Backward echo in two-level systems

The possible three-pulse echoes in two-level systems of solids and gases are discussed with particular emphasis on backward-wave echoes. One example of the backward echoes was realized with the atomic sodium D line. The detection of this echo requires neither optical shutters nor magnetic field. The decay rate of the echo due to Na-Ar collisions was measured. It is shown that the effect of velocity changing collisions is negligible compared with that of phase interrupting ones.