Nondestructive Detection of an Optical Photon

All optical detectors to date annihilate photons upon detection, thus excluding repeated measurements. Here, we demonstrate a robust photon detection scheme that does not rely on absorption. Instead, an incoming photon is reflected from 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 % are 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 exotic quantum states of light. More than a century ago, Planck’s idea ofa quantized energy exchange betweenlight and matter and Einstein’s conclu-sion that a light beam consists of a stream of par-ticles have revolutionized our view of the world. The explanation of the photoelectric effect in terms of a photon-absorption process is the basis of the theoretical description of light with nor