Abstract

Quantum key distribution allows two parties (Alice and Bob) to exchange an unconditionally secure key that can be used for classical cryptography. As the first real application of quantum information methods, first commercial systems are available for fibre links. Our system sends photons directly through a free space link. Compared to fibre based systems, this allows many new applications of quantum key distribution like rapidly deployable inner city systems, key exchanges with handheld devices or key distribution with satellites. The advantage of a free space link is, that no fibre connection is required, only a direct line of sight from Alice to Bob is needed. The disadvantage however is that such links are exposed to outside weather-conditions and daylight. Since single photon detectors can easily be saturated even by small light intensities, until recently our system could only be operated during night. Improvements in the optical filtering systems allowed us to overcome this restriction.

© 2009 IEEE

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