Creating a Stable Molecular Condensate Using a Generalized Raman Adiabatic Passage Scheme

Abstract

We study the Feshbach resonance assisted stimulated adiabatic passage of an effective coupling field for creating stable molecules from atomic Bose condensate. By exploring the properties of the coherent population trapping state, we show that, contrary to the previous belief, mean-field shifts need not to limit the conversion efficiency as long as one chooses an adiabatic passage route that compensates the collision mean-field phase shifts and avoids the dynamical unstable regime.PACS numbers: 03.75. Mn, 05.30.Jp, 32.80.Qk Molecules offer a whole new dimension in the study of ultracold atomic physics. In particular, resonant photo-or magneto-association (Feshbach resonance [1]) of cold atoms to molecules represents a matter-wave analog of second harmonic generation and has become a new paradigm of coupled macroscopic quantum systems. Due to energy conservation, such processes generally produce molecules in a vibrationally and/or electronically excited quasibound level and hence are not energetically stable. This does not seem to be a serious problem in the case of fermionic atoms due to the suppression of molecular decay by Pauli blocking. In fact, several groups have now successfully Bose condensed so-formed molecules [2]. In contrast, although evidence of macroscopic coherence has been observed in several experiments, excited molecules formed by a pair of bosonic atoms have very short lifetime ( a few ms) and the atom-molecule conversion efficiency is limited to 10% [3,4]. Hence it is very important to be able to create deeply-bound ground state molecules from atomic Bose condensates with high efficiency, which is the focus of the current work.First proposed for photoassociating nondegenerate atoms into stable molecules [7], stimulated Raman adiabatic passage (STIRAP) aided by Feshbach resonance is considered a more efficient way of converting atomic condensates into molecular ones [5,6] than the bare STI-RAP implementation of photoassociation [7,8]. In this scheme, the free atomic, the quasibound and the ground molecular states form the three-level Λ-system to which STIRAP can apply [10]. The success of STIRAP relies on the existence of the coherent population trapping (CPT) state [9]. In a linear Λ-system, the CPT state exists when the two-photon resonance condition is satisfied, hence STIRAP can be straightforwardly implemented by appropriately choosing the laser frequencies. In the case of condensate, collisions between particles give rise to nonlinear mean-field shifts which dynamically changes when population is transferred from atomic state to molecular state. This poses a serious problem for STIRAP as collisions shift the system out of the two-photon resonance. For typical experimental parameters, a conversion efficiency of only ∼ 20% is predicted with a somewhat complicated laser sequence containing seven Raman pulses [5]. A possible remedy is to use a low-density atomic condensate where the effect of collisions can be minimized [11]. This, however, creates new problems as stronger coupling fields...

Quantum Noise in the Collective Abstraction ReactionA+B2→AB+B

“…The terms proportional to χ ′ i,j describe s-wave collisions between these species. We remark that trimer formation via an atom-dimer resonance is actively studied in ongoing experiments, and that the Feshbachresonance-aided photoassociation considered here could also be reformulated as a laser frequency modulation scheme [9,15]. Finally, the Bose-enhanced selectivity of dissociation channels in coherent photodissociation of the heteronuclear trimers ABC was also studied by Moore and Vardi [7].…”

“…The maintenance of dark or unpopulated trimer state is important for an efficient conversion even with a very short molecular lifetime [15]. For ψ b (0) = ψ ab (0) = 0, Eqs.…”

Coherent population trapping and dynamical instability in coupled atom-molecule condensates

“…Evidently, the success of these applications depends on our ability to avoid the unstable regimes of the CPT state. For example, high efficiency coherent population transfer between the atomic and molecular condensate using STIRAP can only be achieved if these unstable regimes are not encountered during the transfer process [23]. With this in mind, we believe that our work will have important implications in the coupled atom-molecule condensates [7], a system of significant current interest, with ramifications in other nonlinear systems.…”

“…It was generally believed that the nonlinear phase shifts arising from particle collisions would defeat the two-photon resonance condition [13,15], a prerequisite for the existence of CPT states and hence the key for the successful implementation of STIRAP. In a recent work [23], we have shown that by appropriately chirping the optical frequency and the Feshbach detuning, the collision-induced nonlinear phase shifts can be dynamically compensated, hence generalizing the concept of the CPT state from the collisionless limit [13] to situations where collisions cannot be ignored. In addition, we have also shown that collisions may lead to dynamical instabilities in the nonlinear CPT state, in contrast to the CPT states in the linear Λ-system which are always stable.…”

Bose-Einstein condensation of trapped atoms with dipole interactions

“…However, understanding new kinds of systems will require the investigation of the effects of additional interactions, since Bose-Einstein condensation of molecules has now been observed using new techniques for creating ultracold molecules [5]. In addition, Ling et al [6] proposed an efficient method to convert atomic condensates into stable molecules using a generalized Raman adiabatic passage scheme. Very recently, the generation of a BEC in a gas of 52 Cr atoms was reported [7].…”