Alignment of mobile free-space quantum key distribution transmitting and receiving apparatus

摘要

This invention concerns communication between a transmitter and a receiver over a classical channel (such as radio) using a key established using a quantum cryptography channel. The transmitter may be a mobile apparatus such as mobile phone or PDA. In order to perform quantum key distribution (QKD) it is necessary to ensure that the transmitter and receiver are aligned. The transmitter must be pointing at the receiver, and the polarisation axis of the transmitter must be aligned with the polarization access of the receiver. The transmitter emits an alignment beam to facilitate this. The alignment beam may be in the form of a visible cross to provide the user with an indication of the degree of alignment. The user may be provided with an additional indication of how he should move the transmitter to achieve alignment, e.g. via the receiver screen 66. The receiver may include active optics which adapt in response to the alignment beam. For example the receiver may include a movable lens 54 and movable polarising filters 58, 59 62, 63.