Atom-Molecule Dark States in a Bose-Einstein Condensate

Abstract

We have created a dark quantum superposition state of a Rb Bose-Einstein condensate (BEC) and a degenerate gas of Rb2 ground state molecules in a specific ro-vibrational state using two-color photoassociation. As a signature for the decoupling of this coherent atom-molecule gas from the light field we observe a striking suppression of photoassociation loss. In our experiment the maximal molecule population in the dark state is limited to about 100 Rb2 molecules due to laser induced decay. The experimental findings can be well described by a simple three mode model. PACS numbers: 34.50.Rk, 32.80.Pj, 03.75.Nt, 42.50.Gy The phenomenon of coherent dark states is well known in quantum optics and is based on a superposition of long-lived system eigenstates which decouples from the light field. Since their discovery [1] dark states have found numerous applications. Prominent examples are electromagnetically induced transparency and lasing without inversion [2], sub-recoil laser cooling [3], and ultra-sensitive magnetometers [4]. A particular application is the coherent transfer of population between two long-lived states by a stimulated Raman adiabatic passage (STIRAP) [5].In the emerging field of ultracold molecules, the conversion of atomic into molecular BECs is a central issue. A series of recent experiments on the creation of molecular quantum gases rely on the application of Feshbach resonances [6]. This coupling mechanism, however, is restricted to the creation of molecules in the highest ro-vibrational level and is only practicable for a limited number of systems. As a more general method a stimulated optical Raman transition can directly produce deeply bound molecules as demonstrated a few years ago [7,8]. STIRAP was proposed as a promising way for a fast, efficient and robust process to convert a BEC of atoms into a molecular condensate [9,10,11,12,13,14]. The central prerequisite for this kind of STIRAP is a dark superposition state of a BEC of atoms and a BEC of molecules.In this Letter, we report the observation of such a collective multi-particle dark state in which atoms in a BEC are pairwise coupled coherently to ground state molecules. This dark atom-molecule BEC shows up in a striking suppression of photoassociative loss, as illustrated by the spectra in Fig. 1. In one-color photoassociation, the excitation of a molecular transition produces a resonant loss feature that reflects the optical transition linewidth, see Fig. 1(a). The presence of a second laser field coupling the electronically excited molecular state to a long-lived ground-state level can drastically reduce this loss, as shown in Fig. 1(b) and (c). In (b), for example, we observe a striking loss suppression by about a factor of 70 on resonance.Already the mere observation of an atom-molecule dark resonance in a BEC proves that a coherent, quantum degenerate gas of molecules has been formed. This follows from the facts that 1) the dark state is by definition a coherent superposition of atoms and molecules and 2) the atomic BEC is a coh...

Rabi Oscillations between Atomic and Molecular Condensates Driven with Coherent One-Color Photoassociation

“…It has been demonstrated with magnetic-field ramps near a magnetic Feshbach resonance (MFR) [2,3] and with different configurations of two-color photoassociation (PA) [4][5][6][7]. Coherent onecolor PA has typically been considered inaccessible due to the short lifetime of excited molecular states in alkalimetal atoms [8].…”

“…Two-color PA, in which two light fields couple atomic scattering states to a ground molecular state through an intermediate, excited molecular level, is often used to create molecules in quantum degenerate gases through stimulated Raman transitions [5,27], creation of coherent atom-molecule superpositions in a dark state [4], or stimulated Raman adiabatic passage (STIRAP) [6,28]. These two-color processes are typically coherent, as demonstrated, for example, by the suppression of PA loss [4].…”

“…[6,28]. These two-color processes are typically coherent, as demonstrated, for example, by the suppression of PA loss [4]. Rabi oscillations between atomic and ground molecular condensates driven by twocolor PA [12,[14][15][16] have never been observed, even in optical lattices [5], although two-photon Raman coupling was recently used to observe Rabi oscillations in a degenerate Fermi gas [29].…”

“…Two-color PA, in which two light fields couple atomic scattering states to a ground molecular state through an intermediate, excited molecular level, is often used to create molecules in quantum degenerate gases through stimulated Raman transitions [5,27], creation of coherent atom-molecule superpositions in a dark state [4], or stimulated Raman adiabatic passage (STIRAP) [6,28]. These two-color processes are typically coherent, as demonstrated, for example, by the suppression of PA loss [4]. Rabi oscillations between atomic and ground molecular condensates driven by twocolor PA [12,[14][15][16] have never been observed, even in optical lattices [5], although two-photon Raman coupling was recently used to observe Rabi oscillations in a degenerate Fermi gas [29].…”

Quantum Storage and Cloning of Light States in Eit-Like Medium

“…In conclusion we briefly discuss appropriate application of the schemes proposed in the paper for the problem of quantum cryptography with continuous variables. In Appendix we also discuss an implementation of the stimulated twophoton Raman photoassociation 13,14 effect under the BEC condition as a tool for experimental realization of the problem under consideration.…”