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arXiv:cond-mat/0502247 · Physical Review A

Laser cooling down to molecular condensate

Jacek Dziarmaga, Maciej Lewenstein

Published Sep 9, 2005• 6 citations
arXivHTMLPDF

Abstract

On the repulsive side of the Feshbach resonance fermionic atoms can bind into diatomic molecules which at low enough temperature condense into molecular Bose-Einstein condensate. Sufficiently close to the resonance the molecules are bound so weakly that a significant fraction of unbound fermionic atoms survives even below the temperature of condensation. In this paper we show that laser cooling of the unbound fermionic atoms can reduce temperature of mixture of atoms and molecules below the condensation temperature. Numerical simulations for reasonable experimental parameters show that it is possible to achieve molecular condensate after a few seconds of laser cooling.

DOI
10.1103/physreva.72.033606

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On the repulsive side of the Feshbach resonance fermionic atoms can bind into diatomic molecules which at low enough temperature condense into molecular Bose-Einstein condensate. Sufficiently close to the resonance the molecules are bound so weakly that a significant fraction of unbound fermionic atoms survives even below the temperature of condensation. In this paper we show that laser cooling of the unbound fermionic atoms can reduce temperature of mixture of atoms and molecules below the condensation temperature. Numerical simulations for reasonable experimental parameters show that it is possible to achieve molecular condensate after a few seconds of laser cooling.
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Besides the enormous opportunities offered by spectroscopy and atom interferometry, laser cooling has opened up the possibility of experimentally realizing and manipulating quantum degenerate gases: Bose–Einstein condensates and degenerate Fermionic systems. This chapter starts with an illustration of key experimental techniques, such as magnetic trapping, evaporative cooling, and Feshbach resonances. It then turns to the physics of atomic Bose–Einstein condensates, discussing their superfluid behaviour, their coherence properties, and their application to precise measurements. It also describes the experimental investigation of ultracold Fermi gases and fermionic superfluidity, as well as ongoing research into ultracold polar molecules.
We have observed Bose-Einstein condensation of molecules. When a spin mixture of fermionic 6Li atoms was evaporatively cooled in an optical dipole trap near a Feshbach resonance, the atomic gas was converted into 6Li2 molecules. Below 600 nK, a Bose-Einstein condensate of up to 900 000 molecules was identified by the sudden onset of a bimodal density distribution. This condensate realizes the limit of tightly bound fermion pairs in the crossover between BCS superfluidity and Bose-Einstein condensation.
A counterintuitive scheme to produce ultracold hydrogen via fragmentation of laser cooled diatomic hydrides is presented where the final atomic H temperature is inversely proportional to the mass of the molecular parent. In addition, the critical density for formation of a Bose-Einstein condensate (BEC) at a fixed temperature is reduced by a factor (mH/mMH)3/2({m}_{H}/\phantom{\rule{0.0pt}{0ex}}{m}_{MH}{)}^{3/2}(mH​/mMH​)3/2 over directly cooled hydrogen atoms. The narrow Feshbach resonances between a 1S0^{1}S_{0}1S0​ atom and hydrogen are well suited to a tiny center of mass energy release necessary during fragmentation. With the support of ab initio quantum chemistry, it is demonstrated that BaH is an ideal diatomic precursor that can be laser cooled to a Doppler temperature of $\ensuremath{\sim}26\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$ with just two rovibronic transitions, the simplest molecular cooling scheme identified to date. Preparation of a hydrogen atom gas below the critical BEC temperature Tc{T}_{c}Tc​ is feasible with present cooling technology, with optical pulse control of the condensation process.
Abstract The methods of laser cooling of atoms have long been applied to obtain cold and ultracold atomic gases, including degenerate states and the atomic Bose–Einstein condensate in particular. Until recently the application of laser cooling methods to molecules was assumed to be impossible because of the complex structure of molecular levels and the absence of closed cooling cycles for transitions between the electron levels of molecules in the general case. However, it has recently become clear that laser cooling can be performed for a large class of molecules, including not only the simplest diatomic molecules but also polyatomic molecules. We here present the general principles for identifying suitable molecules and discuss current studies on and further developments in the laser cooling of molecules.