Experimental observation of transient velocity-selective coherent   population trapping in one dimension
  We report the observation of transient velocity-selective coherent population trapping (VSCPT) in a beam of metastable neon atoms. The atomic momentum distribution resulting from the interaction with counterpropagating sigma_+ and sigma_- radiation which couples a J_g=2 <-> J_e=1 transition is measured via the transversal beam profile. This transition exhibits a stable VSCPT dark state formed by the two |J=2,m=+/-1> states, and a metastable dark state containing the |J=2,m=+/-2> and |J=2,m=0> states. The dynamics of the formation and decay of stable and metastable dark states is studied experimentally and numerically and the finite lifetime of the metastable dark state is experimentally observed. We compare the measured distribution with a numerical solution of the master equation. 
Frank Vewinger and Frank Zimmer