Quantum cryptography, originally proposed and demonstrated by Charles Bennett at the IBM T.J. Watson Research Center, provides a way to communicate with complete security over an insecure channel such as an unguarded optical fiber. The security is guaranteed by the fundamental quantum properties of light rather than by computational complexity or physical barriers to interception.

Alice

Bob

Eve, a potential eavesdropper is foiled by the quantum properties of light 

In quantum cryptography, two parties, traditionally called Alice and Bob, wish to create a shared secret key that they can use to encrypt and decrypt their messages to each other. To establish this key, Alice encodes information in the quantum states of individual optical photons and transmits them to Bob. Bob's receiver sorts the photons according to their states and sends them to detectors capable of registering the arrival of single photons with high efficiency and low noise. Potential eavesdroppers are foiled by the quantum properties of light, which dictate that any attempts to intercept the quantum information will necessarily disturb it in a way that Alice and Bob can detect and evade.

At the IBM Almaden Research Center scientists are developing a quantum cryptography system based on standard telecommunications optical fiber, lasers, and detectors. The system operates over a 10-km long optical fiber link, and automatically compensates for changes in the photon states caused by variations in the properties of the optical fiber. The development of working quantum cryptography systems at IBM and other laboratories shows that by using quantum states of individual particles to represent information - that is, by using quantum information - a practical task can be accomplished that is impossible by classical means.