In a quantum key distribution (QKD) system, a method of autocalibrating the gating of a single-photon detector (SPD) unit is disclosed. The method includes exchanging photon signals (P1, P2) to establish a bit-error rate (BER). The method also includes performing a detector gate signal timing scan (314) that varies the signal arrival time (T) to establish an optimum arrival time (T.sub.MIN) corresponding to an optimum (e.g., minimum) BER. Detector gate signal timing dithering is then performed (318). This involves varying the detector gate signal arrival times about the optimum arrival time to ensure that the QKD system operates at or near an optimum BER.