Abstract

When single photons are used to code for individual bits, security can be guaranteed by exploiting Heisenberg's uncertainty principle. Binary information can be coded into two orthogonal states, such as polarization or relative phase (in an interferometer). To receive this information we require an analyzer able to discriminate between the two states in an error-free way. In a linear-polarization system a polarizing beam splitter is used with its axes aligned to the two possible polarization directions. However, using a polarizer at 45° to these directions will ensure a random output uncorrelated to the single-photon-coded bit stream. The principle of secure key sharing, or quantum cryptography, is that a random bit stream is encoded onto individual photons while randomly switching the coding basis between the 0° and 45° polarization bases (in general, any non-orthogonal measurement bases). The receiver also randomly switches between these basis, and information is shared only when a photon is received and the same basis is used for coding and for measurement. An initial experiment of this type was carried out by using faint pulses of polarized visible light over a 40-cm transmission distance.¹

© 1994 Optical Society of America