  All optical detectors to date annihilate photons upon detection, thus
excluding repeated measurements. Here, we demonstrate a robust photon detection
scheme which does not rely on absorption. Instead, an incoming photon is
reflected off an optical resonator containing a single atom prepared in a
superposition of two states. The reflection toggles the superposition phase
which is then measured to trace the photon. Characterizing the device with
faint laser pulses, a single-photon detection efficiency of 74% and a survival
probability of 66% is achieved. The efficiency can be further increased by
observing the photon repeatedly. The large single-photon nonlinearity of the
experiment should enable the development of photonic quantum gates and the
preparation of novel quantum states of light.
