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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Efficiency of the VSCPT subrecoil laser cooling

Boris G. Matisov, I. E. Mazets

Published May 6, 1996• 0 citations
Publisher

Abstract

We represent a new solution of the set of equations describing laser cooling of atoms by velocity selective coherent population tapping. Our solution is valid at large interaction time t in a full range of atomic kinetic momenta. Referring to this analytic expression we obtain the formula for lowest achievable temperature of cooled atoms for given experimental conditions. We demonstrate that the cooling efficiency decreases as t -3/4, when t is greater than or equal to (Gamma) 12-1, where (Gamma) 12 is the low-frequency atomic coherence decay rate. Also the case of subrecoil laser cooling in vertical direction is examined and the gravitational force influence is estimated.

DOI
10.1117/12.239870

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We represent a new solution of the set of equations describing laser cooling of atoms by velocity selective coherent population tapping. Our solution is valid at large interaction time t in a full range of atomic kinetic momenta. Referring to this analytic expression we obtain the formula for lowest achievable temperature of cooled atoms for given experimental conditions. We demonstrate that the cooling efficiency decreases as t -3/4, when t is greater than or equal to (Gamma) 12-1, where (Gamma) 12 is the low-frequency atomic coherence decay rate. Also the case of subrecoil laser cooling in vertical direction is examined and the gravitational force influence is estimated.
This paper presents new experimental realizations of sub‐recoil laser cooling by velocity selective coherent population trapping (VSCPT). Starting from a cloud of trapped and precooled metastable helium atoms, it has been possible to achieve a VSCPT interaction time of 500 μs. This has enabled the momentum distribution to be compressed along one or two orthogonal axes to better than δp = ħk/4 where ħk is the photon momentum. The corresponding temperature is at least 16 times smaller than the single photon recoil limit, and the de Broglie wavelength of the atoms is at least 4 times larger than the wavelength of the laser used to cool the atoms. Recent theoretical developments are also described, establishing a connection between subrecoil cooling and Lévy flights and allowing one to get analytical expressions for the proportion of cooled atoms in the long time limit. Finally, the possibility of increasing the efficiency of VSCPT by Sisyphus precooling is briefly mentioned.
A theoretical analysis of one-dimensional cooling of atoms is presented. The method of cooling is based on velocity-selective coherent population trapping. Both internal and translational degrees of freedom of the atoms are treated quantum mechanically. The analytic expression for atomic density matrix is obtained in the limit of large interaction time and small kinetic momentum of atom. It is demonstrated that: (i) there is a limit on the lowest temperature that can be achieved by such a method and it is determined by the small but finite rate of decay of the coherence between two low-lying states in the three-level Lambda -configuration; (ii) the fraction of atoms involved in the cooling process decreases to zero as the atom-field interaction time increases, because of the presence of such a relaxation which always takes place in real experimental systems.
ABSTRACT In this work we present an investigation of velocity-selective coherent population trapping (VSCPT) in asym- metric three-level schemes. The light force in such schemes is of an intrinsically transient character and shouldnot be described in a quasiclassical approach. Instead, results of the full quantum-mechanical calculations are presented with emphasis on their meaning to the efficiency of subrecoil laser cooling. 1 INTRODUCTION Laser cooling of neutral atoms has received significant advances by the introduction of velocity-selectivecoherent population trapping ((VSCPT) some years ago.1 Since it leads to atomic temperatures well below therecoil limit, studies of ultracold atoms and atomic interactions may be improved by magnitudes.The effect of VSCPT relies on optical pumping of atoms into a state WNC) which is a trap state for a certaincenter-of-mass velocity vO. This trap state is created by a destructive superposition of atomic states within amultilevel quantum system. Once being in such a state LWNC) the laser light does not interact anymore with theatom which leads to a decoupling with the rest of the system at vo. Atoms located outside this small velocityregion get a chance to be trapped by the momentum diffusion due to spontaneous emission of photons. Aftera sufficient long interaction time nearly all atoms are trapped inside a narrow momentum distribution whichcan be interpreted as cooling. The theoretical and experimental investigation of VSCPT is usually consideredby use of a three-level A-system as shown in Fig.1. Such a system can be found for example in a symmetrical
One-dimensional laser cooling based on velocity-selective coherent population trapping (VSCPT) has been investigated numerically through the solution of the optical Bloch equations and through a Monte Carlo analysis. The 1\ensuremath{\rightarrow}1 and 2\ensuremath{\rightarrow}2 transitions have been examined as a function of the atomic recoil frequency, the spontaneous-emission decay rate, and the Rabi frequency of the cooling laser. It has been found that for a large set of those parameters, the VSCPT cooling process may be described through scaling-law relations. The scaling laws are not valid at long atom-laser interaction times or large Rabi frequencies, where the atomic Doppler shift plays a significant role in the atomic motion evolution. Similar results for two atomic transitions suggest the validity of the scaling law for any one-dimensional VSCPT process. \textcopyright{} 1996 The American Physical Society.