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ACTA PHOTONICA SINICA

Atomic Coherence Effect on the Formation of Molecules via Photoassociation

王璞玉 WANG Pu-yu, 杨国建 YANG Guo-jian

Published Jan 1, 2010• 0 citations
Publisher

Abstract

The influence of atomic coherence on the formation of a ultracold molecule from ultracold two-state atoms is discussed via photoassociation.By using the method of quantum statistics,it is found that under the action of atomic coherence,the molecular number vibrates with an approximate period and with reduced amplitude.Atomic coherence has a strong influence on the temporary of the whole photoassociation process,but the molecules are always subject to sub-Poisson statistical distribution.

DOI
10.3788/gzxb20103903.0543

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The influence of atomic coherence on the formation of a ultracold molecule from ultracold two-state atoms is discussed via photoassociation.By using the method of quantum statistics,it is found that under the action of atomic coherence,the molecular number vibrates with an approximate period and with reduced amplitude.Atomic coherence has a strong influence on the temporary of the whole photoassociation process,but the molecules are always subject to sub-Poisson statistical distribution.
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We show that coherence between two excited rovibrational states belonging to the same molecular electronic configuration arises quite naturally due to their interaction with electromagnetic vacuum. For initial preparation of a molecule in the desired rovibrational states, we propose to employ the method of ultracold photoassociation. Spontaneous decay of the excited molecule then gives rise to vacuum-induced coherence between the excited ro-vibrational states. We demonstrate theoretically an interesting interplay of effects due to vacuum-induced coherence and photoassociation. We apply our theory to photoassociation of bosonic ytterbium (174{}^{174}174Yb) atoms, which appear to be a promising system for exploring such interplay. The effects discussed here can be important for controlling decoherence and dissipation in molecular systems.
We calculate the momentum distribution and the second-order correlation function in momentum space, ${g}^{(2)}(\mathbf{p},{\mathbf{p}}^{\ensuremath{'}},t)$ for molecular dimers that are coherently formed from an ultracold atomic gas by photoassociation or a Feshbach resonance. We investigate using perturbation theory how the quantum statistics of the molecules depend on the initial state of the atoms by considering three different initial states: a Bose-Einstein condensate (BEC), a normal Fermi gas of ultracold atoms, and a BCS-type superfluid Fermi gas. The cases of strong and weak coupling to the molecular field are discussed. It is found that BEC and BCS states give rise to an essentially coherent molecular field with a momentum distribution determined by the zero-point motion in the confining potential. On the other hand, a normal Fermi gas and the unpaired atoms in the BCS state give rise to a molecular field with a broad momentum distribution and thermal number statistics. It is shown that the first-order correlations of the molecules can be used to measure second-order correlations of the initial atomic state.
We propose a method for copiously producing ultracold ground electronic-state molecules by two-photon absorption to a Rydberg molecular level (that spontaneously radiates to the ground molecular state) during collisions of ultracold atoms in a laser trap. Using the Na2{\mathrm{Na}}_{2}Na2​ molecule as an example, the sequential two-photon absorption is via an intermediate high-lying vibrational level of the excited 1g{1}_{\mathit{g}}1g​ state and results in the formation of a given vibrational level of a $^{1}\mathrm{\ensuremath{\Pi}}_{\mathit{u}}$ Rydberg molecular state. Realistic calculations demonstrating the validity of the method are presented.
We study coherent photoassociation, phenomena analogous to coherent optical transients in few-level systems, which may take place in photoassociation of an atomic Bose-Einstein condensate but not in a nondegenerate gas. We develop a second-quantized Hamiltonian to describe photoassociation, and apply the Hamiltonian both in the momentum representation and in the position representation (field theory). The solution of the two-mode problem, including only one mode each for the atomic and molecular condensates, displays analogs of Rabi oscillations and rapid adiabatic passage. A classical version of the field theory for atoms and molecules is used to demonstrate that, in the presence of photoassociating light, a joint atom-molecule condensate is unstable against the growth of density fluctuations. Experimental complications, including spontaneous emission and unwanted ``rogue'' photodissociation from a photoassociated molecule, are analyzed. A two-color Raman scheme is studied as a method to set up an effective two-mode scheme with reduced spontaneous-emission losses. We discuss photoassociation rates and photoassociation Rabi frequencies for high-lying vibrational states in alkali dimers both on the basis of molecular-structure calculations, and by comparing with an experiment [Wynar et al., Science 287, 1016 (2000)].