PRL 105, 203001 (2010)

PHYSICAL REVIEW LETTERS

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Coherent Transfer of Photoassociated Molecules into the Rovibrational Ground State
K. Aikawa,1,* D. Akamatsu,2,† M. Hayashi,1,‡ K. Oasa,1 J. Kobayashi,2 P. Naidon,3 T. Kishimoto,4M. Ueda,3,5 and S. Inouye2,31
Department of Applied Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Institute of Engineering Innovation, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan
3
JST, ERATO, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan
4
Center for Frontier Science and Engineering, University of Electro-Communications, Chofu, Tokyo 182-8585, Japan
5
Department of Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan(Received 16 July 2010; published 10 November 2010)2

We report on the direct conversion of laser-cooled 41 K and 87 Rb atoms into ultracold 41 K87 Rb
molecules in the rovibrational ground state via photoassociation followed by stimulated Raman adiabatic
passage. High-resolution spectroscopy based on the coherent transfer revealed the hyperfine structure of
weakly bound molecules in an unexplored region. Our results show that a rovibrationally pure sample ofultracold ground-state molecules is achieved via the all-optical association of laser-cooled atoms, opening
possibilities to coherently manipulate a wide variety of molecules.
DOI: 10.1103/PhysRevLett.105.203001

PACS numbers: 37.10.Mn, 33.15.Pw, 42.50.Ct, 82.80.Ms

Progress in cooling and manipulating atoms has led todiverse applications. Even more possibilities will open up
if molecules are cooled and prepared in the rovibrational
ground state because then the translational, vibrational,
and rotational degrees of freedom of the molecules can be
manipulated with greater accuracy. The potential applications range from novel quantum phases to ultracold chemistry, quantum computation, and precision measurements [1].
Various schemes to achieve cold molecules have been demonstrated thus far. Direct cooling of molecules has been
limited in terms of temperature [2]. Recently, quantum
gases of molecules in the rovibrational ground state
have been achieved [3–5] with a coherent optical transfer
of weakly bound molecules produced via the magnetoassociation [6] of quantum degenerate atoms. However, thisscheme is only applicable to atomic species for which magnetoassociation is available. An alternative general method
for producing weakly bound molecules from ultracold atoms
is photoassociation (PA) [7]. Up to now, only the incoherent
formation of molecules in the vibrational ground state via
photoassociation has been reported [8–10]. Although thedirect photoassociation yielded molecules in the v00 ¼ 0,
J 00 ¼ 2 level predominantly [10], the state-selective photoassociative formation of cold molecules in the rovibrational
ground state is yet to be achieved.
Here, we report that a rotationally and vibrationally puresample of ultracold 41 K87 Rb molecules in the rovibrational
ground state can be directly produced in a magneto-opticaltrap of 41 K and 87 Rb via photoassociation followed bystimulated Raman adiabatic passage (STIRAP) [11]. In
spite of the low laser intensity due to the large volume ofour molecular gas, we have reached a STIRAP transfer
efficiency of more than 70% by employing an intermediate
state with a narrow natural linewidth. In order to identify
the two-photon resonance precisely, we developed a new
0031-9007=10=105(20)=203001(4)detection scheme, spontaneous-decay-induced double
resonance (SpIDR), which provided the high-resolution
spectra of one-photon transitions from PA molecules andallowed us to observe a two-photon dark resonance [12].
Our results show that a rovibrationally pure sample
of ultracold molecules in the ground state is achieved
via the all-optical association of laser-cooled atoms, opening novel possibilities to coherently manipulate a wide
variety of cold molecules that can be produced via
photoassociation.
Figure 1 shows the schematic representation of the coherent transfer of ultracold 41 K87 Rb molecules. The potential
curves are taken from Ref. [13]. Details of the experimental
setup for the production and detection of weakly boundmolecules are found elsewhere [14]. For the present study,
we used 41 K87 Rb molecules in the v00 ¼ 91, J 00 ¼ 0 level of
X1 þ produced via photoassociation of 41 K and 87 Rb atoms
in a magneto-optical trap. The typical numbers, densities,
and temperatures of atoms were 1  108 , 2  1011 cm3 ,and 400 K for 41 K and 2  108 , 4  1011 cm3 , and100 K for 87 Rb, respectively. The PA laser resonant to
the J 0 ¼ 1 level of  ¼ 0þ (wave number 12570 cm1 ,intensity 1  103 W cm2 ) was applied for 10 ms.
Molecules were ionized through resonance enhanced multiphoton ionization by a pulsed dye laser and detected by
microchannel plates. The entire experimental procedure
was operated at 9 Hz.
Despite the successful demonstrations on quantum
gases, the STIRAP transfer into the rovibrational groundstate was not straightforward for PA molecules. Any residual magnetic field could cause decoherence for Raman
coupling between two molecular states since our molecularsample was not spin polarized. In addition, untrapped,
high-temperature PA molecules occupied a volume larger( 1 mm3 ) than that occupied by the trapped quantum

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Ó 2010 The American Physical Society

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FIG. 1 (color online). Relevant molecular potential energy
curves of KRb. Weakly bound molecules were produced by
the photoassociation of laser-cooled 41 K and 87 Rb atoms.
Molecules in the v00 ¼ 91, J00 ¼ 0, F00 ¼ 2 level of X 1 þ
were coupled to the v00 ¼ 0, J00 ¼ 0 level of X 1 þ by a twophoton transition with wavelengths near 875ðL1Þ and641ðL2Þ nm via the v0 ¼ 41, J 0 ¼ 1 level of ð3Þ1 þ . For depletion spectroscopy and SpIDR spectroscopy, two-photon ionization through the ð4Þ1 þ state was used. For the detection of therovibrational ground-state molecules, a three-photon transition
mediated by the v0 ¼ 6 level of ð1Þ1  was used.

gases ( 104 mm3 ). This limited the available laser
intensity for driving the transition.
These difficulties were circumvented by starting fromsinglet molecules which were insensitive to magnetic
fields. Utilizing singlet excited states with a narrow naturallinewidth as an intermediate state, we realized an efficient
Raman transfer of PA molecules into a singlet ground state.
This was in contrast with previous works on the twophoton transfer of heteronuclear molecules [3,8], whichstarted from triplet molecules and transferred the PA molecules into the singlet ground state via the  ¼ 1 state (thespin-mixed state of ð2Þ3 þ , ð1Þ3  and ð1Þ1 ). A transition
strength comparable to the  ¼ 1 state was obtained withthe v0 ¼ 41 level of ð3Þ1 þ [14,15], whereas the narrow
natural linewidth of this state (  2  300 kHz, calculated from Ref. [16]) allowed us to obtain an efficienttransfer with a relatively low laser intensity.
Our optical system for the Raman transition consistedof four lasers. Two diode lasers (875 nm and 641 nm)
were locked to an ultralow expansion cavity with a dualwavelength coating. These lasers worked as master lasers.
Two slave lasers, a ring dye laser (641 nm) and a diodelaser (875 nm), were then locked to master lasers throughan optical phase-locked loop (OPLL) [17] and used for the

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Time (µs)

5060FIG. 2. STIRAP transfer from the v00 ¼ 91, J 00 ¼ 0, F00 ¼ 2
level to the v00 ¼ 0 level of X1 þ . (a) Transfer to the J00 ¼ 0
level. The population in the v00 ¼ 0 level of X 1 þ is plottedagainst the frequency of the down transition laser (L2).
(b) Transfer to the J 00 ¼ 2 level. The asymmetry observed in
the J 00 ¼ 2 spectrum is expected to be a result of the hyperfine
structure induced by the nuclear electric quadrupole moments of
87 Rb. (c) Time evolution of the population in the v00 ¼ 0, J 00 ¼ 0
level of X1 þ during the multiple STIRAP transfer process. The
error bars indicate the standard deviation. (d) Time variation of
the intensities of the Raman lasers during the multiple-transfer
process. Both intensities are normalized to unity. During thewaiting time of 20 s set between the transfers, the remaining
population that is not transferred by STIRAP is removed by theresonant Raman lasers.

experiments. The scanning of the Raman lasers was realized by sweeping the microwave source for OPLL. In thisreport, the microwave frequency for OPLL was noted as anoffset frequency. The typical short-term linewidth of theRaman lasers was less than 10 Hz, whereas the long-term
stability of the ultralow expansion cavity was estimated tobe less than 100 kHz by monitoring the Cs D2 transition.
The 1=e2 diameter of the Raman lasers was set to 1.5 mm
which was larger than the size of atomic clouds. For the
STIRAP transfer, the power of the laser L1 and the laser L2
was set to 9 mW and 25 mW, respectively.
Figure 2(a) shows the population in the v00 ¼ 0 level of1 þX  after the STIRAP transfer into the J 00 ¼ 0 level as afunction of the frequency of the down transition laser (L2).
The observed two-photon linewidth of 200 kHz was consistent with a simple three-level theoretical model [11]when we took into account the Rabi frequency (2 
400 kHz) and the transfer duration (10 s). In order toconfirm that Fig. 2(a) indicated the rovibrational groundstate, we also performed a STIRAP transfer into the v00 ¼ 0,
J 00 ¼ 2 level [Fig. 2(b)] and determined the rotational
constant to be 1095:4ð1Þ MHz  h (h is the Planck’s

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constant). The uncertainty was due to the unresolved hyperfine structure of the J 00 ¼ 2 level. The obtained value
was in good agreement with the calculated value of1095:362ð5Þ MHz  h obtained from the mass scalingof the experimental value for 40 K87 Rb [18]. The typicalnumber, density, and temperature of the produced groundstate molecules were 103 , 106 cm3 , and 130 K, respectively. The production rate in the rovibrational ground state
was as high as 104 s1 .
The direct evidence of the STIRAP transfer was obtained by studying the time evolution of the population in
the ground state [Fig. 2(c)] during the multiple-transfer
process [Fig. 2(d)]. The ground-state molecules after asingle transfer were used for measuring the efficiency oftransfer. We typically retrieved 53% of the ground-state
molecules after the double-transfer process, which implieda single-step efficiency of 73%. Currently, the efficiency islimited by the difference in the Doppler shift for the twolasers.
The STIRAP transfer into the rovibrational ground state
can be used as new precision spectroscopy for weakly
bound molecules in an unexplored region (Fig. 3). By
mapping the population in the initial hyperfine levels onto
the ground state, we could measure the hyperfine splitting of
PA molecules with an accuracy of 10 kHz  h. A higher
resolution could be achieved by using a longer transfer
duration and smaller intensities for the Raman lasers. The
observed energy levels of the F00 ¼ 3, 2, 1 levels withrespect to the F00 ¼ 0 level were 55.960(9), 27.089(7), and8:852ð7Þ MHz  h, respectively. The values of these energy levels as obtained by a coupled channel calculation
were 56.180, 27.176, and 8:870 MHz  h, which were in
reasonable agreement with our observations.
The rest of this report is devoted to SpIDR spectroscopy,
which is an essential improvement of the one-photon
spectroscopy of PA molecules. This improvement wasnecessary for identifying the structure of the excited statesand observing a two-photon dark resonance. To date,

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Offset frequency of L1 (MHz)FIG. 3. STIRAP spectroscopy of weakly bound molecules (thev00 ¼ 91, J00 ¼ 0 level of X 1 þ ). An efficient transfer into therovibrational ground state enables the precision spectroscopy ofthe hyperfine structure of photoassociated molecules in an unexplored region with a typical accuracy of 10 kHz. The labeling isbased on the total angular momentum including the nuclear spin.

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depletion spectroscopy has been widely used for analyzing
the transition from PA molecules [15]. A continuous-wave(cw) laser depletes the number of PA molecules, while theloss is monitored with resonance enhanced multiphoton
ionization. Although this spectroscopy was straightforward, we encountered a serious problem related to thesignal-to-noise ratio when we attempted to record data at
a low intensity in order to avoid saturation broadening. At arelatively high intensity, where we could observe a large
signal, the spectral width broadened to tens of megahertz,
washing out all the structure from the hyperfine levels. In
addition, saturation broadening smoothed out the transparency peak of the two-photon dark resonance and made itdifficult to identify the two-photon resonance.
In order to improve the signal-to-noise ratio of depletion
spectroscopy, we developed a method to monitor not the
depletion but the creation of molecules in the excited state.
In practice, we monitored the population in one of thevibrational levels in the ground state to which a significant
fraction of excited molecules decays. Thus, we could
obtain a background-free spectrum, thereby increasing
the signal-to-noise ratio to observe the narrow linewidthof the ð3Þ1 þ state.
Figure 4(a) shows the SpIDR spectrum of the v0 ¼ 41,
0
J ¼ 1 level of ð3Þ1 þ . A considerably smaller power(5 W) and shorter time (400 s) than those requiredfor depletion spectroscopy (> 100 W, 10 ms) were sufficient to detect the excitation. This led to a significantly
narrow linewidth close to the natural linewidth. The coarse
splitting resulted from the hyperfine structure in the groundstate, whereas the fine splitting implied the existence of thehyperfine structure in the excited state. For the sake ofcomparison, the depletion spectrum in the same frequency
range is shown in Fig. 4(b).
The SpIDR spectroscopy enabled us to observe a twophoton dark resonance [Fig. 4(c)]. The dip indicatedthe emergence of the two-photon dark state with the laser
L2 [12]. By comparing the two frequencies at the dip, we
determined the binding energy of the v00 ¼ 0, J 00 ¼ 0 levelof X1 þ with respect to the v00 ¼ 91, J 00 ¼ 0 level of
X1 þ to be 124 955:92ð4Þ GHz  h, where the accuracy
was limited by the uncertainty of our wave meter. Taking
into account the binding energy of the v00 ¼ 91, J 00 ¼ 0
level with respect to the threshold FK ¼ 1 þ FRb ¼ 1,
374:75ð3Þ GHz  h, the binding energy of the v00 ¼ 0,
J 00 ¼ 0 level with respect to the atomic level without the hyperfine structure was determined to be125 335:11ð5Þ GHz  h. From the width of the transparency peak, we determined the transition dipole moment
for the down transition to be 0.0098(7) ea0 , assuming thenatural linewidth of the excited state to be 2  300 kHz.
When we locked the laser L2 to the dip frequency andscanned the laser L1, we obtained a well-known darkresonance spectrum [Fig. 4(d)].
In conclusion, we realized an efficient STIRAP transfer
of PA molecules into the rovibrational ground state. Highresolution spectroscopy of the hyperfine structure of PA

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repetition rate makes our method a promising candidate
for high-precision spectroscopy. The precision measurement of the electron-to-proton mass ratio is within reachas microwave spectroscopy [20] or optical spectroscopy
combined with a frequency comb [21].
We thank M. Kozuma for helpful discussions, andM. Falkenau, Y. Tanooka, and K. Mori for experimental
assistance in the early stages of the experiment. K. A. and
D. A. acknowledge support from the Japan Society for the
Promotion of Science.

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Offset frequency of L1 (MHz)FIG. 4 (color). SpIDR spectroscopy of photoassociated molecules. (a) SpIDR spectrum of the v0 ¼ 41, J 0 ¼ 1 level ofð3Þ1 þ taken by exciting molecules in the v00 ¼ 91, J 00 ¼ 0
level of X 1 þ . Molecules formed by spontaneous decays fromthe excited state are counted against the offset frequency of thelaser L1. The coarse splitting results from the hyperfine structureof the v00 ¼ 91, J00 ¼ 0 level of X1 þ . The fine splitting in eachline implies the existence of hyperfine structure in the excitedstate. For the two-photon transition, the largest peak (F00 ¼ 2) is
employed as an initial state. (b) Depletion spectrum. In orderto observe any signal, we had to increase the laser intensity by
3 orders of magnitude as compared to (a). Transitions from thev00 ¼ 91, J 00 ¼ 0 level are barely observable since the ion counts
are dominated by molecules in the v00 ¼ 91, J 00 ¼ 2 level.
(c) Two-photon dark resonance between the v00 ¼ 91, J00 ¼ 0,
F00 ¼ 2 level and the v00 ¼ 0, J 00 ¼ 0 level of X1 þ . The downtransition laser (L2) is scanned while the up transition laser (L1)is held on resonance. The dots indicate experimentally obtained
data and the solid curve is a fit to the data points. (d) Two-photon
dark resonance obtained by scanning the laser L1. The laser L2
is held on resonance. The blue dashed curve indicates a SpIDR
spectrum for the F00 ¼ 2 level, while the red solid one indicatesa spectrum obtained with the laser L2.

molecules in an unexplored region was demonstrated using
coherent transfer. We developed a highly sensitive detection scheme which enabled us to take a high-resolution
spectrum of one-photon transitions and observe a twophoton dark resonance for PA molecules. Our method isreadily extended to other atomic species for which laser
cooling and photoassociation are available. Trapping
molecules into an optical dipole trap [19] is the next key
issue. The trapped sample of molecules will constitutea test tube for ultracold chemistry. Moreover, the high*ka_cypridina@atomtrap.t.u-tokyo.ac.jp†
Present address: National Metrology Institute of Japan,
Tsukuba 305-8563, Japan.
‡
Present address: Nikon Corporation, Kanagawa 252-0328,
Japan.
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