Best-Kept Secrets; January 2005; Scientific American Magazine; by Gary Stix; 6 Page(s)

At the IBM Thomas J. Watson Research Laboratory, Charles Bennett is known as a brilliant theoretician--one of the fathers of the emerging field of quantum computing. Like many theorists, he has not logged much experience in the laboratory. His absentmindedness in relation to the physical world once transformed the color of a teapot from green to red when he left it on a double boiler too long. But in 1989 Bennett and colleagues John A. Smolin and Gilles Brassard cast caution aside and undertook a groundbreaking experiment that would demonstrate a new cryptography based on the principles of quantum mechanics.

The team put together an experiment in which photons moved down a 30-centimeter channel in a light-tight box called "Aunt Martha's coffin." The direction in which the photons oscillated, their polarization, represented the 0s or 1s of a series of quantum bits, or qubits. The qubits constituted a cryptographic "key" that could be used to encrypt or decipher a message. What kept the key from prying eavesdroppers was Heisenberg's uncertainty principle--a foundation of quantum physics that dictates that the measurement of one property in a quantum state will perturb another. In a quantum cryptographic system, any interloper tapping into the stream of photons will alter them in a way that is detectable to the sender and the receiver. In principle, the technique provides the makings of an unbreakable cryptographic key.