Extended photon correlation in a negative-temperature medium

Abstract

We show how two-photon correlation of a two-level amplifier in the small signal regime depends on the atomic populations and the propagation length. We predict that inverted population associated with negative temperature gives a very long correlation time even in the presence of decoherence, a useful asset for quantum communication. The correlation vanishes for very dilute atomic gas. Analytical solutions for the field operators obtained by Fourier transform and Laplace transform (with initial condition) of the time variable appear very different but yield identical numerical results except for certain parameters. The physical explanation behind the deviation is given. The presence of thermal photons is found to reduce the correlation time.