instability inthe dissipation which we observe.

7.5

Bragg spectroscopy in the BEC-BCS crossoverIn the previous sections we discussed using a 1D moving lattice to probe excitations at all
wavevectors, through collisions with thermal impurities. Such a lattice can also be used
to study excitations at a single wavevector, given by the spatial period of the lattice. In
Figure 3-8 we saw that Bragg diffraction could impart a quantity (two photons worth) of
momentum unto either a single atom or bound diatomic molecule. Distinguishing between
the two was straightforward as for a molecule the doubled mass halves the recoil energy andrecoil velocity. The molecular Bragg resonance occurs at half the frequency of the atomic
peak. It is interesting to ask what happens to the spectrum when we can tune continuously
between these two cases, i.e. in the BEC-BCS crossover. We have taken some preliminary106

data to address this question.
Traditionally, in Bragg spectroscopy , one relies on the out-coupled fraction which appears in time of flight, as in Figure 3-2c, for a signal. This is not possible in the stronglyinteracting regime, because the distinct momentum component collide violently as the pass
one another. However, after collisions, a small out-coupled fraction manifests as a protuberance, and the width of the expanded cloud can be used as a measure of the Bragg diffraction
efficiency.
Figure 7-9 we show Bragg spectra taken throughout the crossover. On the BEC side, we
observe a peak corresponding to the Bragg scattering of molecules. As we move towards the
BCS limit, the peak shifts towards the atomic resonance, but also broadens significantly.
This may be related to a broad pair breaking feature such as that observed in Ref. [21].
This interesting behavior certainly warrants greater attention. In particular, one may wish
to vary the recoil energy with respect to EF (by changing the angle of the Bragg beams,
perhaps).

7.6

Conclusion and Outlook

In this chapter we have explored the limits of superfluidity in the BEC-BCS crossover. The
technique we use may find application in identifying superfluidity in other systems, suchas degenerate Fermi gases in lower dimensions. The role of the thermal component, as
well as a greater understanding of the dissipation mechanism, in general, requires further
investigation.

107

1

Width (a.u.)

Width (a.u.)

1

0

0
0

20

a)

40

60

80

100

120

0

Detuning (khz)

b)

1

40

60

80

100

120

Detuning (khz)

Width (a.u.)

Width (a.u.)

1

0

0
0

c)

20

20

40

60

80

100

120

0

Detuning (khz)

d)

20

40

60

80

100

120

Detuning (khz)

Figure 7-9: Bragg spectroscopy in the BEC-BCS crossover. Strong collisions prevent theidentification of the out-coupled fraction as a signal. Rather, the width of the cloud inexpansion, which is distorted due to the collisions, provides a measure of the Bragg diffraction efficiency. The Fermi energy is EF = h×12.4 khz. The recoil energy for a pair is
Er = h × 7.3 khz. Measurements of the BEC side of the resonance, at a) 750 G and b) 820
G, show a peak at the molecular resonance h δν = 4 Er . On the BCS side of the resonance,at c) 890 G and d) 960 G, the spectra shift towards an atomic resonance, in which the
Bragg process breaks a pair and the momentum is carried by a single atom. The resolution,
given by the finite pulse duration, is 3.5 khz.

108

Epilogue“I don’t want to achieve immortality through my work ...
I want to achieve it through not dying.”- Woody Allen

After five years, and countless late nights in the lab, I can attest to the incompatibilityof these two aims. Graduate school has been a remarkable experience; one of great intensityand freedom. I have been extremely fortunate to have had this opportunity to have rubbed
elbows with some brilliant individuals, to have witnessed a number of impressive milestonesin the advancement of science, and to have played my own small role in this great theater.

109

Appendix A

High-contrast Interference in a
Thermal Cloud of Atoms
D. E. Miller, J. R. Anglin, J. R. Abo-Shaeer, K. Xu,
J. K. Chin, and W. Ketterle“High-contrast interference in a thermal cloud of atoms,”
Phys. Rev. A 71, 043615 (2005).

110

PHYSICAL REVIEW A 71, 043615 共2005兲

High-contrast interference in a thermal cloud of atoms
D. E. Miller, J. R. Anglin, J. R. Abo-Shaeer, K. Xu, J. K. Chin, and W. Ketterle*
Department of Physics, MIT-Harvard Center for Ultracold Atoms,
and Research Laboratory of Electronics, MIT, Cambridge, Massachusetts 02139, USA
共Received 23 December 2004; published 29 April 2005兲
The coherence properties of a gas of bosonic atoms above the Bose-Einstein-condensation transition temperature were studied. Bragg diffraction was used to create two spatially separated wave packets, whichinterfere during expansion. Given sufficient expansion time, high fringe contrast could be observed in a cloudof arbitrary temperature. Fringe visibility greater than 90% was observed, which decreased with increasing
temperature, in agreement with a simple model. When the sample was “filtered” in momentum space using
long, velocity-selective Bragg pulses, the contrast was significantly enhanced in contrast to predictions.
DOI: 10.1103/PhysRevA.71.043615

PACS number共s兲: 03.75.Hh, 34.20.Cf, 32.80.Pj, 33.80.Ps

Images of interfering atomic clouds are widely considereda hallmark of Bose-condensed systems and a signature oflong-range correlations 关1兴. A thermal atomic cloud is often
regarded as an incoherent source, with a coherence length
too short to obtain high-contrast interference patterns when
two clouds are overlapped. Here we show that ballistic expansion can increase the coherence length such that “BoseEinstein-condensation-type” 共“BEC-type”兲 interference canbe observed in a thermal cloud. Currently, there is considerable interest in characterizing the coherence properties ofnoncondensed systems including ultracold fermions 关2兴, fermion pairs 关3–5兴, and ultracold molecules 关6–8兴. In this paper we show that an interferometric autocorrelation technique, previously only applied to condensates 关9,10兴, can beused to study the coherence properties of samples at finite
temperature.
We studied the first-order spatial coherence of a trapped
thermal cloud of atoms. After release from the trap, the atom
cloud expanded ballistically, and Bragg diffraction was used
to create an identical copy of the initial cloud displaced by adistance d. Therefore, our study was analogous to Young’s
double-slit experiment 关11兴. We investigate the conditions
under which two such overlapping clouds produce a highcontrast interference pattern. Our result is that for sufficiently
long expansion times, there will always be high contrast, but
the required time of flight becomes longer at higher temperatures.
The experiment used a magnetically trapped thermal
cloud of ⬃5 ⫻ 107 sodium atoms, prepared in a manner similar to our previous work 关12兴. Atoms in the 兩F = 1 , mF = −1典
state were loaded from a magneto-optical trap 共MOT兲 into amagnetic trap, where they were further cooled by radio frequency 共rf兲 evaporation. The rf evaporation was stopped before the critical temperature for Bose-Einstein condensation,
Tc, was reached, yielding a thermal cloud at a controlledtemperature. Shortly after being released from the trap
共2 ms兲, the cloud was exposed to two successive Bragg
pulses, separated by wait time ␶w. The effect of the Bragg
beams 关13,14兴 was to couple two momentum states 兩បk0典 and*Website: cua.mit.edu/ketterleគgroup1050-2947/2005/71共4兲/043615共4兲/$23.00

兩ប共k0 + kr兲典 via a two-photon transition, where kr = 共k1 − k2兲and k1, k2 are the wave vectors of the two Bragg beams. The
coupling-induced Rabi oscillations and the pulse area wereexperimentally chosen to correspond to ␲ / 2 共i.e., an atom
originally in a well-defined momentum state was taken to an
equal superposition of the two states兲. During the wait time
␶w the two states accrued different phases before a second
␲ / 2 pulse mixed the states again. Considering only two momentum states, this is equivalent to Ramsey spectroscopy
关15兴 共see Fig. 1兲.
One can regard a thermal cloud as an ensemble of atoms,
each having its initial amplitude spread over a range of momentum states centered about zero, with rms width h / ␭T,
where ␭T = h / 冑mkBT is the thermal de Broglie wavelengthand m the atomic mass. The relative detuning of ␦␯= 45 kHz between the Bragg beams and pulse duration ␶ p= 10 ␮s were chosen to couple all initial momentum components to those centered at kr. During ␶w the relative phaseaccumulated between coupled states is proportional to k0,
and the momentum distribution shows a sinusoidal modulation 共Ramsey fringes兲 after the Bragg sequence. In long timeof flight 共TOF兲, the spatial density simply mimics this momentum distribution. Interference fringes were observed at a
spacing ␭ f = httof / md, whered = vr共␶w + 4/␲␶ p兲

共1兲

is the cloud separation discussed in the equivalent picture of
two overlapping clouds. vr is the two-photon recoil velocity.
The factor of 4 / ␲ emerges from a simple Rabi oscillation
model and includes the extra phase accumulated while theBragg beams effect a ␲ / 2 pulse. The sum of single-particle
interference patterns results in a density along the x direction
with reduced contrast C:冋冉n共x兲 = f共x兲 1 + C sin

2␲
x+␾
␭f

冊册

,

共2兲

where f共x兲 is an envelope function.
To find the expected contrast we consider the k-space distribution of noninteracting bosons in a harmonic trap assuming the high-temperature 共Maxwell-Boltzmann兲 limit. Thismay be written exactly as an ensemble of single-particle

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PHYSICAL REVIEW A 71, 043615 共2005兲

MILLER et al.

FIG. 2. Emerging contrast during ballistic expansion. The coherence length ᐉc grew larger than the initial separation d = 2 ␮m as
the cloud expanded for 共a兲 14 ms, 共b兲 20 ms, and 共c兲 25 ms 关see Eq.
共4兲兴.

FIG. 1. Interference of spatially separated thermal clouds: 共a兲
An absorption image, taken after 48 ms time of flight, shows
fringes with a spatial period of ␭ f = 340 ␮m. 共b兲 A cross section of
the optical density 共black circles兲 taken through the center of theimage was fit 共gray line兲 to extract the fringe contrast. 共c兲 Schematic
depiction of the Bragg pulse sequence. The first ␲ / 2 pulse at t = 0
created a superposition of stationary and moving clouds. At t = ␶ p+ ␶w the clouds had separated by distance d, and a second pulse
created a superposition of cloud pairs: one pair moving with recoil
momentum pr, the other stationary. Each pair developed an interference pattern.

Gaussian states, incoherently averaged over their positions.
Thus, it is appropriate to regard the thermal cloud as a collection of wave packets of Gaussian width ␭T, distributed inspace according to Maxwell-Boltzmann statistics.
The incoherent sum over all particles results in contrast

冉C = exp −2␲2RT2
␭2f

冊 冉 冊= exp −

d2
,
2ᐉ2c

共3兲

where RT = 冑kBT / m␻2 is the thermal size of a cloud in aharmonic trap of frequency ␻ and where we have defined thecoherence length asᐉc =␭T size in TOF.
2␲ size in trap

共4兲

Equation 共3兲 gives two different, but equivalent criteria
for the loss of contrast. While the interference pattern of asingle-particle quantum state 共as well as that of a pure condensate兲 should always show perfect contrast, the incoherent
sum over a thermal cloud washes out the fringe visibility.
Contrast is lost when the single-particle interference patternis smeared out over an initial size RT that is larger than thefringe spacing ␭ f . However, because the fringe spacing
grows as the cloud expands, contrast will always emerge
with enough time of flight. Alternatively, Eq. 共3兲 states thatinterference is lost when the separation d of the two sources
exceeds the coherence length ᐉc. Here it is important to notethat the coherence length increases with time of flight 关Eq.
共4兲兴. The coherence length is inversely proportional to the
local momentum spread which decreases in ballistic expansion as atoms with different velocities separate from eachother. For very long expansion times, the coherence length
becomes arbitrarily large resulting in high-contrast interference 共Fig. 2兲. This can also be understood by the conservation of local phase-space density during ballistic expansion,
where the decrease in density is accompanied by a decreasein momentum spread.
We repeated the experiment over a range of temperatures
for several values of the cloud separation d. The temperature
was controlled by varying the final value of the rf evaporation. Each temperature was calibrated by measuring the sizeof a cloud in expansion without pulsing on the Bragg beams.
The temperature calibration was consistent with the observed
onset of BEC at the calculated temperature of Tc = 0.6 ␮K.
The Bragg beams were detuned 30 GHz from the atomic
transition and heating was demonstrated to be negligible byobserving the effects of the light with the two-photon detuning ␦␯ set far from the Bragg resonance. Absorption imagesof our samples were taken after 48 ms time of flight 关Fig.
1共a兲兴. A cross section of the atomic density was fit to Eq. 共2兲
to determine the contrast. Deviation of the pulse area from␲ / 2 reduces the number of atoms in the out-coupled cloud;however, it does not reduce the contrast assuming the twopulses are equal.
Figure 3共a兲 shows the measured fringe contrast of the outcoupled cloud as a function of the cloud temperature forthree different fringe spacings ␭ f . The data are compared tothe contrast expected from Eq. 共3兲 共dashed line兲 with no freeparameters. At low temperature this equation provides an043615-2

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PHYSICAL REVIEW A 71, 043615 共2005兲

FIG. 3. Contrast vs temperature for 10-␮s Bragg pulses. Different fringe spacings ␭ f were realized by varying the wait time ␶w
between the two Bragg pulses. The time of flight was kept fixed at
48 ms. At lower temperatures the observed contrast agreed wellwith theory 共dashed lines兲 given by Eq. 共3兲. At higher temperature,
however, the contrast was higher than predicted, owing to the velocity selectivity of the Bragg pulses. The BEC transition temperature was at Tc = 0.6 ␮K.

accurate description of the observed fringe visibility. At
higher temperatures, however, the observed contrast is consistently greater than expected. While Bose-Einstein statistics can considerably enhance contrast at temperatures even
above Tc 关16,17兴, our experiment does not achieve the highdensities necessary to make this effect pronounced. One possible explanation for the discrepancy in Fig. 5, below, is that
our model fails when the Bragg diffraction becomes velocity
selective—i.e., when the Doppler width of the atoms exceeds
the Fourier width of the Bragg pulses.
In order to further investigate this effect, we repeated theexperiment with longer, more velocity-selective pulses. The
absorption images showed that these pulses addressed a narrow range of atomic velocities 共Fig. 4兲. In this case the fit
routine was modified to account for the 共Gaussian兲 background of atoms unaffected by the Bragg pulses. While thefraction of out-coupled atoms decreased with increasing temperature, the interference was still clearly visible. In Fig. 5
the measured contrast deviates substantially from theory
共dashed line兲. While velocity selectivity clearly plays a role,
the mechanism for this enhanced contrast is not apparent. We
have shown that it is only the initial cloud size RT that determines the contrast for a given ␭ f 关Eq. 共3兲兴. Since theBragg pulses are not spatially selective, we do not expect
their details to influence the contrast. Similarly, Eq. 共4兲 illustrates why velocity selection does not increase the coherence
length in time of flight: the narrowed momentum distribution
implies a larger effective de Broglie wavelength which isexactly canceled by the reduced expansion in time of flight.
Therefore, the enhanced contrast in Fig. 5 cannot be described by the single-particle free expansion of a thermal gas.
We suspect that particle interactions play a role.
In conclusion, we have shown how the coherence lengthof a trapped gas is modified during ballistic expansion. This

FIG. 4. Velocity-selective Bragg diffraction: 共a兲 Absorption image of atoms subject to 40-␮s Bragg pulses. The fringe spacing was␭ f = 210 ␮m after 48 ms time of flight. These longer Bragg pulses
addressed only a subset of the momentum distribution. 共b兲 Cross
section and fit 共black circles and gray line, respectively兲.

allows for the observation of high-contrast interference in asample at any temperature and is not limited to condensates.
For the technique employed here, the only advantage of a
BEC is its larger initial coherence length, which is equal to
the size of the condensate 关1,9,16兴. However, it is the abilityof two independent condensates to interfere that sets thisstate of matter apart 关18兴. Two independent thermal clouds
would not interfere. The self-interference technique characterized here can be used to study the coherence properties ofother novel quantum degenerate systems. One example ismolecular clouds created by sweeping an external magnetic

FIG. 5. Measured contrast for ␶ p = 30 ␮s velocity-selective
Bragg pulses. The contrast was significantly higher than that predicted by Eq. 共3兲 共dashed line兲. The fringe spacing was ␭ f= 170 ␮m after 48 ms expansion time.

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MILLER et al.

field across a Feshbach resonance. While the rapid decay ofsuch a sample precluded thermalization, an interferometricmethod has already been used to demonstrate coherence inthis system 关19兴.

The authors thank D. E. Pritchard for a critical reading of
the manuscript, M. W. Zwierlein for contributions to the data
analysis, and W. Setiawan for experimental assistance. This
research was supported by NSF, ONR, ARO, and NASA.

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Miller, and W. Ketterle, Phys. Rev. Lett. 91, 210402 共2003兲.
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Deng, E. W. Hagley, K. Helmerson, S. L. Rolston, and W. D.

Phillips, Phys. Rev. Lett. 85, 2040 共2000兲.
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043615-4

Appendix B

Coherent Molecular Optics using
Sodium Dimers
J. R. Abo-Shaeer, D. E. Miller, J. K. Chin, K. Xu,
T. Mukaiyama and W. Ketterle“Coherent Molecular Optics Using Ultracold Sodium Dimers,”
Phys. Rev. Lett. 94, 40405 (2005).

115

PRL 94, 040405 (2005)

PHYSICAL REVIEW LETTERS

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Coherent Molecular Optics Using Ultracold Sodium Dimers
J. R. Abo-Shaeer,* D. E. Miller, J. K. Chin, K. Xu, T. Mukaiyama, and W. Ketterle†
Department of Physics, MIT-Harvard Center for Ultracold Atoms, and Research Laboratory of Electronics, MIT,
Cambridge, Massachusetts 02139, USA(Received 13 September 2004; published 3 February 2005)
Coherent molecular optics is performed using two-photon Bragg scattering. Molecules were producedby sweeping an atomic Bose-Einstein condensate through a Feshbach resonance. The spectral width of themolecular Bragg resonance corresponded to an instantaneous temperature of 20 nK, indicating that atomic
coherence was transferred directly to the molecules. An autocorrelating interference technique was used toobserve the quadratic spatial dependence of the phase of an expanding molecular cloud. Finally, atoms
initially prepared in two momentum states were observed to cross pair with one another, forming
molecules in a third momentum state. This process is analogous to sum-frequency generation in optics.
DOI: 10.1103/PhysRevLett.94.040405

PACS numbers: 03.75.Be, 32.80.Pj, 33.80.Ps, 34.20.Cf

Similar to the field of optics, where the high intensity
and coherence of lasers allow for the observation of effects
such as frequency doubling and wave mixing, atom optics
has benefited greatly from the realization of Bose-Einsteincondensates (BECs). High phase-space density (atoms permode) and a uniform phase [1,2] give the condensate its
laserlike qualities. Although not fundamentally required[3,4], BECs have led to the observation of such phenomena
as four-wave mixing [5], matter wave amplification [6,7],
and atom number squeezing [8].
The current state of molecular optics is similar to atom
optics prior to the realization of BECs. Diffraction andinterferometry of thermal molecular beams has been demonstrated [9–12], yet monoenergetic beams lack the density necessary to observe nonlinear effects. However,recent experiments using Feshbach resonances have demonstrated the conversion of degenerate atomic bosons [13–15] and fermions [16 –20] into ultracold molecules. Thesesources have the potential to greatly advance molecular
optics. Furthermore, atom-molecule coupling can bestudied as the first steps towards ‘‘superchemistry,’’ where
chemical reactions are stimulated via macroscopic occupation of a quantum state [21].
The coherent manipulation of atomic matter waves via
stimulated light scattering has been crucial in the development of nonlinear atom optics (for a review, see [22]). Here
we demonstrate the ability to apply Kapitza-Dirac andBragg diffraction [23] to cold molecules. Using opticalstanding waves of suitably chosen frequencies, sodium
dimers were coherently manipulated with negligible heating or other incoherent processes. First, we characterized
the coherence of our ‘‘source’’ molecules, created via
Feshbach resonance. By measuring the Bragg spectrumof the molecules immediately after their creation, theconversion from atoms to molecules was shown to becoherent —the matter wave analog to frequency doublingin optics. The quadratic spatial dependence of the phase of
the expanding molecules was observed using an autocorrelation interference technique. By creating a duplicate
sample of molecules and overlapping it with the original,
0031-9007=05=94(4)=040405(4)$23.00

matter wave interference was observed. Finally, the matterwave analog to sum-frequency generation was demonstrated. Atoms prepared in two momentum states, prior
to creating molecules, were observed to cross pair, generating a third momentum state.
To produce molecules, sodium condensates were prepared in a crossed optical dipole trap in the jF; mF i j1; 1i state. Trap frequencies of !x ; !y ; !z  
2  146 105 23 Hz yielded a typical peak density ofn0 ’ 4:3  1014 cm3 for 25  106 atoms. Atoms werethen spin-flipped to the j1; 1i state, in which a 1 G wide
Feshbach resonance exists at 907 G [24].
The magnetic field sequence used to create and detect
Na2 molecules was identical to our previous work [15,25].
Briefly, the axial (z axis) magnetic field was ramped to903 G in 100 ms. In order to prepare the condensate on thenegative scattering length side of the resonance, the field
was stepped up to 913 G as quickly as possible ( 1 s) tojump through the resonance with minimal atom loss. After
waiting 1200 s for the transient field to die down, thefield was ramped back down to 903 G in 50 s to form
molecules. In order to remove nonpaired atoms from thetrap, the sample was irradiated with a 10 s pulse ofresonant light. Because 903 G is far from the Feshbachresonance, the mixing between atomic and molecularstates was small, and therefore molecules were transparent
to this ‘‘blast’’ pulse. By ramping the field back to 913 G,
molecules were converted back to atoms. Absorption images were taken at high fields (either at 903 G or 913 G),with the imaging light incident along the axial direction of
the condensate. Bragg scattering of atoms and molecules
was carried out using two nearly orthogonal beams ( B 84 ), aligned such that particles were scattered along the xaxis of the trap. The beams were far detuned from atomicmolecular transitions to limit spontaneous scattering. For
atoms the detuning was ’ 4 nm from the sodium D lines.
To find a suitable transition for the molecules, we scanned
the laser wavelength and measured the Rabi frequency for
Bragg transitions. Several narrow transitions were observed, but not carefully characterized. For this work the040405-1

 2005 The American Physical Societylaser was set to 590.159 nm and detuned 50 MHz fromthe apparent resonance, yielding a Rabi frequency of
2 kHz.
Figure 1 shows time-of-flight images of Bragg scattering
for atoms and molecules. Because the kinetic energy of thescattered particles was much larger than their mean-fieldenergy, individual momentum states were well resolvedin ballistic expansion. Both atoms and molecules
receive equal two-photon recoil momentum, pr 
2h sin B =2=L , where L is the wavelength of the
Bragg beams. However, scattered molecules move awayfrom the central peak with half the velocity of atoms,
owing to their doubled mass. Figures 1(c) and 1(d) show
Kapitza-Dirac scattering, where multiple atomic and molecular momentum states were populated due to the broadfrequency distribution of the short pulse (5 s).
In order to study the coherence properties of the sample,
Bragg spectra [1] were taken with 1 kHz resolution bypulsing on the two laser beams (250 s square pulse)
before releasing the particles from the trap. For noninteracting particles, the Bragg resonance occurs at a relative
detuning of 0  p2r =2mh between the beams, where thesign of 0 dictates the direction of outcoupling. Interactions in a condensate give rise to a mean-field shift of
the resonance   4n0 U=7h, where U  4h 2 a=m anda is the scattering length. Figure 2 shows three spectra for(a) atoms above the Feshbach resonance, as well as (b)atoms and (c) molecules after sweeping through the reso--2

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PHYSICAL REVIEW LETTERSPRL 94, 040405 (2005)

Atomic Momentum (pr)
-1

0

1

2

a)

nance. The reduced mean-field shift for atoms below theresonance [Fig. 1(b)] can be attributed to inelastic losses
caused by passing through the resonance. Atoms below theresonance coexisted with a small fraction of molecules(2%). The peak output for each set of data is normalized
to 1. The actual peak outcoupled fractions were 0.06 for theatoms and 0.5 for the molecules. The atomic signal was
kept intentionally low to minimize collisions which makethe data analysis difficult [see halos in Fig. 1(c)]. As
expected from the resonance condition, molecular resonances occur at half the frequency of atomic resonances.
Two mechanisms contribute to the fundamental width of
the Bragg resonance [1,26]. For a parabolic density distribution, the finite size of the sample yields a momentum
spreadprp  1:58
;
(1)
2mx0
where x0 is the Thomas-Fermi radius. In addition, theinhomogeneous density distribution of the sample produces a spread in mean-field energys
8 n0 U
n 
:
(2)
147 h
The fundamental width is approximately given by thequadraturesum of these two broadening mechanismsq  2n 2p .
The fundamental width, , and measured rms
width, , are compared for each case in Table I. n0
and x0 were determined from the size of the condensate
in time of flight. The measured widths cannot be accounted
for by fundamental broadening mechanisms alone.
For atoms above resonance, the fundamental width is  1:39 kHz, compared to the measured value of

b)

b)

d)

c)
-2

-1

0

1

1

Outcoupled Fraction (a.u.)

a)

c)

0

1

0

1

0

2

Molecular Momentum (p r )

-100

-50

0

50

100

Detuning (kHz)FIG. 1. Bragg diffraction of (a) atoms and (b) molecules.
Scattered particles recoil with identical momenta, pr .
However, during ballistic expansion, diffracted molecules expand with half the velocity of atoms, due to their doubled mass.
The pulse duration in each image was 200 s. A shorter pulse(5 s) populated multiple (c) atomic and (d) molecular momentum states. The halos in (c) are due to collisions between
different momentum states [36]. The time of flight in each image
is 17 ms.

FIG. 2. Bragg spectra for atoms and molecules. In (a) the
spectrum is taken for a pure atomic sample above the
Feshbach resonance (913 G). (b) and (c) are spectra of atoms
and molecules, respectively, below the resonance (903 G). In (c)atoms were removed from the trap with resonant light to limit
losses due to atom-molecule collisions. The Bragg resonancecondition for molecules occurs at half the frequency of theatomic resonance.

040405-2

4.46 kHz. Therefore, another broadening mechanism must
contribute 4 kHz to the width. Most likely this is due toDoppler broadening caused by random center-of-mass motion and other collective excitations of the condensate. Thiswas investigated by mixing two frequencies into each
Bragg beam to outcouple particles in both x directions.
For particles at rest, the outcoupling should always be
symmetric. However, we observe the ratio of outcoupled
particles in either direction to fluctuate. In addition, we
measure a small, consistent shift in the Bragg spectrum,indicating a drift velocity. A line broadening of 4 kHz
corresponds to a velocity of ’ 2 mm=s, or a vibrational
amplitude of ’ 2 m (compared to x0  13 m). This is
not unreasonable, because the field ramping scheme used
to bring the atoms to high field is violent and may induce
collective excitations such as breathing modes.
Despite vibrational noise making the dominant contribution to the width of the spectra, the measured values arestill narrow enough to indicate quantum degeneracy. For agiven , the corresponding temperature for a thermal
distribution of particles isT

mh2 2:
kB p2r(3)

Thus, for an rms width of 4.5 kHz, the temperature for athermal distribution of molecules would be 20 nK, comparable to a previous value obtained using a time-of-flighttechnique [15]. The BEC transition temperature for our
trap parameters [27] and 5  104 molecules is much higher(115 nK). This demonstrates a deeply degenerate, purely
molecular sample, where as previous experiments havedemonstrated coherent admixture of molecular character
into an atomic BEC [28].
The molecular Bragg spectrum showed a surprisingly
large shift of   625 Hz. If interpreted as a mean-field
shift, this would imply either a very high density (possible
due to a spatial collapse) or an anomalously large molecular scattering length outside the Feshbach resonance.
Further study is necessary.
The spatial phase of the expanding molecular cloud was
directly imaged using an autocorrelation method [29],which gives rise to the self-interference of the molecularsample (see Fig. 3). To accomplish this, two identicalcopies of the sample were made using a short Kapitza-

Dirac pulse (10 s), applied after 2 ms of ballistic expansion. The copies, with momentum pr , moved away fromthe zero momentum peak for time t before an identicalpulse recombined them with the original. This type ofinterferometer has three readout ports, with momenta 0,
pr . The straightline interference fringes are characteristic of a quadratic spatial phase. We observe fringe contrast as high as 50% and a fringe spacing consistent withf  ht=md [30], where d  pr t=m is the distance thecopies moved between pulses [31]. Interference fringes canonly be resolved for small d. Therefore, this method cannot
be used to observe coherence lengths longer than thoseinferred from Bragg spectroscopy. It should be noted thatthe appearance of interference fringes does not imply thatthe sample is condensed. Rather, it demonstrates only thatthe coherence length in time of flight is longer than theseparation d. Therefore, similar interference can also be
observed for a cloud of thermal atoms [32,33].
The conversion of atoms to molecules may be viewed as
the atom optic equivalent of frequency doubling [34]. The
relevant Hamiltonian for the atom-molecule coupling has
the same form as that for the optical frequency doublingprocess:
ay2m am am ;Atoms (above)
Atoms (below)
Moleculesa (kHz) (kHz)T (nK)

1.39
1.03
0.36a

4.46(17)4.50(60)
4.53(14)10
1020This lower bound assumes that the molecules have the same
spatial profile as the atoms, which our results indicate is not the
case.

(4)

where am is the annihilation operator for the atomic field
and ay2m is the creation operator for the molecular field. The
measurement of the Bragg spectrum shows that the sharp‘‘linewidth’’ of the seed (atom) laser is inherited by themolecular laser. In nonlinear optics, photon interactions are
a)

b)

c)

TABLE I. Fundamental width (), measured rms width (),and the corresponding temperature (T), assuming a thermal
distribution.
Spectrum

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PHYSICAL REVIEW LETTERSPRL 94, 040405 (2005)

-1

0

1

Momentum (pr)FIG. 3. Matter wave interference of molecules. (a) The molecular sample is split with a short Kapitza-Dirac pulse, creating
two identical copies with momenta pr . (b),(c) After the copies
have separated for time t  100 s, a second pulse recombines them, giving rise to interference in each momentum
component. The time of flight in each image is 12 ms.

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PHYSICAL REVIEW LETTERS

Atomic Momentum (pr)01week ending
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manuscript. This research is supported by NSF, ONR,
ARO, and NASA.

a)

b)

0
1
2
Molecular Momentum (pr)FIG. 4. Sum-frequency generation of atomic matter waves.
(a) Atoms were initially prepared in momentum states 0, 1.
(b) By sweeping through the Feshbach resonance, atoms combine to form molecules with momenta 0, 1, and 2. Momentum
state 1 is the sum frequency of the two atomic matter waves. The‘‘nonlinear medium’’ is provided by the atomic interactions. The
time of flight in each image is 17 ms.

typically mediated by a refractive medium. Here, the nonlinearity arises from the atoms themselves, in the form ofs-wave interactions. The high density, or ‘‘brightness,’’ ofthe source, together with the enhanced interactions at the
Feshbach resonance provide the means to combine twomatter waves. By combining two disparate matter waves,rather than identical ones, we extend the analogy of frequency doubling to the more general process of sumfrequency generation. To do this, atoms were initiallyprepared in momentum states 0, 1 (in units of pr ). By
sweeping through the resonance, molecules were producedin three momentum states: 0, 1, and 2 (see Fig. 4). States 0
and 2 are simply the frequency doubled components of thetwo initial matter waves. State 1, however, results fromcross pairing between the initial momentum states and isthus their sum frequency. This is the first time that a
Feshbach resonance was observed between atoms colliding
with a controlled nonvanishing momentum. The Feshbachresonance should be slightly shifted compared to the resonance for atoms at rest, which reflects the same physics
encountered in the temperature dependence of the positionof the resonance [35].
In conclusion, we have demonstrated coherent molecular optics using standing light waves. The ability to coherently convert atoms into molecules makes molecular optics
even richer than atom optics. In addition, the techniquesdemonstrated in this Letter could prove useful for probing
molecules formed in quantum-degenerate Fermi systems,
and possibly even Cooper pairs.
The authors would like to acknowledge M. Boyd and W.
Setiawan for experimental contributions and thank A. E.
Leanhardt and M. W. Zwierlein for critical readings of the*Present
address:
Lawrence
Berkeley
National
Laboratories, MS88R0192, Berkeley, CA 94720-8101,
USA.
Email address: jrabo-shaeer@lbl.gov†
Electronic address: http://cua.mit.edu/ketterle_group[1] J. Stenger et al., Phys. Rev. Lett. 82, 4569 (1999).
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[27] For an optical dipole trap using 1064 nm light, the trap
parameters for both atoms and molecules are the same.
[28] E. A. Donley, N. R. Claussen, S. T. Thompson, and C. E.
Wieman, Nature (London) 417, 529 (2002).
[29] J. Simsarian et al., Phys. Rev. Lett. 85, 2040 (2000).
[30] M. R. Andrews et al., Science 275, 637 (1997).
[31] Strictly speaking, this analysis applies only to the outcoupled ports. The center port is comprised of threecopies.
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040405-4

Appendix C

Evidence for Superfluidity of
Ultracold Fermions in an Optical
Lattice
J.K.Chin, D.E. Miller, Y. Liu, C. Stan, W. Setiawan,
C. Sanner, K.Xu, W. Ketterle“Evidence for Superfluidity of Ultracold Fermions in an Optical Lattice,”
Nature 443, 961 (2006).

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LETTERS
Evidence for superfluidity of ultracold fermionsin an optical lattice
J. K. Chin1, D. E. Miller1, Y. Liu1, C. Stan1{, W. Setiawan1, C. Sanner1, K. Xu1 & W. Ketterle1

three-dimensional optical lattice. A broad Feshbach resonancecentred at 834 G enabled tuning of the interatomic interactions overa wide range. On resonance, a bound molecular state becomes degenerate with the open atomic scattering channel, leading to a divergencein the scattering length a. Here we explore the region of strong interactions, also known as the BEC–BCS (Bardeen–Cooper–Schrieffer)crossover, in which the magnitude of the interaction parameter jkFaj
is greater than unity, and kF is defined as the peak Fermi wavevectorof a two-component non-interacting mixture of 6Li atoms. In the
crossover region, pairing occurs as a result of many-body interactions. Below resonance, for strong interactions, the bare two-bodystate has a bond length larger than the interatomic spacing and isirrelevant. In a lattice, atom pairs above and below the resonance canbe confined to one lattice site11, and crossover physics may require an
occupation larger than or equal to one.
The peak pair filling factor of the lattice was about unity. At
this density in the bulk, the fermion pair size is on the order of
1/kF 5 170 nm, comparable to the lattice spacing of 532 nm. To probe
the momentum distribution, we ramped the magnetic field out of thestrongly interacting regime as fast as technically possible (about
150 ms) and then turned off the confining potential. Absorption
Interaction parameter, kF a
10

0.3

1

a

Magnetic field (G)The study of superfluid fermion pairs in a periodic potential has
important ramifications for understanding superconductivity incrystalline materials. By using cold atomic gases, various models ofcondensed matter can be studied in a highly controllable environment. Weakly repulsive fermions in an optical lattice could
undergo d-wave pairing1 at low temperatures, a possible mechanism for high temperature superconductivity in the copperoxides2. The lattice potential could also strongly increase the critical temperature for s-wave superfluidity. Recent experimental
advances in bulk atomic gases include the observation of fermion-pair condensates and high-temperature superfluidity3–8.
Experiments with fermions9–11 and bosonic bound pairs12,13 in
optical lattices have been reported but have not yet addressedsuperfluid behaviour. Here we report the observation of distinct
interference peaks when a condensate of fermionic atom pairs isreleased from an optical lattice, implying long-range order (aproperty of a superfluid). Conceptually, this means that s-wave
pairing and coherence of fermion pairs have now been establishedin a lattice potential, in which the transport of atoms occurs by
quantum mechanical tunnelling and not by simple propagation.
These observations were made for interactions on both sides of a
Feshbach resonance. For larger lattice depths, the coherence waslost in a reversible manner, possibly as a result of a transition fromsuperfluid to insulator. Such strongly interacting fermions in an
optical lattice can be used to study a new class of hamiltonians withinterband and atom–molecule couplings14.
Previous experiments showing long-range phase coherence inBose–Einstein condensates (BECs) and in fermion superfluids usedballistic expansion to observe the interference of two independent
condensates15, vortex lattices8,16,17 or interference peaks after releasefrom an optical lattice18,19. However, for strongly interacting fermions, elastic collisions can change the momentum distribution
and wash out interference peaks. For an initially superfluid cloud,such dissipative dynamics corresponds to superfluid flow faster thanthe critical velocity. Consistent with this expectation is the observation that a strongly interacting Fermi superfluid initially containing distinct momentum components yielded a broad diffuse cloudafter expansion (Fig. 1). This issue was addressed by using a magnetic
field ramp that quickly increased the detuning from a Feshbach resonance, taking the system out of the strongly interacting regime andenforcing ballistic expansion. In previous studies of strongly interacting Fermi gases, magnetic field sweeps were applied to prevent
fermion pairs above the Feshbach resonance from dissociating6,7,20 .In
contrast, our experiment required a magnetic field sweep both aboveand below the Feshbach resonance to avoid elastic collisions.
Our experiments used a balanced mixture of 6Li fermions in the
two lowest hyperfine states. Evaporative cooling produced a nearlypure fermion pair condensate that was adiabatically loaded into a

b

c

822

x

730

470

//
//

0

0.15
2
Time (ms)468

Figure 1 | Dissipative collisions during expansion of a strongly interacting
fermionic superfluid. The schematic shows the time sequence of themagnetic field ramp used throughout this paper. A one-dimensional opticalstanding wave was pulsed onto the superfluid at different magnetic fields Bp(indicated by arrows at 822 G (a), 749 G (b) and 665 G (c)) during expansion,
creating particles at twice the photon recoil30. Absorption images taken at thetime marked with the cross show distinct momentum peaks only at magnetic
fields Bp # 750 G, where kFa # 1. At higher magnetic fields, the peaks blurred
into a broad diffuse cloud as a result of the larger collision cross-section.

1
Department of Physics, MIT-Harvard Center for Ultracold Atoms, and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139,
USA. {Present address: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA.

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NATURE | Vol 443 | 26 October 2006

a

c

b

c

d

e

f

g

h

Figure 3 | Interferograms of fermion pairs released from different lattice
depths V0 at a field of 822 G. Values of V0 are 0Er (a), 2.5Er (b), 4Er (c), 5Er(d), 6Er (e), 7Er (f), 9Er (g) and 2.5Er (h). a–g were taken after an adiabatic
ramp up to the final V0, whereas h was taken after first ramping up to 10Er
before ramping down to 2.5Er.

dFigure 2 | Observation of high-contrast interference of fermion pairs
released from an optical lattice below and above the Feshbach resonance.
a, The orientation of the reciprocal lattice, also with respect to the imaging
light. b–d, Interference peaks are observed for magnetic fields of 822 G(b), 867 G (c) and 917 G (d). The lattice depth for all images is 5Er, and each
image is the average of three shots. The field of view is 1 mm 3 1 mm.
Density profiles through the vertical interference peaks are shown for each
image.

images taken after 6.5 ms of expansion reveal sharp peaks at thereciprocal lattice vectors—the signature of long-range coherence, astrong indicator for superfluidity.
We observed these interference peaks at magnetic fields both aboveand below the Feshbach resonance (Fig. 2). The six first-order diffracted peaks are clearly visible around the zero momentum fraction
and their positions correspond to the expected momentum quanta of
2BkL carried by molecules of mass 2m, where kL is the lattice wavevector. At high magnetic fields (Fig. 2d) the visibility of the interference peaks decreased and some additional heating was observed. Thisdegradation could be due to a higher fraction of thermal atoms as weapproached the BCS limit, but it was not studied in detail.
The narrow interference peaks clearly reveal the presence of amacroscopic wavefunction possessing long-range phase coherence.
The separation between the interference peaks relative to their width
gives an estimate of the coherence length of about ten lattice sites.
This estimate is a lower bound, because effects of finite resolution andmechanisms of residual broadening have been neglected. With unity
occupation, and in the absence of any discernible background at
magnetic fields near the Feshbach resonance, this implies a minimum
phase space density of 103 and shows that our samples are deep in thequantum-degenerate regime. In previous studies of ultracold Boseand Fermi gases, the appearance of a condensate fraction and longrange phase coherence was shown to occur concurrently with thepossibility to excite superfluid flow8,16,17,21. Superfluid hydrodynamics is usually regarded as the direct proof for superfluidity.
However, all reports of superfluidity of bosons in three-dimensional
optical lattices have relied solely on observations of sharp interference peaks and inferred superfluidity from the established connection between long-range coherence and superfluidity19,22. Similarly,
our observations directly show long-range coherence and indirectly
show superfluidity of fermion pairs in an optical lattice.
For deep lattices, breakdown of superfluid behaviour has been
observed for weakly interacting BECs of different bosonic species19,23.
This phase transition to the Mott-insulator state occurs when on-site
interactions start to suppress atom number fluctuations and thesystem undergoes a transition from a delocalized superfluid

described by a macroscopic wavefunction to a product of Wannier
states tightly localized at each lattice site. Experimentally, this ismanifested as a smearing of the distinct 2BkL interference peaks.
Figure 3 shows the change in the coherence properties when thelattice depth was increased. The interference peaks became more
pronounced initially, because of increased modulation of the wavefunction. The interference peaks began to smear out, rapidly giving
way to a featureless cloud, beyond a critical lattice depth Vc < 6Er,
where Er 5 B2kL2/4m 5 h 3 15 kHz is the recoil energy. This indicatesthat all phase coherence had been lost. On subsequent ramping downof the lattice, interference peaks became visible again (Fig. 3h), showing reversibility of the lattice ramp.
We repeated this sequence for a wide range of initial magnetic
fields, both above and below the resonance, and observed the same
marked change in the interference pattern. Figure 4 displays the peak
optical density of the interference peaks for different lattice depths at
representative fields. Across all fields, the sharp decrease in peak
optical density occurred between 5Er and 6Er. A further increase in
the magnetic field resulted in decreasing overall visibility, until interference peaks could no longer be observed regardless of lattice depth.
The loss of phase coherence with increasing lattice depth is consistent with the qualitative description of the superfluid to Mottinsulator transition. However, the usual single-band description is
no longer applicable, because in the strong-coupling regime theon-site interaction strength should be comparable to the band gap
Bv, where v is the onsite trap frequency. Furthermore, Pauli blocking forbids the multiple occupation of the lowest state of an

Peak optical density (a.u.)

b

a

0.4
0.3
0.2
0.1
0

2

3

4
Lattice depth (Er)

5

6

7

Figure 4 | Peak optical density of interference peaks for increasing lattice
depths at different magnetic fields. Values of magnetic fields are 842 G(filled circles), 892 G (open squares) and 942 G (filled triangles). Peak opticaldensities were estimated from fits to the peaks, including backgroundsubtraction. The inset shows a sample density profile of the central and one
pair of interference peaks (dotted line), with a bimodal fit to one side peak(solid line). Each point is the average of three different images with six
interference peaks per image. Error bars show s.d.

962
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NATURE | Vol 443 | 26 October 2006

better understanding of the unitarity-limited interactions in such a
Fermi system. Recent theoretical work14,26 predicts that stronglyinteracting fermions in an optical lattice feature multiband couplingsand next-neighbour interactions and can realize the important t–J
and magnetic XXZ models of condensed-matter theory. This demonstrates that such atomic systems are an ideal laboratory for the
exploration of novel condensed-matter physics.

160

Central peak width (µm)

140
120
100

METHODS
80
60
0

200

400

600

800

1,000

Ramp-down time (µs)

Figure 5 | Restoring coherence from a deep lattice. The width of the central
peak is used as a measure of phase coherence after an adiabatic ramp up to
8Er, followed by a fast ramp down to 2.5Er at a fixed magnetic field of 822 G.
Filled circles were extracted with the use of a gaussian fit, and diamonds witha bimodal fit. Also plotted for comparison is the gaussian width of the central
peak for a dephased sample, in which a field gradient was applied during theramp up of the lattice (crosses). All points were taken for 6.5 ms time offlight.

individual lattice site by identical fermions, and modification of thesingle-particle tunnelling rate is expected as a result of virtual pairbreaking transitions14. One may still be tempted to use the standard
bosonic Hubbard model and estimate the critical lattice depth Vc foran assumed value of onsite interaction energy U 5 Bv and noninteracting, single-particle tunnelling J, but the obtained Vc < 3Er
is significantly smaller than our observation, which is in turn muchsmaller than the Vc . 10Er observed for weakly interacting atomic
BECs19,23. Together with the observed insensitivity of Vc to the magnetic field, this shows that models based on weak interactions are
inadequate.
Figure 3h shows the reversibility of the transition from a longrange coherent state to a state without strong coherence. We now
study the timescale for this recoherence, by analogy with similar
measurements performed across the transition from superfluid toMott insulator in atomic BECs19. Figure 5 shows that phase coherence was restored on a submillisecond timescale, on the order of thesingle-particle tunnelling time of about 500 ms (for a shallow lattice of
2.5Er). When the same lattice ramp sequence was applied to a superfluid that had been dephased by a magnetic field gradient19, thesystem did not regain phase coherence on the timescales that weprobed. Evaporative cooling is therefore negligible during this time.
The short recoherence time of the condensate is evidence that thesystem stayed in its ground state or at least in a low-entropy state
when the lattice was ramped up.
Figure 5 also provides evidence that the system could not recohere
during the 150-ms magnetic field ramp. In Fig. 3h, the central peak iswell fitted by a bimodal distribution with a width of 35 mm, in clear
contrast to the gaussian width of 105 mm obtained from Fig. 5 after
150 ms. We therefore conclude that the observed interference patternsin Fig. 1 reflect the coherence of the cloud at the initial magnetic field,in the strongly interacting regime.
We have shown long-range phase coherence of fermion pairs in an
optical lattice in the BEC–BCS crossover region by observing sharp
interference peaks during ballistic expansion. This indicates that wehave achieved s-wave pairing and superfluidity in a lattice potential.
Further studies will reveal how the pair wavefunction is affectedby confinement24, and whether the lattice shifts the BEC–BCS crossover away from the Feshbach resonance25. The loss of coherence
during the lattice ramp up and the rapid recoherence are characteristic of a Mott insulator. However, definitive proof will require a

Clouds of superfluid fermion pairs were created in a new experimental setup27,28
by using techniques similar to those described elsewhere8. In brief, a combination of laser cooling and sympathetic cooling of spin-polarized fermions bybosonic 23Na was followed by a spin transfer to create a two-component
Fermi gas, allowing further cooling through direct evaporation of the fermions.
As the fermions cooled, they formed pairs that Bose-condensed.
Estimates of the scattering length, and hence the interaction parameter, fromthe magnetic field were obtained with a(B) 5 21,405a0[1 1 300/(B 2 834)][1 1 0.0004(B 2 834)] (ref. 29), where B is measured in gauss and a0 is theBohr radius. The calibration of the magnetic field in our system had an uncertainty
of about 5 G.
Evaporation was performed at a magnetic field of 822 G, at which strong
interactions permitted efficient evaporation. An estimated average final
number of N < 2 3 105 6Li pairs and harmonic trapping frequencies ofnx,y,z 5 (270,340,200) Hz gave a trap depth of 1.7 mK and a Fermi energy ofEF 5 kB 3 1.4 mK, where EF 5 B (6N)1/3 andis the average trapping frequency. After evaporation, the magnetic field was brought to a desired value
B0 in 20 ms and the condensate was allowed to equilibrate for a further 200 ms.
Before ramping to values of B0 on the BCS side, we also recompressed the opticaltrap to (340,440,270) Hz and 2.2 mK depth in 100 ms to accommodate the larger
Fermi clouds above the resonance7.
A three-dimensional optical lattice was formed from three optical standing
waves, oriented such that the resulting unit cell had a sheared cubic structure,with one axis tilted about 20u from the normal for reasons of optical access (seeFig. 1a)23. The incident laser beams were focused down to the condensate withwaists of about 90 mm, then retroreflected and overlapped at the condensate to
generate the standing-wave potentials. All lattice light was derived from a 1,064nm single-frequency fibre laser, and each beam was detuned by tens of MHz withrespect to the others to eliminate interference between different beams.
The lattice potential was imposed on the condensate by adiabatically increasing the intensity of the laser beams to a variable final value V0. The calibration of
V0 had an uncertainty of about 20%. A simple linear ramp with a constant rate
dV0/dt of 0.5Er ms21 was used unless otherwise specified. This satisfies the interband adiabaticity condition of dV0/dt = 16Er2/B.
Ballistic expansion for the detection of the different momentum components
was provided by a magnetic field sequence (shown in Fig. 1) that quickly brought
the system out of the strongly interacting regime when all confinement was
switched off. During the magnetic field ramp of about 150 ms, the lattice potential
was kept on. The first 2 ms of expansion took place at 470 G, at which themolecules are tightly bound, before the field was ramped back up to 730 G in
the next 4.5 ms, at which the weakly bound molecules strongly absorb light near
the atomic resonance line and could be observed by absorption imaging. The
specific magnetic field sequence was chosen to minimize collisions within technical capabilities.
Received 29 June; accepted 25 August 2006.
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Acknowledgements We thank E. Demler, Z. Hadzibabic and M. Zwierlein for
discussions. This work was supported by the NSF, the Office of Naval Research andNASA.
Author Information Reprints and permissions information is available atwww.nature.com/reprints. The authors declare no competing financial interests.
Correspondence and requests for materials should be addressed to J.K.C.
(jitkee@mit.edu).

964©2006 Nature Publishing GroupAppendix D

Critical velocity for superfluid flow
across the BEC-BCS crossover
D. E. Miller, J. K. Chin, C. A. Stan, Y. Liu, W. Setiawan,
C. Sanner and W. Ketterle“Critical velocity for superfluid flow across the BEC-BCS crossover,”Phys. Rev. Lett. 99, 070402 (2007) .

125

PRL 99, 070402 (2007)

PHYSICAL REVIEW LETTERS

week ending
17 AUGUST 2007

Critical Velocity for Superfluid Flow across the BEC-BCS Crossover
D. E. Miller, J. K. Chin, C. A. Stan,* Y. Liu, W. Setiawan, C. Sanner, and W. Ketterle†
Department of Physics, MIT-Harvard Center for Ultracold Atoms, and Research Laboratory of Electronics, MIT,
Cambridge, Massachusetts 02139, USA(Received 11 July 2007; published 16 August 2007)
Critical velocities have been observed in an ultracold superfluid Fermi gas throughout the BEC-BCS
crossover. A pronounced peak of the critical velocity at unitarity demonstrates that superfluidity is mostrobust for resonant atomic interactions. Critical velocities were determined from the abrupt onset of
dissipation when the velocity of a moving one-dimensional optical lattice was varied. The dependence ofthe critical velocity on lattice depth and on the inhomogeneous density profile was studied.
DOI: 10.1103/PhysRevLett.99.070402

PACS numbers: 03.75.Kk, 03.75.Lm, 03.75.Ss, 05.30.Jp

The recent realization of the BEC-BCS crossover inultracold atomic gases [1] allows one to study how bosonicsuperfluidity transforms into fermionic superfluidity. The
critical velocity for superfluid flow is determined by the
low-lying excitations of the superfluid. For weakly boundfermions, the (Landau) critical velocity is proportional tothe binding energy of the pairs, which increases monotonically along the crossover into the Bose-Einstein condensation (BEC) regime. However, the speed of sound, whichsets the critical velocity for phonon excitations, is almost
constant in the BCS regime, but then decreases monotonically on the BEC side, since the strongly bound molecules
are weakly interacting. At the BEC-BCS crossover, one
expects a rather narrow transition from a region where
excitation of sound limits superfluid flow, to a region where
pair breaking dominates. In this transition region, thecritical velocity is predicted to reach a maximum [2 – 4].
This makes the critical velocity one of the few quantities
that show a pronounced peak across the BEC-BCS crossover in contrast to the chemical potential, the transition
temperature [5], the speed of sound [6,7], and the frequencies of shape oscillations [8], which all vary monotonically.
In this Letter, we report the first study of critical velocities across the BEC-BCS crossover, where a Feshbachresonance allows the magnetic tuning of the atomic interactions, and find that superfluid flow is most robust near the
resonance. Our observation of a pronounced maximum of
the critical velocity is in agreement with the predictedcrossover between the two different mechanisms fordissipation.
Critical velocities have been determined before in
atomic BECs perturbed by a stirring beam [9–11] as well
as by a 1D moving optical lattice [12]. In both cases, theinhomogeneous density of the harmonically trapped sample had to be carefully accounted for in order to makequantitative comparisons to theory. Here we mitigate thisproblem by probing only the central region of our sample
with a tightly focused moving lattice formed from twointersecting laser beams. For decreasing lattice depths,
the critical velocity increases and, at very small depths,
0031-9007=07=99(7)=070402(4)approaches a value that is in agreement with theoreticalpredictions.
In our experiments, we first create a superfluid of 6 Li
pairs according to the procedure described in previous
work [13]. Forced evaporative cooling of an even mixtureof the two lowest hyperfine states is performed at a magnetic field of 822 G, on the BEC side of a broad Feshbachresonance centered at B0  834 G. This results in a nearlypure Bose-Einstein condensate of N  5  105 pairs in across optical dipole trap with harmonic trapping frequencies x;y;z  65; 45; 50 Hz. The Fermi energy of the sys1=3  h  7:6 kHz. To form the
tem is EF  h6N
moving lattice, we focus two phase-locked 1064 nm laser
beams to intersect at the sample with an angle of 90(see Fig. 1). The resulting 1D lattice has a spatial periodof L  0:75 m. A frequency difference between thetwo beams of  causes the lattice to move withvelocity vL  L . The beams have e2 waists of 20
and 60 m, respectively, and address a relatively homogeneous region at the center of the cloud which has ThomasFermi radii Rx;y;z  63; 91; 82 m. The minimum density at the position of the e2 waist is 42% of the central
density.
The lattice which necessarily varies in depth across the
sample, is characterized by its peak depth V0 specified inunits of EF or the recoil energy Er  h2 =8m2L   h 
7:3 kHz, where m is the molecular mass. The lattice depth
is calibrated using Kapitza-Dirac scattering. Because of theinhomogeneity of the lattice, the uncertainty is 40%. The
lattice depths explored in this Letter are sufficiently small
such that motion induced in the laboratory frame is negligible, in contrast to [14].
The lattice moving at a constant velocity is adiabaticallyramped up and held for a time t up to 2 s, after which thelattice is ramped down and all confinement is switched off.
As in previous work [13], a fast magnetic field ramp is used
to reduce strong interactions in order to probe the centerof-mass momentum distribution of the pairs. Subsequently,
absorption imaging is done on the atomic resonance line at
730 G. A bimodal fit reveals the number of pairs remaining

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Lattice Velocity

FIG. 1. Onset of dissipation for superfluid fermions in a moving optical lattice. (Inset) Schematic of the experiment in which
two intersecting laser beams produced a moving optical lattice at
the center of an optically trapped cloud (trapping beams notshown). Number of fermion pairs which remained in the condensate Nc after being subjected to a V0  0:2EF deep opticallattice for 500 ms, moving with velocity vL , at a magnetic fieldof 822 G (1=kF a  0:15). An abrupt onset of dissipation occurred above a critical velocity vc , which we identify from a fit
to Eq. (1).

in the condensate Nc , providing a measure of the heating
incurred during application of the moving lattice.
Figure 1 illustrates the characteristic dependence ofdissipation on the velocity of the moving lattice. At low
velocities, the sample is unaffected. Above some criticalvelocity vc , dissipation sets in abruptly. We determine vc
from a fit of Nc to the intersection of two lines with slopes 0
and :
Ncond v  Ncond 0f1  max 0; v  vc g:(1)

The critical velocity that we obtain from this procedure isconsistent for a large range of hold times, varying by lessthan 15% when the hold time t is changed by a factor of 20.
We explore the BEC-BCS crossover by adiabatically ramping the magnetic field to different values after evaporation
and repeating the measurement as before. The crossover isparametrized by the interaction parameter 1=kF a, where kF
is the Fermi wave vector and a is the B-field dependents-wave scattering length [15]. Again, we observe a threshold for dissipation.
Figure 2 shows the measured critical velocity throughout
the BEC-BCS crossover. The maximum near resonance isconsistent with the picture of a crossover between two
different types of excitation, as discussed in the introduction, and confirms that superfluidity is most robust on
resonance.
To illuminate the role of the inhomogeneous density
distribution, we performed experiments in which the entire
sample was perturbed by a uniform lattice. Lattice beams
with 80 m waists probed a more tightly confined sample

FIG. 2. Critical velocities throughout the BEC-BCS crossover.
A pronounced maximum was found at resonance. Data areshown for a V0  0:2EF deep lattice, held for t  500 ms.
The solid line is a guide to the eye.

of 2  105 pairs, with spatial extent RTF ’ 37 m. The
onset of dissipation seen in Fig. 3 is still striking, but now
loss is observed at much lower lattice velocities, in spite ofa larger Fermi energy EF  h  12:4 kHz. Moreover, theonset of dissipation is slightly more gradual. When themagnetic field was varied across the Feshbach resonance,
we again found a maximum of the critical velocity nearresonance. The lowering of the critical velocity due to theinhomogeneous density profile is expected, since at lower
density, both the speed of sound and (on the BCS side) the
pairing energy decrease. Although the critical velocity
should approach zero in the low density wings of the cloud,we still observe a sudden onset of dissipation at a finite
velocity, similar to studies in Refs. [9–11], where a laser
beam pierced through the whole condensate, but in contrast
to studies reported in [12].
In the limit of vanishing perturbation, the critical velocity should be given by the Landau criterion. In Fig. 4 weaddress the effects of a finite lattice potential in the originallattice configuration, as depicted in Fig. 1. The criticalvelocity is shown to be a decreasing function of V0 , saturating in the limit of low lattice depth (V0 0:03EF ). Thisbehavior is consistent with numerical simulation [4,16].
Measurements at the smallest lattice depths had large uncertainties, as the hold time required to observe a heating
effect of the lattice approached the natural lifetime of oursample. For this reason, we studied the field dependence(Fig. 2) at an intermediate lattice depth, where vc was morewell defined.
For comparison with theory we reference the local Fermi
velocity at the trap center vF  v0F 1 1=4 p
39 mm=s, where v0F  2EF =m is the Fermi velocity ofa noninteracting gas at the trap center, and   0:58 is auniversal parameter characterizing unitarity limited inter-070402-2

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PHYSICAL REVIEW LETTERS

FIG. 3 (color online). Effects of density inhomogeneity on thecritical velocity. A configuration in which the lattice beams(80 m) were larger than the trapped sample (37 m) resultsin loss in the condensate number Nc at significantly lower
velocity. Data are shown for a V0  0:15EF deep optical latticeheld for 200 ms at a magnetic field of 822 G.

actions [17–19]. For vanishing lattice depth, the observed
critical velocity at unitarity approaches vc =vF  0:25. If
we use the local Fermi velocity vF;w at the e2 waist of thelattice, we obtain vc =vF;w  0:34. The difference between
these values indicates the uncertainty due to residual density inhomogeneity. The local speed of sound in a Fermi
gas at unitarity isp
cs  vF;loc 1 1=2 = 3  0:37vF;loc :(2)
The critical velocity for pair breaking isq
vpair   2 2  =m1=2  0:34vF;loc ;(3)with   0:50v2F;loc =2m [17,19] and   1
v2F;loc =2m. These two values should provide approximate upper bounds to the critical velocity at unitarity[2,3]. It seems natural that the combination of both excitation mechanisms lowers the critical velocity further.
Within these uncertainties, and those of the density, thetheoretical predictions agree with the experimental results.
Up until now, we have deferred a discussion of how themoving lattice couples to the excitations. In a pure systemat zero temperature, one would expect the excitation spectrum to exhibit discrete resonances, where the perturbation
couples only to modes with the k vector of the lattice. On
the other hand, at finite temperature, it is possible that thelattice drags along thermal atoms which are pointlike
perturbations and can create excitations at all k vectors.
Our observation that the dissipation sets in at a certain
threshold velocity and increases monotonically with velocity is consistent with the participation of the thermal
component.

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FIG. 4. Critical velocities at different lattice depths. The results show vc to be a decreasing function of lattice depth V0 . In
the limit of low V0 , vc converges to a maximum value of 0.25 vF .
Data were taken near resonance, at 822 G (1=kF a  0:15) for
hold times t  250, 500, 1000, 2000 ms (squares, diamonds,
circles, triangles).

We further elucidated the role of thermal excitations byvarying the temperature. Gradually reducing the trap depth
from U0 to Umin , during exposure to a lattice moving above
vc , will suppress the accumulation of a thermal component. The lifetime in this case exceeded that for a sample
held at a fixed depth of either U0 or Umin . For BoseEinstein condensates, theoretical papers emphasized therole of the thermal component in the Landau damping
process in a moving lattice [20,21]. This was confirmed
qualitatively in an experiment at Florence [12] in which the
lifetime of the sample was drastically improved by eliminating the thermal atoms.
In our experiments, the clouds heated up during theexposure to the moving lattice. Figure 5 shows the increasein the number of thermal atoms and the loss in the total
number of atoms due to evaporative cooling. In an idealized model, where density is fixed, constant dissipation
would result in a linear decrease in the number of atoms
due to evaporative cooling. Our data show an accelerated
decrease, possibly reflecting increased dissipation due tothe increasing fraction of thermal atoms. However, anaccurate model should include the change in density (andtherefore critical velocity) during the exposure time.
Additional impurity atoms (e.g., sodium atoms) could
cause dissipation even at zero temperature and would allow
more controlled studies of the dissipation mechanism.
Unpaired atoms in clouds with population imbalance
may not play this role because of phase separation effects[22].
Another possible dissipation mechanism in a lattice isthe creation of two excitations through a dynamical or
modulational instability. Such an instability [23] occurs
already for weakly interacting particles moving through alattice with momentum q, when they collide and scatter
into states with momenta q q, analogous to opticalparametric generation [24]. This process is energetically

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PHYSICAL REVIEW LETTERS

FIG. 5. Number of pairs that remained in the condensate Nc(solid circles) and thermal component Nth (open circles) after
being held in a V0  0:35EF deep optical lattice moving withvelocity vL  6 mm=s for a variable hold time. The thermal
component shows a linear increase (dashed line), whereas Ncond
showed an accelerated loss, and is fit to a quadratic function(solid line).

possible only above 0:5qB , where qB  h=2L is the Bragg
momentum that defines the edge of the Brillouin zone. This
corresponds to a velocity v  11 mm/s for fermion pairs(and twice this value for single atoms). Since the highest
critical velocities we observe are slightly below this threshold, and strongly decrease already for relatively small V0 , itis very unlikely, that dynamical instabilities play a role inour experiments. Moreover, such instabilities should bestrongly modified by Pauli blocking. For our ratio of local
Fermi momentum to the Bragg momentum of 0.9, the firstband is nearly full in the center of the cloud. For BoseEinstein condensates, it has been recently predicted [25]and experimentally shown [14] that strong interactions canlower the threshold for the dynamical instability, close to
the Mott-insulator transition. The range of 1D lattice
depths explored here (V0 2Er ) is far from the 1D
Mott-insulator regime. We have observed the loss of coherence which typically accompanies the superfluid toMott insulation transition to occur only beyond V0 ’ 25Er .
In conclusion, we have used a novel optical latticegeometry to determine critical velocities in the BEC-BCS
crossover without the complications of strong density inhomogeneity. This configuration could be applied to studies in atomic Bose gases which so far have been limited by
the inhomogeneous density [9–12]. In addition, it would beinteresting to study dynamical instabilities for fermionsand the role of Pauli blocking.
The authors would like to thank Aviv Keshet for experimental assistance. This research has been supported by the
NSF and the U.S. Office of Naval Research.

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17 AUGUST 2007

*Present address: Department of Chemistry and Chemical
Biology, Harvard University, Cambridge, MA 02138,
USA.
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