Evolution of coherent dark states

One-dimensional velocity-selective coherent population trapping (VSCPT) was studied into two domains of parameters namely applied magnetic field and optical excitation. The effects of laser detuning and the atom-laser interaction time in determining the shape and width of VSCPT velocity distribution was investigated. A beam of metastable helium atoms in the 2³S state of about 10¹⁴ atoms/s sr, was generated in a dc discharge followed by a set of apertures. Results indicated that magnetic field applied along the direction of light propagation resulted in shift of the center of VSCPT peaks in the atomic velocity distribution by an amount proportional to the magnitude of the applied field.