Science 342, 1349–1351 (2013)

Optical detectors usually annihilate a photon on detection, with the photon being absorbed by the detector material. However, Andreas Reiserer and co-workers from the Max-Planck-Institut für Quantenoptik in Germany have now demonstrated a novel non-destructive scheme for detecting photons. The photon detector consists of a Fabry–Pérot resonator containing a single trapped 87Rb atom. The cavity induces strong coupling between a light pulse and the atom when it is one of two states, but not when the atom is in the other state. These atomic states are controlled by Raman lasers. When the atom is not coupled to the incoming light, the photon enters the cavity before being reflected. Although there is no atom–photon interaction, the photon induces a phase shift of π in the state of the atom. In contrast, when the atom is coupled, the photon is reflected without entering the cavity, and hence it does not induce a phase shift. By reading out the atomic phase, it is possible to detect a photon without it being absorbed. This detector achieved a single-photon detection efficiency of 74%.