Abstract

In quantum optical computing and quantum cryptography a very important problem is that of developing an efficient photon-number resolving detector. Such a device is critical for the development of linear optical quantum computing (LOQC) [1], and can be also used in quantum cryptography [2]. A recent paper claims to have demonstrated the operation of a superconducting, number-resolving photon detector, developed by a group at NIST [3]. However, this device operates with a quantum efficiency of only about 20%, and we show here that it cannot discriminate photon number, even in principal. However, the NIST device has a very low dark-count rate, much less than 1%, which means that it can quite efficiently discriminate against the absence of photons of particular numbers. We call such a device a number-discriminating non-photon detector. Interestingly enough, even though this detector cannot yield photon number—its ability to discriminate against photons of certain numbers allows it to be used, under suitable assumptions, to reconstruct the ensemble photostatistics of the incoming photon field. However, such a low-efficiency device cannot be used, say, in LOQC, where single-shot photon number discrimination is required.

© 2003 Optical Society of America