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Optical Fiber Communication Conference

Free space Quantum Cryptography at 1.9km range

Paul R. Tapster

Published Jan 1, 2001• 0 citations
Publisher

Abstract

We have recently demonstrated a free space quantum cryptography system which performed secure communication over a range of 1.9 kilometers. We exchanged a few thousand bits of key (after error correction) at rates of around a hundred bits per second. I will give details of the experimental system and discuss the results obtained. Plans for further experiments will also be discussed.

DOI
10.1364/icqi.2001.fqipb3

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We have recently demonstrated a free space quantum cryptography system which performed secure communication over a range of 1.9 kilometers. We exchanged a few thousand bits of key (after error correction) at rates of around a hundred bits per second. I will give details of the experimental system and discuss the results obtained. Plans for further experiments will also be discussed.
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.
Coding data bits in the phase or polarisation state of light allows us to exploit the wave particle duality for novel communication protocols. Using this principle the first practical quantum communication systems have been built. These are the fibre and free-space quantum cryptography apparatus used for secure exchange of keys. To date free space key exchange has aimed at long range with 144km range achieved and future experiments aiming to extend this range to 1000km exchanging keys with low earth orbit satellites. At the other end of the spectrum we are developing low cost hand held systems. These systems could be an effective way for the user to generate a store of secrets shared with a central repository. These secrets can then be used up to protect a wide variety of sensitive classical communications. Examples include on-line PIN protection for consumer transactions and password protection in secure access schemes.
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.