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arXiv:1002.0664 · New Journal of Physics

Efficient production of polar molecular Bose–Einstein condensates via an all-optical R-type atom–molecule adiabatic passage

Jing Qian, Lu Zhou, Keye Zhang, Weiping Zhang

Published Mar 5, 2010• 5 citations
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Abstract

We propose a scheme of 'R-type' photoassociative adiabatic passage (PAP) to create polar molecular condensates from two different species of ultracold atoms. Due to the presence of a quasi-coherent population trapping state in the scheme, it is possible to associate atoms into molecules with a low-power photoassociation (PA) laser. One remarkable advantage of our scheme is that a tunable atom–molecule coupling strength can be achieved by using a time-dependent PA field, which exhibits larger flexibility than using a tunable magnetic field. In addition, our results show that the PA intensity required in the 'R-type' PAP could be greatly reduced compared to that in a conventional 'Λ-type' one.

DOI
10.1088/1367-2630/12/3/033002

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We propose a scheme of 'R-type' photoassociative adiabatic passage (PAP) to create polar molecular condensates from two different species of ultracold atoms. Due to the presence of a quasi-coherent population trapping state in the scheme, it is possible to associate atoms into molecules with a low-power photoassociation (PA) laser. One remarkable advantage of our scheme is that a tunable atom–molecule coupling strength can be achieved by using a time-dependent PA field, which exhibits larger flexibility than using a tunable magnetic field. In addition, our results show that the PA intensity required in the 'R-type' PAP could be greatly reduced compared to that in a conventional 'Λ-type' one.
We generalize the idea of chainwise stimulated Raman adiabatic passage (STIRAP) [Kuznetsova et al., Phys. Rev. A 78, 021402(R) (2008)] to a photoassociation-based chainwise atom-molecule system, with the goal of directly converting two-species atomic Bose-Einstein condensates (BEC) into a ground polar molecular BEC. We pay particular attention to the intermediate Raman laser fields, a control knob inaccessible to the usual three-level model. We find that an appropriate exploration of both the intermediate laser fields and the stability property of the atom-molecule STIRAP can greatly reduce the power demand on the photoassociation laser, a key concern for STIRAPs starting from free atoms due to the small Franck-Condon factor in the free-bound transition.
We propose an efficient scheme to avoid collision-induced dynamical instabilities in the generation of heteronuclear molecular Bose–Einstein condensates based on an R-type photoassociative adiabatic passage. This scheme allows us to completely compensate the intraspecies collisions by tunable interspecies collisions, and thereby achieve a robust atom–molecule conversion which is equivalent to that in a collision-free case. By taking the formation of stable 41K87Rb molecules as an example, we study the feasibility of the scheme.
We theoretically explore photoassociation by adiabatic passage of two colliding cold 85Rb^{85}\mathrm{Rb}85Rb atoms in an atomic trap to form an ultracold Rb2{\mathrm{Rb}}_{2}Rb2​ molecule. We consider the incoherent thermal nature of the scattering process in a trap and show that coherent manipulations of the atomic ensemble, such as adiabatic passage, are feasible if performed within the coherence time window dictated by the temperature, which is relatively long for cold atoms. We show that a sequence of $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{7}$ pulses of moderate intensities, each lasting $\ensuremath{\sim}750\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$, can photoassociate a large fraction of the atomic ensemble at temperature of $100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ and density of 1011atoms∕cm3{10}^{11}\phantom{\rule{0.3em}{0ex}}\text{atoms}∕{\mathrm{cm}}^{3}1011atoms∕cm3. Use of multiple pulse sequences makes it possible to populate the ground vibrational state. Employing spontaneous decay from a selected excited state, one can accumulate the molecules in a narrow distribution of vibrational states in the ground electronic potential. Alternatively, by removing the created molecules from the beam path between pulse sets, one can create a low-density ensemble of molecules in their ground rovibrational state.
We theoretically examine Raman photoassociation of a Bose-Einstein condensate, revisiting stimulated Raman adiabatic passage (STIRAP). Due to collisional mean-field shifts, efficient molecular conversion requires strong coupling and low density, either of which can bring about rogue photodissociation to noncondensate modes. We demonstrate explicitly that rogue transitions are negligible for low excited-state fractions and photodissociation that is slower than the STIRAP time scale. Moreover, we derive a reduced-parameter model of collisions, and thereby find that a gain in the molecular conversion efficiency can be obtained by adjusting the atom-atom scattering length with off-resonant magnetoassociation. This gain saturates when the atom-atom scattering length is tuned to a specific fraction of either the molecule-molecule or atom-molecule scattering length. We conclude that a fully optimized STIRAP scheme may offer the best chance for achieving coherent conversion from an atomic to a molecular condensate with photoassociation.
Quantum degenerate molecules represent a new paradigm for fundamental studies and practical applications. Association of already quantum degenerate atoms into molecules provides a crucial shortcut around the difficulty of cooling molecules to ultracold temperatures. Whereas association can be induced with either laser or magnetic fields, photoassociation requires impractical laser intensity to overcome poor overlap between the atom pair and molecular wave functions, and experiments are currently restricted to magnetoassociation. Here we model realistic production of a quantum degenerate gas of stable molecules via two-photon photoassociation of Bose-condensed atoms. An adiabatic change of the laser frequency converts the initial atomic condensate almost entirely into stable molecular condensate, even for low-intensity lasers. Results for dipolar LiNa provide an upper bound on the necessary photoassociation laser intensity for alkali-metal atoms  ~ 303030 W/cm2{}^{2}2, indicating a feasible path to quantum degenerate molecules beyond magnetoassociation.
We perform a detailed study of pulse sequences in a photoassociation via adiabatic passage (PAP) process to transfer population from an ensemble of ultracold atomic clouds to a vibrationally cold molecular state. We show that an appreciable final population of ultracold NaCs molecules can be achieved with optimized pulses in either the 'counter-intuitive' ( t P > t S ) or 'intuitive' ( t P < t S ) PAP pulse sequences, with t P and t S denoting the temporal centers of the pump and Stokes pulses, respectively. By investigating the dependence of the reactive yield on pulse sequences, in a wide range of t P – t S , we show that there is not a fundamental preference to either pulse sequence in a PAP process. We explain this no-sequence-preference phenomenon by analyzing a multi-bound model so that an analogy can be drawn to the conventional stimulated Raman adiabatic passage.