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

There is an increasing need for identifying the number of photons that are contained within a weak pulse of time interval or a weak continuous-wave light field. A multichannel detector has been constructed using a single avalanche photodiode and a fiber-loop delay line. Detection probabilities of the channels can be set using a variable-ratio coupler. The performance of the detector is demonstrated on its capability to distinguish multi-photon states (containing two or more photons) from the one-photon state and the vacuum state. In this work we use only the information whether single or multiple detections occurred regardless of the information in which channels the photons were detected. Since the channels exhibit different detection probabilities, there is an additional information which might be used. In general, it is possible to obtain the photon-number statistics of a detected quantum state to some extent