Colloquia

Quantum cryptography, or more accurately quantum key distribution (QKD), uses single-photon transmissions to generate the shared, secret random number sequences, known as cryptographic keys, which are used to encrypt secret communications. Appealing features of QKD are that its security is based on principles of quantum physics and attempted eavesdropping can be detected. (Heisenberg’s uncertainty principle ensures that an adversary can neither successfully tap the key transmissions, nor evade detection because eavesdropping raises the key error rate above a threshold value). I shall describe two quantum cryptography systems, based on the transmission of non-orthogonal single-photon states to generate shared key material, at Los Alamos. In one experiment we are generating key material over a 48-kilometer optical fiber path, and in the other by transmitting photons over a 1.6-km atmospheric path in daylight. In both cases, key material is built up using the transmission of a single-photon per bit of an initial secret random sequence. A quantum-mechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender. The atmospheric results show that QKD could be used for surface to satellite transmissions.