arXiv:cond-mat/0609259 v1 11 Sep 2006

Dual-species quantum degeneracy of 40K and
87 Rb on an atom chip
M. H. T. Extavour, L. J. LeBlanc, T. Schumm, B. Cieslak, S. Myrskog,
A. Stummer, S. Aubin, J. H. Thywissen
Department of Physics and Institute for Optical Sciences, University of Toronto, 60 St. George
Street, Toronto, Canada
Abstract. In this article we review our recent experiments with a 40 K-87 Rb mixture. We demonstrate rapid sympathetic cooling of a 40 K-87 Rb mixture to dual quantum degeneracy on an atom chip.
We also provide details on efficient BEC production, species-selective magnetic confinement, andprogress toward integration of an optical lattice with an atom chip. The efficiency of our evaporation
allows us to reach dual degeneracy after just 6 s of evaporation - more rapidly than in conventional
magnetic traps. When optimizing evaporative cooling for efficient evaporation of 87 Rb alone weachieve BEC after just 4 s of evaporation and an 8 s total cycle time.
Keywords: Ultracold atoms, Bose-Einstein condensation, degenerate Fermi gas, sympathetic cooling, atom chips, species-selective potentials, microfabrication
PACS: 39.25.+k, 34.20.-b, 32.80.Pj, 3.75.Ss, 3.75.Nt, 85.40.Hp

INTRODUCTION
Ultra-cold gases of neutral fermionic atoms are of great interest in atomic and condensed
matter physics, yet they remain challenging to produce experimentally. This article describes the production and manipulation of ultra-cold fermionic and bosonic gases using
an atom chip and a simple, single-chamber vacuum system. We describe the observation of efficient evaporation to a Bose-Einstein condensate (BEC) in 87 Rb, along withthe rapid sympathetic evaporative cooling of a 40 K-87 Rb mixture to dual degeneracy - a
BEC of 87 Rb and a degenerate Fermi gas (DFG) of 40 K. We discuss the use of radio frequency (RF) dressed potentials to create species-selective magnetic microtraps. Finally,
we detail progress toward incorporating an optical lattice into our experiment. Althoughsome of the details of our setup and DFG-BEC have been reported elsewhere [1, 2], thisarticle provides an overview and update on important figures of merit.
The article begins with an outline of our initial optical and magnetic trapping andcooling stages for 87 Rb and 40 K. Next, we give a description of our observation ofquantum degeneracy in both species, and further details of our evaporative coolingtrajectories. We then describe the use of RF-dressed adiabatic potentials to produce a
species-selective microtrap. The article ends with a discussion of our plans to integrate
optical lattices into the experiment, and a discussion of the technical improvements being
made to our atom chip.

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FIGURE 1. MOT loading via LIAD We load the MOT from the atomic vapour formed via lightinduced atomic desorption (LIAD) from the interior walls of our Pyrex vacuum cell. (a) As the 87 Rb
MOT is repeatedly loaded from atomic vapour, we observe a one-hundred-fold increase in atom number(greyed area), which is lost after the LEDs are switched off. (b) The MOT loading rate as function of timeafter the LEDs have been switched off. Once the LEDs are off, the MOT loading rate undergoes a sharp
initial decrease from the LED-on value of 3 × 108 s−1 , followed by a slower decay to the LED-off valueof ∼ 106 s−1 in approximately 300 s.

LOADING AND TRAPPING
We begin by trapping neutral atoms directly from atomic vapour with a six-beammagneto-optical trap (MOT). The dual species MOT consists of two overlapping sets
beams: six counterpropagating beams at 780 nm to trap 87 Rb, and six beams at 767 nm
for 40 K [1]. With 87 Rb MOT loading times as short at 950 ms, we can achieve BEC .
Working with the 40 K-87 Rb mixture requires longer loading times due to the low isotopic
abundance of 40 K in our dispensers1: we first load 40 K alone into the MOT for 10-20 s,after which 87 Rb is loaded for an additional 1-5 s, while maintaining the 40 K population.
Both sets of MOT beams operate with a detuning of -26 MHz until the last 10 ms of
the load, when the 40 K is compressed by reducing the detuning of the 40 K beams to -10
MHz. With this procedure we load 109 87 Rb atoms and 107 40 K atoms into the MOT.
The background atomic 87 Rb and 40 K vapours are enhanced via light-induced atomic
desorption (LIAD) of alkali atoms from the inner walls of a 75 mm × 75 mm × 165
mm rectangular Pyrex vacuum cell [1]. By illuminating the cell using approximately
600 mW of incoherent 405 nm LED light2, we observe a 100-fold increase in 87 Rb MOT
atom number compared to loading from the background vapour alone (see Figure 1). The
increase in 40 K MOT atom number is unknown; we cannot observe a 40 K MOT without
LIAD. A coating of 87 Rb and 40 K atoms on the interior walls of our vacuum chamber isreplenished every few weeks by running a commercial Rb dispenser and a home-madeK dispenser.
After MOT loading, 3 ms of optical molasses cooling is applied to the 87 Rb. Finally,both species are optically pumped into weak-field-seeking hyperfine ground states:|F = 2, mF = 2i for 87 Rb and |F = 9/2, mF = 9/2i for 40 K.
1 We use KCl powder whose 40 K isotopic abundance has been enriched to 5% from the naturally-occurring 0.012%. For comparison, the natural abundance of 87 Rb is 28%.
2 We use ten Epitex L405 series UV illuminators.

Proceedings of the 20th International Conference on Atomic Physics (2006)2

We use a magnetic transfer procedure to transport the atoms from the site of the MOT
to the atom chip, the site of evaporative cooling. Immediately following optical pumping,
all optical fields are extinguished and atoms are confined in a two-coil quadrupole
magnetic trap formed at the site of the MOT. A biasing magnetic field generated bya third coil shifts the magnetic trap centre vertically to just below the surface of the atom
chip, which is mounted horizontally in the vacuum chamber 5 cm above the MOT site.
Once near the surface of the atom chip, atoms are smoothly transferred from thequadrupole magnetic trap into an anisotropic, Ioffe-Pritchard-type magnetic microtrap.
The chip trap is formed by combining the static magnetic fields generated by DC current
in a ‘Z’-shaped wire on the atom chip with an external, uniform magnetic field parallelto the chip surface [3]. This combination produces a non-zero magnetic field minimum
just below the chip surface, in which atoms in weak-field-seeking hyperfine states are
confined. Atoms are transferred into the microtrap by ramping up the Z-wire current andexternal field while ramping down the quadrupole trap.

QUANTUM DEGENERACY
We cool to quantum degeneracy using forced RF evaporative cooling of 87 Rb. Whether
cooling 87 Rb alone or a 40 K-87 Rb mixture [4, 5, 6], RF radiation acts only on the
87 Rb, ejecting hot atoms from the magnetic trap and allowing the remaining atoms
to rethermalize to a new, lower temperature. The the process of indirectly cooling theone species by direct RF evaporation of the other is referred to as sympathetic cooling[7, 8, 9, 10, 11]. In our case 40 K is cooled indirectly via thermalizing elastic collisions
with the 87 Rb.
Evaporative cooling of bosons alone. In the 87 Rb-only case we achieve a BEC of
2 × 105 atoms when empirically optimizing for evaporation efficiency. We first load the
MOT for 2 s, magnetically trap and transfer to the chip in 950 ms, and load atoms intoa chip microtrap in 310 ms at T ∼ 300 µ K. Next, we perform forced RF evaporative
cooling in which the RF frequency νRF is swept from 15.0 MHz to 3.6 MHz in 4.3 s,bringing the the total cycle time to ∼ 7.6 s. Evaporation is carried out in an anisotropic
trap with trap frequencies of ω⊥ /2π = 560 Hz and ωℓ /2π = 32 Hz. The log-slope
efficiency ηevap of the evaporation, defined as −∂ [log(ρ )]/∂ [log(N)], where ρ is thephase space density3 and N the atom number, is equal to 4.0 ± 0.1 when we optimize
evaporation for 87 Rb alone. For comparison, ηevap = 2.9 ± 0.4 when evaporation is
optimized for sympathetic cooling to dual degeneracy (see Figure 2). Our largest BECs
are achieved with cycle times of 15 s, and contain 3 × 105 atoms.
When optimizing for minimum cycle time, we sacrifice some evaporation efficiency,
but achieve a BEC of 2 × 104 atoms with cycle times as short as 5.7 s. In this case,
we load the MOT for 950 ms, and transfer to the atom chip as described above. Evaporative cooling is carried out in two empirically optimized steps. The initial 700 ms RF
3 Phase space density is defined as n λ 3

thermal deBroglie wavelength.

0 dB in this context, where n0 is the peak atomic density, and λdB the

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FIGURE 2. Evaporative cooling in a chip trap. (a) The 87 Rb evaporative cooling trajectory optimized
for 87 Rb alone is more efficient than that used for sympathetic cooling. The line of best fit establishes
the log-slope efficiency (defined in the text). (b) Spin-polarized fermions without a bosonic bath cannot
be evaporatively cooled (diamond). However, if bosonic 87 Rb (squares) is evaporatively cooled, thenfermionic 40 K is sympathetically cooled (circles) to quantum degeneracy (greyed area). For bosonic
87 Rb, the vertical axis is the occupation of the ground state; for fermionic 40 K, the vertical axis is the
fugacity. These two quantities are equivalent in the non-degenerate limit. A typical run-to-run spread in
atom number is shown on the right-most point; all vertical error bars are smaller than the marker size.

frequency sweep from νRF = 33.5 MHz to νRF = 6.0 MHz is carried out in a compressed
trap, which has ω⊥ /2π = 2.8 kHz at its centre and a transverse magnetic field gradient of
4800 G/cm at its edge. Next, the microtrap is decompressed until ω⊥ /2π = 861 Hz andωℓ /2π = 32 Hz. A second evaporative νRF sweep is carried out here, from 27.9 MHz to
3.5 MHz in 2030 ms.
We detect the phase transition from thermal gas to BEC by tracking the peak phasespace density of the gas. We fit time-of-flight absorption images with a composite peakfunction to capture the separate behaviour of the condensed and non-condensed fractions: a Thomas-Fermi parabola describes the condensate fraction, and a Bose function
non-condensed fraction (see Figure 3f). These fits yield reliable atom number and temperature data, from which we determine the phase (thermal atoms or BEC) of the cloud.
Once below Tc we also observe an inversion in the aspect ratio of the expanding cloud[12]. Figures 3e-g show absorption images of expanding condensates at three successively colder temperatures: above Tc , where the gas is entirely non-condensed; near Tc ,where the BEC begins to emerge; and much below Tc , where the condensed fraction
dominates the cloud.
Sympathetic cooling of the Bose-Fermi mixture. When working with the 40 K-87 Rb
mixture, we find that RF sweep times faster than ∼ 6 s are not successful in achieving
dual degeneracy. This is because the 40 K-87 Rb rethermalization time lags that of 87 Rb87 Rb at high temperatures. This empirical observation was studied in the context of
the Ramsauer-Townsend effect in [2]. We are also unable to sympathetically cool in
the compressed trap used for rapid BEC production, since the higher densities in thistrap lead to three-body loss, preventing efficient cooling of the 40 K-87 Rb mixture [4, 6].
Figure 2 shows the relative evaporation efficiencies 87 Rb and 40 K during sympathetic
cooling, as well as during single-species evaporation of 87 Rb alone.
We first load 2×105 40 K and 2×107 87 Rb doubly spin-polarized atoms into a 1.1 mKdeep microtrap at a temperature & 300 µ K. The 40 K radial and longitudinal trapping
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FIGURE 3. Observation of Fermi statistics. Due to Fermi pressure, Fermi degenerate 40 K clouds
appear to stop getting colder, even when the reservoir temperature approaches zero. (a) The apparent
temperature of the fermions, as measured by Gaussian fits to images of 40 K clouds, is plotted versus
temperature of both thermal (diamonds) and Bose-condensed (circles) 87 Rb. Data is compared to a
Gaussian fit of a theoretically generated ideal Fermi distribution (solid line) and its classical expectation(dashed line). Both temperatures are scaled by the Fermi energy EF of each 40 K cloud. Error bars
are statistical (one standard deviation), with uncertainty smaller than the sizes of symbols for lower
temperature data. Absorption images are shown for kB T /EF = 0.35 (b) and 0.95 (c), including a circle
indicating the Fermi energy EF . (d) A closer look at the fermion cloud shape reveals that it does not follow
a Boltzmann distribution. The fit residuals of a radially averaged cloud profile show a strong systematicdeviation when assuming Boltzmann (circles) instead of Fermi (diamonds) statistics. A degenerate Fermi
cloud is flatter at its centre than a Boltzmann distribution, and falls more sharply to zero near its edge.
(e-g) Absorption images of 87 Rb atoms after 20 ms of time-of-flight expansion: above (e), just below (f),and well below Tc (g), which shows 2.5 × 105 atoms in a quasi-pure BEC. Tc ≈ 400 nK in this data. The
BEC is anisotropic in-trap, with its long axis aligned horizontally. This alignment is inverted at long timesof flight as evident in these images.

frequencies are measured to be ω⊥ /2π = 826 ± 7 Hz and ωℓ /2π = 46.2 ± 0.7 Hz,
respectively4. νRF is swept from 28.6 MHZ to 3.6 MHz in 6.15 s.
As with 87 Rb, we assess the degree of quantum degeneracy in 40 K by fitting absorption
data with ideal gas theory. In the case of 40 K, fits using ideal fermion density distributions yield temperature, density, Fermi energy and fugacity. However, since there is nophase transition to the quantum degenerate state in cold, spin-polarized non-interacting
fermions, distinguishing degenerate from non-degenerate Fermi gases is much morechallenging. Nevertheless, we observe a clear signature of Fermi degeneracy in 40 K
below T ≈ 1 µ K in the shape of the time-of-flight distribution.
We compare the rms cloud sizes of the 40 K and the non-condensed 87 Rb fraction
by fitting the density profiles to the Gaussian shapes predicted by Boltzmann statistics. Figure 3a shows that the apparent (i.e., Gaussian-estimated) 40 K temperature approaches a finite value while the 87 Rb temperature approaches zero, even though thetwo gases are in good thermal contact. This deviation is evidence of the Fermi pressure expected of a gas obeying Fermi-Dirac statistics, and of the Pauli exclusion prin4 The corresponding 87 Rb trap frequencies are a factor ofpmRb /mK ≈ 1.47 smaller than those for 40 K,
87
40where mRb and mK are the atomic masses of Rb and K, respectively.
Proceedings of the 20th International Conference on Atomic Physics (2006)5

ciple [13]: at zero temperature, fermions fill all available energy states up to the Fermi
2 ω )1/3 , where N is the number of fermions. We plot data with therenergy EF = h̄(6N ω⊥ℓmal (diamonds) and Bose-condensed (circles) 87 Rb separately, to show that the densitydependent attractive interaction between 40 K and 87 Rb does not significantly affect therelease energy. For our typical parameters, EF ≈ kB × 1.1 µ K. After all 87 Rb atoms havebeen evaporated, we use Fermi-Dirac fits to measure temperature, and find kB T /EF as
low as 0.09 ± 0.05 with as many as 4 × 104 40 K atoms.

SPECIES-SELECTIVE MICROTRAPS
A common practice among experimental realizations of 40 K-87 Rb mixtures [4, 6, 5](and Bose-Fermi mixtures generally) is the use of a single potential which confines
both species. The ability to separately control the external potential of each species is a
new and useful tool for studying such mixtures, which we demonstrate here. Tunable,
species-selective magnetic confinement of our 40 K-87 Rb mixture is accomplished by
combining a transverse field oscillating at radio frequency (RF) with the static magnetic
microtrap [14, 15, 16, 17, 18].
When the RF frequency is resonant with the magnetic hyperfine splitting of one
atomic species, the RF radiation induces a coupling between the mF levels in thatspecies, just as in forced RF evaporative cooling. Unlike in evaporative cooling, however,
we ramp the RF frequency from below to above resonance. Doing so dynamicallytransforms the potential from the |F = 2, mF = 2i single-well (νRF below resonance) intoa dressed state double-well (νRF above resonance)5 . The central barrier height and well
separation of the double-well are controlled by the RF frequency and amplitude [16].
We achieve species-selective dressed potentials when the RF radiation is resonant
to the ground state hyperfine splitting of 87 Rb, but well detuned from that of 40 K. In
a magnetic field the energy separation between the five F = 2 hyperfine levels in the
52 S1/2 87 Rb ground state is roughly twice the separation of that between the ten F = 9/2
hyperfine levels in the 42 S1/2 40 K ground state6 . The 87 Rb double well is formed byincreasing the RF frequency from 3.6 MHz to a final RF frequency between 4.0 MHz
and 6.0 MHz (see Figure 4). The single-well 87 Rb static trap deforms into a double-wellas the RF frequency passes through the 87 Rb trap bottom, located at 3.8 MHz, whilethe 40 K potential remains almost unaffected by the RF dressing: we estimate the Rabi
frequency ΩR to be as high as 1 MHz, whereas the static magnetic bias field provides aminimum detuning of 2.1 MHz between 40 K and 87 Rb resonances.
Figure 4 shows absorption images of the 40 K-87 Rb mixture 0.3 ms after being released
from the RF-dressed trap. As expected, the 87 Rb atoms split primarily into two clouds
as the double-well RF frequency is increased. The 40 K atoms, still experiencing thebare static trap, remain confined to a single, central cloud. This species separation
5 In evaporative cooling, the RF frequency is ramped down starting from above resonance, so that atoms

experience a single-well dressed state potential at all times during evaporation.
6U
magnetic = mF gF µB B, where µB is the Bohr magneton and B the magnitude of magnetic field. The
Landé g-factor gF is 2/9 for F=9/2 in 40 K and 1/2 for F = 2 in 87 Rb.
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FIGURE 4. Species-selective manipulation. A static chip trap loaded with a 40 K-87 Rb mixture iscoupled to an RF field to induce a double-well dressed potential for 87 Rb only; the 40 K portion of the
mixture remains unsplit. The final RF coupling frequency increases from 4.0 MHz (a and e) to 6.0 MHz(f and i) in steps of 0.5 MHz, deforming the 87 Rb dressed potential minimum. (a-e) Absorption images ofthe mixture with probe light resonant to 87 Rb only. (f-i) Absorption images of the mixture taken with twocolour probe light resonant to both species. Both species are visible, with the 40 K atoms nearly unaffectedby the RF dressing.

was observed via absorption imaging with a two-colour probe beam, the two opticalfrequencies corresponding to the imaging transitions in 87 Rb and 40 K.

NEXT GENERATION ATOM CHIP: ON-CHIP OPTICALLATTICES
With an eye to future experiments, we are in the midst of developing a new atom chip
which can support an optical lattice. The atom chip will be covered by a dielectric
mirror, enabling the formation of a 1D optical lattice near the chip surface with a single,
retroreflected laser beam. Improved chip optical access will allow the incorporation of
additional beams to form 2D and 3D lattices.
Mirrorizing is carried out following Reichel’s replica technique [19]. Once the chip
wires have been fabricated on the substrate, a planarizing epoxy is applied to the atomchip. A dielectric mirror on a glass substrate is then carefully lowered onto the epoxy
layer, pressed, and held in place while the epoxy cures. Once the epoxy has set andcured, the mirror substrate is carefully removed, leaving the smooth dielectric mirror
adhered to the atom chip.
The new atom chip will also contain smoother wires with higher current capacity,
fabricated with photolithography, evaporative deposition, and lift-off. The chip substrate
is an AlN wafer with filled vias connecting the front (wire) side to the backside of thechip. This allows for electrical contacts to be made on the back side of the chip, removing
them as obstacles and thus greatly increasing our optical access. After evaporative
deposition of Cr and Ti adhesion layers, a 2.7 µ m-thick layer of Ag and a thin capping
layer of Au are evaporated directly onto the substrate to form the chip wires. Evaporative
deposition was chosen over electroplating to reduce the wire roughness, which in turnwill reduce the rugosity of the resulting magnetic potential [20, 21, 22]. High DC current
capacity in these wires is enabled by the combination of the high electrical conductivityof Ag and the high thermal conductivity of AlN. Recent tests have shown that as muchas 7.5 A of current can be passed through a 20 µ m-wide, 2.7 µ m-tall, 4.6 mm-long wire;
this corresponds to a current density of greater than 107 A/cm2 .
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ACKNOWLEDGMENTS
We would like to thank M. Häffner, I. Leroux and D. McKay for their early contributions
to this work. We would also like to thank the Groupe d’Optique Atomique in Orsay fortechnical collaborations. This work is supported by the NSERC, CFI, the Province ofOntario, CRC, and Research Corporation. S. A. and L. J. L. acknowledge support fromNSERC. M. H. T. E. acknowledges support from OGS.

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