  We demonstrate a way to prepare single photons with a temporal envelope that
resembles the time reversal of photons from the spontaneous decay process. We
use the photon pairs generated from a time-ordered atomic cascade decay: the
detection of the first photon of the cascade is used as a herald for the
ground-state transition resonant second photon. We show how the interaction of
the heralding photon with an asymmetric Fabry-Perot cavity reverses the
temporal shape of its twin photon from a decaying to a rising exponential
envelope. This single photon is expected to be ideal for interacting with two
level systems.

Quantum Physics