Quantum key distribution over a 48 km optical fibre network

The secure distribution of the secret random bit sequences known as 'key' m aterial, is an essential precursor to their use for the encryption and decr yption of confidential communications. Quantum cryptography is a new techni que for secure key distribution with single-photon transmissions: Heisenber g's uncertainty principle ensures that act adversary can neither successful ly tap the key transmissions, nor evade detection (eavesdropping raises the key error rate above a threshold value). We have developed experimental qu antum cryptography systems based on the transmission of non-orthogonal phot on states to generate shared key material over multi-kilometre optical fibr e paths and over line-of-sight links. In both cases, key material is built up using the transmission of a single-photon per bit of an initial secret r andom sequence. A quantum-mechanically random subset of this sequence is id entified, becoming the key material after a data reconciliation stage with the sender. Here we report the most recent results of our optical fibre exp eriment in which we have performed quantum key distribution over a 48 km op tical fibre network at Los Alamos using photon interference states with the B92 and BB84 quantum key distribution protocols.