  Quantum cryptography is an emerging technology in which two parties may
simultaneously generate shared, secret cryptographic key material using the
transmission of quantum states of light. The security of these transmissions is
based on the inviolability of the laws of quantum mechanics and
information-theoretically secure post-processing methods. An adversary can
neither successfully tap the quantum transmissions, nor evade detection, owing
to Heisenberg's uncertainty principle. In this paper we describe the theory of
quantum cryptography, and the most recent results from our experimental
free-space system with which we have demonstrated for the first time the
feasibility of quantum key generation over a point-to-point outdoor atmospheric
path in daylight. We achieved a transmission distance of 0.5 km, which was
limited only by the length of the test range. Our results provide strong
evidence that cryptographic key material could be generated on demand between a
ground station and a satellite (or between two satellites), allowing a
satellite to be securely re-keyed on orbit. We present a feasibility analysis
of surface-to-satellite quantum key generation.
