Enhancement of quantum interference effects

We present a method of excitation which enhances quantum beats of certain frequencies and diminishes those at other frequencies. This approach can even produce an effect when the original quantum beats appear to be washed out in large molecules because of the average over many pairs of interfering levels. The excitation of the system involves the absorption of a single photon from a coherent pair of pulses, leading to an additional interference effect with a nonzero quantum yield. The derivations are presented for the atomic-like few level systems, but the more general molecular cases are readily obtained by superposition of all pairs of interfering levels. The analysis incorporates the position and nature of the photon detector.