Reversible storage of a weak light pulse in a thermal atomic medium

Abstract

We investigate the storage and retrieval of a weak signal pulse in a medium composed of $N \ensuremath{\Lambda}$-type three-level atoms in thermal motion. Both atomic internal and translational degrees of freedom are treated quantum mechanically, based on which momentum families that are closed with respect to the action of the interaction Hamiltonian are defined. We introduce momentum-dependent collective atomic density operators by sorting the atoms into groups according to their momenta and establish Heisenberg equations for these atomic operators and the signal field. Because of the thermal-motion-dependent dephasing effect in the establishment of electromagnetically induced transparency of the medium, the efficiency of transfer between the signal pulse and the medium is reduced. The numerical simulations show that the performance of the scheme can be improved by applying the copropagation configuration of the light fields and preparing the atoms initially with a small width of the thermal momentum distribution, which is assumed to be the Gaussian function, or in the state where the Raman-Nath approximation is tenable.