Experimental multi-photon-resolving detector using a single avalanche photodiode

There is an increasing need for identifying the number of photons that is contained within a weak light pulse or time interval of a weak cw light field. A device capable of photon-number resolution would contribute both to fundamental research in the area of quantum optics and to more-or-less practical quantum communication systems, such as quantum key distribution schemes [1]. In the former case it would significantly help in the analysis and preparation of quantum states with prescribed statistics [2], in the latter it would prolong communication distances and increase the rate of secure communication [3–5]. A detector capable to resolve the vacuum state, one-photon Fock state and states containing more than one photon was recently constructed [6,7], but it requires operation under extreme conditions at present and it hardly becomes a common laboratory tool at least in near future. A similar