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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Developments towards practical free-space quantum cryptography

P. R. Tapster, P.M. Gorman, David M. Benton, D.M. Taylor, Brian S. Lowans

Published May 25, 2005• 4 citations
Publisher

Abstract

We describe a free space Quantum cryptography system which is designed to allow continuous unattended key exchanges for periods of several days, and over ranges of a few kilometres. The system uses a four laser faint pulse transmission system running at a pulse rate of 10MHz to generate the required four alternative polarization states. The receiver module similarly automatically selects a measurement basis and performs polarization measurements with four avalanche photodiodes. The controlling software can implement the full key exchange including sifting, error correction, and privacy amplification required to generate a secure key.

DOI
10.1117/12.605539

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We describe a free space Quantum cryptography system which is designed to allow continuous unattended key exchanges for periods of several days, and over ranges of a few kilometres. The system uses a four laser faint pulse transmission system running at a pulse rate of 10MHz to generate the required four alternative polarization states. The receiver module similarly automatically selects a measurement basis and performs polarization measurements with four avalanche photodiodes. The controlling software can implement the full key exchange including sifting, error correction, and privacy amplification required to generate a secure key.
We have designed and built a free space quantum cryptography system. It operates using weak laser pulses with polarization modulation by acousto-optic switching. The system also incorporates full key sifting and error correction between computers connected by a serial port. We have used this system to exchange keys over ranges up to 1.9km with absolute security. The system is robust against transmission (and diffraction) losses up to 20 dB. Building from this initial result we analyse the feasibility of exchanging keys to a low earth orbit satellite.
Quantum key distribution (QKD)1 is the first method of quantum information science that will find its way into our everyday life. It employs fundamental laws of quantum physics to ensure provably secure symmetric key generation between two parties. The key can then be used to encrypt and decrypt sensitive data with unconditional security. Here, we report on a free space QKD implementation over a distance of 480 m using strongly attenuated laser pulses. It is designed to work continuously without human interaction. Until now, it produces quantum keys unattended at night for more than 12 hours with a sifted key rate of more than 50 kbit/s on average and a quantum bit error rate between 3% and 5%.
A brief overview of the current status of quantum cryptography is given. The results are presented of our preliminary experiments with free-space quantum-key distribution by means of single linearly polarized photons with coded polarization states according to the BB84 protocol, for which purpose a quantum-cryptography communication system is designed. Single photons are detected with a 50% probability using specially designed high-speed photodetectors based on silicon avalanche photodiodes, operated in Geiger mode with active avalanche quenching. A key generation rate of 3.8 kbit/s is obtained, the mean photon number per pulse being 0.2.
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.