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Physical Review A

Coherent two-color photoassociation in a spinor-1 Bose-Einstein condensate: Single-spin versus mixed-spin cases

Hui Jing, Jianlong Fu, Z. Geng, W.M. Liu

Published Apr 1, 2009• 6 citations
Publisher

Abstract

We investigate the coherent two-color photoassociation (PA) process in a spinor-1 atomic Bose-Einstein condensate by comparing the single-spin and mixed-spin cases. We find that, by tuning the PA light applied on different spin components, one can observe quite different features for the atomic spin-mixing dynamics (with weak PA field) or coherent atom-molecule conversion (with strong PA field), indicating a research field of so-called superchemistry or coherent PA in a spinor gas.

DOI
10.1103/physreva.79.045601

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We investigate the coherent two-color photoassociation (PA) process in a spinor-1 atomic Bose-Einstein condensate by comparing the single-spin and mixed-spin cases. We find that, by tuning the PA light applied on different spin components, one can observe quite different features for the atomic spin-mixing dynamics (with weak PA field) or coherent atom-molecule conversion (with strong PA field), indicating a research field of so-called superchemistry or coherent PA in a spinor gas.
We study the coherent spin-mixing dynamics in a spin-1 atomic condensate coupled with a molecular condensate. A model is proposed to include the photoassociation (PA) interaction in a spinor condensate. If only the ${m}_{F}=\ensuremath{-}1$ spin component can be converted into molecules, the combined dynamics of spin-mixing and PA can be described by nonlinearly coupled pendulums. We performed detailed numerical calculations on the antiferromagnetic spin-1 systems. We find the spin-mixing dynamics can be greatly controlled by the PA field. The PA field can enhance or suppress the period and amplitude of the spin oscillations, which provide more degrees of freedom to control the spin-mixing dynamics.
We consider the process of photoassociation (PA) that occurs when a pair of colliding atoms interacts with a photon, thereby making a transition from the two-atom continuum to a bound molecular state. Such free-bound transitions are described by quantizing the molecule's dissociation continuum, and coupling each of these quasicontinuum (QC) levels to a similarly discrete molecular level. The correct free-atom results are recovered no later than at the end of a given calculation by taking the limit as the QC spacing tends to zero. From the quasicontinuum model, we are led to adapt the few-level and second-quantization methods of photon optics to analyze free-bound transitions. In particular, we establish the PA density parameter as nearly identical to the phase-space parameter as a Bose-Einstein condensate (BEC), illustrate the near-unit efficiency of one- and two-color photoassociation of a BEC, and develop coherent atom-molecule transitions as the matter optics version of second-harmonic generated photons. ^
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)].
Bose-Einstein condensation (BEC) is a phenomenon in which identical bosons occupy the same quantum state below a certain critical temperature. A hallmark of BEC is the coherence between particles every particle shares the same quantum wavefunction and phase. This coherence has been demonstrated for the external (motional) degrees of freedom of the atomic condensates by interfering two condensates. In this thesis, the coherence is shown to extend to the internal spin degrees of freedom of a spin-1 Bose gas evidenced by the observed coherent and reversible spin-changing collisions. The observed coherent dynamics are analogous to Josephson oscillations in weakly connected superconductors and represent a type of matter-wave four-wave mixing. Control of the coherent evolution of the system using magnetic fields is also demonstrated. The studies on spinor condensates begin by creating spinor condensates directly using all-optical approaches that were first developed in our laboratory. All-optical formation of Bose-Einstein condensates (BEC) in 1D optical lattice and single focus trap geometries are developed and presented. These techniques offer considerable flexibility and speed compared to magnetic trap approaches, and the trapping potential can be essentially spin-independent and are ideally suited for studying spinor condensates. Using condensates with well-defined initial non-equilibrium spin configuration, spin mixing of F = 1 and F = 2 spinor condensates of rubidium-87 atoms confined in an optical trap is observed. The equilibrium spin configuration in the F = 1 manifold confirms that 87Rb is ferromagnetic. The coherent spinor dynamics are demonstrated by initiating spin mixing deterministically with a non-stationary spin population configuration. Finally, the interplay between the coherent spin mixing and spatial dynamics in spin-1 condensates with ferromagnetic interactions is investigated.