Paul R. Tapster

Ground to satellite secure key exchange using quantum cryptography

We examine the possibility of secure key exchange between a ground station and a low earth orbit satellite using the technique of quantum cryptography. The study suggests there are no technical obstacles to building a system that could exchange keys at kilobaud rates between a metre diameter telescope on the ground and a satellite with a 10xa0cm diameter lightweight telescope.

Long-distance free-space quantum cryptography

Quantum cryptography bases the security of key exchange on the laws of quantum physics and will become the first application of quantum information methods. Here we present the design of novel hardware components which enabled the demonstration of secure key exchange over a 23.4 km free-space link.

Quantum communications in space

Some of the detailed designs of a system exchanging a key to a low earth orbit satellite and remaining technical challenges to be overcome are discussed.

Developments towards practical free-space quantum cryptography.

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.

Quantum cryptography to satellites for global secure key distribution

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.

ATLAS : an airborne active linescan system for high resolution topographic mapping

High resolution ground mapping is of interest for survey and management of long linear features such as roads, railways and pipelines, and for georeferencing of areas such as flood plains for hydrological purposes. ATLAS (Airborne Topographic Laser System) is an active linescan system operating at the eyesafe wavelength of 1.5μm. Built for airborne survey, it is currently certified for use on a Twin Squirrel helicopter for operation from low levels to heights above 500 feet allowing commercial survey in built up areas. The system operates at a pulse repetition frequency of 56kHz with a line completed in 15ms, giving 36 points/m2 at the surface at the design flight speed. At each point the range to the ground is measured together with the scan angle of the system. This data is combined with a system attitude measurement from an integrated inertial navigation system and with system position derived from differential GPS data aboard the platform. A recording system captures the data with a synchronised time-stamp to enable post-processed reconstruction of a cloud of data points that will give a three-dimensional representation of the terrain, allowing the points to be located with respect to absolute Earth referenced coordinates to a precision of 5cm in three axes. This paper summarises the design, harmonisation, evaluation and performance of the system, and shows examples of survey data.