In order to facilitate alignment of a QKD transmitter and QKD receiver, the transmitter is provided with a retro-reflector for returning to the receiver a photon beam originating at the latter. The transmitter is arranged to polarization modulate the retro-reflected beam. The transmitter is provided both with an intensity detector for generating an indication of retro-reflected photon intensity, and an intensity-dependent controller for controlling the QKD transmitter in dependence on the detected photon intensity. In one embodiment, this control involves aborting operation of the QKD transmitter upon an unexpectedly high photon intensity being detected; in another embodiment, the intensity indication is used to control the attenuation of the retro-reflected beam so as stabilize the average retro-reflected photon count per unit time.