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Physical Review A

Molecular vortex generated from an atom-molecule dark state

Hong Yuan Ling, S. Yi, Han Pu, David E. Grochowski, Weiping Zhang

Published May 26, 2006• 4 citations
Publisher

Abstract

We consider a Raman photoassociation model in a field configuration in which a Gaussian and a first-order Laguerre Gaussian laser field are applied between the bound-bound and free-bound transitions, respectively. We show that such a configuration can lead to a coherent population trapping superposition of an atomic condensate and a ground molecular vortex of unit winding number. We develop stimulated Raman adiabatic passages that minimize the effect of mean-field shifts due to collisions, for optimal conversion of an atomic condensate into a ground molecular vortex.

DOI
10.1103/physreva.73.053612

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We consider a Raman photoassociation model in a field configuration in which a Gaussian and a first-order Laguerre Gaussian laser field are applied between the bound-bound and free-bound transitions, respectively. We show that such a configuration can lead to a coherent population trapping superposition of an atomic condensate and a ground molecular vortex of unit winding number. We develop stimulated Raman adiabatic passages that minimize the effect of mean-field shifts due to collisions, for optimal conversion of an atomic condensate into a ground molecular vortex.
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.
Summary form only given. Coherent conversion of an atomic to a molecular Bose-Einstein condensate (BEC) is a first step towards 'superchemistry', which is the atom optics analog of frequency conversion in nonlinear optics. A number of studies of this have shown that direct conversion via Raman photo-association appears feasible, based on stimulated free-bound and bound-bound transitions in the presence of two laser fields E/sup (1)/=E/sub 0//sup (1)/ cos(/spl omega//sup (1)/t) and E/sup (2)/=E/sub 0//sup (2)/ cos(/spl omega//sup (2)/t). Here pairs of atoms from the two-atom continuum are transferred-via an excited bound molecular state-to a bound molecular state of lower energy. Raman photo-association allows coupling to a single molecular state, which can be selected by the Raman laser frequencies /spl omega//sup (1)/ and /spl omega//sup (2)/. However, practical calculations using available lasers and transitions indicate that coherent transfer is often limited by spontaneous emission from the intermediate molecular excited electronic state. A possible route towards minimizing losses and decoherence from spontaneous emission is to use the stimulated Raman adiabatic passage (STIRAP) method (Drummond et al, Phys. Rev. Lett. vol. 81, p. 3055, 1998) in which a counter-intuitive pulse sequence is used where the first input pulse couples the molecular levels, even when there are no molecules present.
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 analyze coherent two-color photoassociation of a Bose-Einstein condensate, focusing on stimulated Raman adiabatic passage (STIRAP) in free-bound-bound transitions from atoms to molecules. This problem raises an interest because STIRAP has been predicted to be absent in the nondegenerate case [Javanainen and Mackie, Phys. Rev. A 58, R789 (1998)]. Nevertheless, we find that Bose stimulation enhances the free-bound dipole matrix element for an atomic condensate, and photoassociative STIRAP turns out to be a viable mechanism for converting an atomic condensate to a molecular condensate with near-unit efficiency.
We consider the adiabatic evolution of an atom-molecule dark state in a collisional two-color Raman photoassociation condensate system. By linking nonadiabaticity with the population growth in the collective excitations of the dark state, we develop an adiabatic theorem where we have consistently addressed the issues related to the Goldstone mode and the biorthonormality of the collective modes. We apply this theorem to specific examples to demonstrate its use in designing pulses that can optimize the yield of ground molecule production in the stimulated Raman adiabatic passage process.