Interference-induced optical gain without population inversion in cold, trapped atoms

Continuous-wave (cw) optical gain of 1.3×10^-2 cm^-1 is obtained on a probe transition in a driven, three-level, V-type atomic system. The atoms exhibit no population inversion between the probe excited state and the dressed ground states of the combined atom-drive Hamiltonian. This gain without population inversion is interpreted as direct evidence of quantum interference, arising from coherences established in the atom by the applied optical fields. Agreement with a simple four-level theoretical model is excellent.