Photon correlation is a kind of spectroscopy designed to identify optical frequency shifts and line-broadening effects in the range of many MHz down to a few Hz. The optical intensity is measured in terms of single photon- detection events which result in current pulses at the output of photomulti- plier tubes. This signal is processed in real time in a special-purpose paral- lel processor known as a correlator. The resulting photon correlation func- tion, a function in the time domain, contains the desired spectral informa- tion, which may be extracted by a suitable algorithm. Due to the...