Quantum cryptography: Public key distribution and coin tossing

Charles H.Bennett and GillesBrassard use a radically different foundation for cryptography, the uncertainty principle of quantum physics. They also present a protocol for coin-tossing by exchange of quantum messages, which is secure against traditional kinds of cheating, even by an opponent with unlimited computing power, but ironically can be subverted by use of a still subtler quantum phenomenon, the Einstein-Podolsky-Rosen (EPR) paradox. In quantum public key distribution, the quantum channel is not used directly to send meaningful messages, but is rather used to transmit a supply of random bits between two users who share no secret information initially, in such a way that the users, by subsequent consultation over an ordinary non-quantum channel subject to passive eavesdropping, can tell with high probability whether the original quantum transmission has been disturbed in transit, as it would be by an eavesdropper. The EPR effect occurs when certain types of atom or molecule decay with the emission of two photons, and consists of the fact that the two photons are always found to have opposite polarization, regardless of the basis used to observe them, provided both are observed in the same basis. Verbal explanation of the EPR effect is to say that the two photons are produced in an initial state of undefined polarization; and when one of them is measured, the measuring apparatus forces it to choose a polarization while simultaneously forcing the other unmeasured photon, no matter how far away, to choose the opposite polarization.