  A photon echo experiment has been performed using accumulated highly
attenuated laser pulses. We show experimentally that the photon echo process
can be performed with, on the average, less than one photon in each pair of
excitation pulses. The results support an interpretation where this non-linear
process can be performed with, on the average, less than one photon shared
between two of the optical fields involved in the degenerate four-wave mixing
process. Further, we argue that the experiment can be interpreted as a form of
delayed self-interference for photon wave packets that do not overlap in both
space and time.
