Abstract

Quantum cryptographic key distribution (QKD) systems¹ transmit cryptographic key data encoded in the quantum states of individual optical photons with security of the data guaranteed by the fundamental quantum properties of light, rather than by computational complexity or barriers to interception. Beamsplitter attacks are avoided by using single photons, while attempts to read and replace the photons with copies are thwarted by randomly switching between two non-orthogonal bases for the photon states. This so-called 4-state protocol¹ forces an eavesdropper trying to read-and-replace photons to induce errors at a minimum rate of about 25%, which can be readily detected by the parties exchanging the key data. Several groups have performed QKD experiments that demonstrate its feasibility².

© 1998 Optical Society of America