Ultrafast Superconducting Single-Photon Optical Detectors and Their Applications

38 LLE Review, Volume 93 Introduction Single-photon detectors (SPD’s) represent the ultimate sensitivity limit for any quantum radiation detectors. In the visible light range, the best known and most widely used are Si avalanche photodiodes (APD’s)1 and photomultiplier tubes (PMT’s).2 The operation of photomultiplier/avalanche devices is based on the electron cascade and multiplication effect, which significantly amplifies the response and allows for an easy measurement of the response pulses. Unfortunately, this method of registration leads to a large time lag and jitter of the device response. In addition, the counting rates of APD’s and PMT’s are well below 100 MHz. Their actual speed is even more limited since, for noise reduction purposes, they have to be used in a time-gated mode, which reduces their counting rates to 10 MHz or below.