  A pair of coherent femtosecond pulse excitations applied to a molecule with
strong electron-phonon coupling creates a coherent superposition of a low
momentum and a high momentum wavepacket in the vibrational states of both the
excited state and the ground state of the coherent transition. As the excited
state is accelerated further, interference between the high momentum ground
state contribution and the low momentum excited state contribution causes a
photon echo. This photon echo is a direct consequence of quantum interference
between separate vibrational trajectories and can therefore provide
experimental evidence of the non-classical properties of molecular vibrations.
