Efficient key distribution in free space quantum cryptography.

Student: Andrew James Grosser

Supervisor: Prof. Halina Rubinsztein-Dunlop & Dr. Andrew White

Category: Computer Systems Engineering Thesis Project

Quantum cryptography is a secure data communication system with demonstrable intrinsic eavesdropping detection. All quantum cryptographic models are adaptations of the Vernan cipher (one-time pad), which is the only mathematically proven cryptographic method resilient to quantum computer attacks. The main hindrance facing the development of quantum cryptography is concerned with key transmission rate. Key distribution is an intrinsic part of the Vernan cipher. The system consists of typically a key distributor (Alice) and a key recipient (Bob) [see diagram below]. The data that Alice sends Bob that is received by Bob becomes the key. For free space systems using the Vernan cipher, key transfer rates have not yet exceeded 10 kilobits per second. This deficiency is partly due to quantum mechanics but mainly affected by physical limitations in equipment. Numerous quantum cryptographic protocols exist. The most efficient commercially viable free space protocol is B92 (Bennett, C.H.; 1992). This encodes qubits into two non-orthogonally polarized states. Signal attenuation can be as high as 99.7% with a theoretical minimum of 50%. This has dramatic ramifications on key transfer limitations. The problem of transmission rate then becomes dependent primarily upon the rate of raw data Alice attempts to communicate. Using the B92 protocol with an optimal signal attenuation of 98.75% the device designed is capable of transferring a key up to 1 Megabit per second. This goes several orders of magnitude beyond existing systems. The increase is due to an active quenching circuit that stops the photon detector (avalanche photodiode) from saturating. It is intended that by the time of publication, an increase in the existing limitation will be demonstrated, although theoretically I have demonstrated it is possible to attain a key rate of up to a Gigabit per second using current technology.