Light detection is usually a destructive process, in that detectors annihilate photons and convert them into electrical signals, making it impossible to see a single photon twice. But this limitation is not fundamental — quantum non-demolition strategies permit repeated measurements of physically observable quantities, yielding identical results.
The non-destructive measurement of a single photon requires an extremely strong matter-radiation coupling. This can be realized in cavity quantum electrodynamics, where the strength of the interaction between an atom and a photon can overwhelm all dissipative couplings to the environment.
In the experiments reported, an atomic interferometer has been used to measure the phase shift in an atomic wavefunction, caused by a cycle of photon absorption and emission.
The method amounts to a restricted quantum non-demolition measurement, which can be applied only to states containing one or zero photons. It may lead to quantum logic gates based on cavity quantum electrodynamics, and multi-atom entanglement.