PCT No. PCT / GB94 / 02067 Sec. 371 Date Apr. 10, 1996 Sec. 102(e) Date Apr. 10, 1996 PCT Filed Sep. 23, 1994 PCT Pub. No. WO95 / 10907 PCT Pub. Date Apr. 20, 1995A cryptographic receiver (10) includes photon detectors (52, 54, 56, 58) arranged to detect photons arriving from filters (22) and (24). A fiber coupler (14) randomly distributes each received photon (16) from an optical fiber toone of two photon channels (18, 20). The filters (22, 24) are each unbalanced Mach-Zehner interferometers with a phase modulator (34, 44) in one arm (28, 38). The filters (22, 24) impose non-orthogonal measurement bases on photons within the respective channels (18, 20). A signal processor (60) derives a cryptographic key-code by analysis of signals received from the photon detectors (52, 54, 46, 58).