In the Oct 1992 issue of Scientific American, the article "Quantum(adsbygoogle = window.adsbygoogle || []).push({});

Cryptography", by Charles Bennet, Gilles Brassard and Artur Ekert

mainlines experimental work on a very powerful cryptosystem that has

been devised that is essentially impossible under QM rules:

"The EPR effect occurs when a spherically symmetric atom emits two photons in opposite directions toward two observers, Alice and Bob. The two photons are produced in an initial state of undefined polarization. But because of the symmetry of the initial state, the polarizations of the photons, when measured, must have opposite values, provided that the measurements are of the same type. For example, if Alice and Bob both measure rectilinear polarizations, they are each equally likely to record either a 0 (horizontal polarization) or a 1 (vertical), but if Alice obtains a 0, Bob will certainly obtain a 1, and vice versa.

The unusual and important aspect of the EPR effect is that the polarization of both photons is determined as soon as, but not before, one of the photons is measured. This happens no matter how far apart the photons may be at the time. This "classical" explanation of the EPR effect is somewhat counterintuitive, and indeed all classical explanations of the EPR effect involve some implausible element, such as instantaneous action at a distance. Yet the mathematical formalism of quantum mechanics accounts for the EPR effect in a straightforward manner, and experiments have amply confirmed the existance of the phenomenon."

The crypto-system of Bennet, Brassard and Ekert is now the basis of several commercial products, so the EPR experiment is being performed on a daily basis.

All the best

John B.

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# The EPR Crypto-system

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