Within the last two decades, the world has seen an exponential increase in the quantity
of data traffic exchanged electronically. Currently, the widespread use of classical
encryption technology provides tolerable levels of security for data in day to day life.
However, with one somewhat impractical exception these technologies are based on
mathematical complexity and have never been proven to be secure. Significant advances
in mathematics or new computer architectures could render these technologies obsolete
in a very short timescale.
By contrast, Quantum Key Distribution (or Quantum Cryptography as it is sometimes
called) offers a theoretically secure method of cryptographic key generation and
exchange which is guaranteed by physical laws. Moreover, the technique is capable of
eavesdropper detection during the key exchange process. Much research and
development work has been undertaken but most of this work has concentrated on the
use of optical fibres as the transmission medium for the quantum channel. This thesis
discusses the requirements, theoretical basis and practical development of a compact,
free-space transmission quantum key distribution system from inception to system tests.
Experiments conducted over several distances are outlined which verify the feasibility
of quantum key distribution operating continuously over ranges from metres to intercity distances and finally to global reach via the use of satellites
