Transient laser cooling of two-level quantum systems with narrow natural linewidths

A scheme of transient laser cooling of quantum system with a narrow transition linewidth by a sequence of counterpropagating ultrashort laser pulses with special frequency chirping is proposed and investigated. The laser cooling scheme consists of two steps. First, a velocity-selective excitation of the ensemble of two-level quantum system is produced by a frequency chirped laser pulse. This then allows the movement of the distributions of the excited and unexcited quantum systems toward each other in velocity space.