Science http://doi.org/qfb (2013)

A single photon can be detected without being destroyed, researchers now show.

Most classical optical detectors work by absorption. The energy carried by the light particle transfers to the electron in an absorbing material, for example. This excited electron is measurable, but the photon is lost. Andreas Reiserer and co-workers instead use the quantum-mechanical properties of atoms to create a non-absorbing detector of light.

Their detector is a single rubidium atom trapped in an optical cavity. Reiserer et al. prepared the atom in a superposition of two quantum states nearly on resonance with the cavity. A resonant photon incident on the atom–cavity system is reflected away, but leaves its mark on the atom by flipping its quantum state. Further manipulation of the atom leaves it in its upper state if a photon was incident on the system, and its lower state if not.

The system detected 74% of incident photons from an attenuated light source. Of course, the advantage of a non-destructive detector is that this efficiency can be improved by repeating the measurement on the same photon many times.