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International Conference on Space Optics (ICSO)

Quantum cryptography to satellites for global secure key distribution

John Rarity, Paul Knight, P.M. Gorman, Kotska Wallace, Paul R. Tapster

Published Nov 21, 2017• 1 citation
Publisher

Abstract

We have designed and built a free space secure key exchange system using weak laser pulses with polarisation modulation by acousto-optic switching. We have used this system to exchange keys over a 1.2km ground range with absolute security. Building from this initial result we analyse the feasibility of exchanging keys to a low earth orbit satellite.

DOI
10.1117/12.2307891

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We have designed and built a free space secure key exchange system using weak laser pulses with polarisation modulation by acousto-optic switching. We have used this system to exchange keys over a 1.2km ground range with absolute security. Building from this initial result we analyse the feasibility of exchanging keys to a low earth orbit satellite.
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.
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.
The University of Canberra and its research collaborators have established a dedicated free-space laser communications test-bed between a national telecommunications facility and four selected monitoring sites at distances ranging from 4 to 47 km. This test-bed is currently being used to simulate and characterize the free-space quantum channel along which secure quantum key exchanges could take place between earth stations and low earth orbit satellites, the latter acting as global key couriers. We present the results of these trials conducted under a variety of atmospheric conditions. We use the results to estimate error rates and secure key exchange rates for ground/satellite key exchange systems employing weak Poissonian pulses with more stringent security requirements than are sometimes quoted.
Summary form only given. Quantum cryptographic systems employ secret keys exchanged between two parties in the form of a stream of polarisation- or phase-encoded single photons. Under ideal conditions the absolute security of the quantum key is guaranteed by the laws of quantum physics, since any attempt at eavesdropping will introduce gross bit errors which will be quickly detected by the legitimate parties to the key exchange. Secure quantum key distribution (QKD) it currently limited to distances of several tens of kilometres. It has been suggested that key distribution over global distances will require the use of free-space optical intersatellite (O-ISL) and ground to satellite links. Free-space terrestrial paths provide a natural propagation medium in which to investigate the viability of ground to satellite single photon QKD links. We describe the use of an optical free-space range based on a satellite laser ranging observatory instrumented for this purpose.
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.