Free-space quantum cryptography and satellite key uploading

Summary form only given. The possibility of generating and exchanging keys between secure sites is of interest to various commercial and military programmes. Conventional key exchange methods generally utilise public key methods and rely on computational complexity as proof against tempering and eavesdropping. One area where future proofing against the rapid improvements in computational power is absolutely essential is in the upload of keys to satellites. Here we discuss the first tentative steps to an optically based system for uploading keys to remote platforms using polarisation encoding of weak pulses of light. This method relies on the rules of physics, those of quantum mechanics, to guarantee the absolute security of key generation and exchange and is known as quantum cryptography. We have built a breadboard based free space quantum cryptography system. The transmitter uses an attenuated pulsed visible laser diode (635 nm) at 1O MHz repetition rate. The four polarisations 0, 90/spl deg/, 45/spl deg/, 135/spl deg/ encoding key bits in two non-orthogonal measurement bases are selected by acousto-optic switches and beams recombine in a collimating telescope. In the receiver the collected beam is reduced in diameter and split into two beams by a non-polarising 50/50 beamsplitter. The beams are then recombined in a polarisation beamsplitter before detection in one of two photon-counting detectors.