Dark count ($r_{\mathrm{dark}}$)

A registered detector event occurring without an incident signal photon. For a single-photon avalanche diode, internally generated carriers can trigger the same avalanche as an absorbed photon. The dark-count rate $r_{\mathrm{dark}}$ is the mean number of these events per unit time under specified operating conditions. Such events can contaminate coincidence counting and increase the quantum bit error rate in quantum key distribution.