The failure of "uncrackable" codes has brought down countless armies and diplomats. But the principles of quantum mechanics offer the cloak-and-dagger community an opportunity to learn whether someone has eavesdropped on their messages. A paper in this week's issue of Nature brings quantum cryptography a step closer to practical application by describing a system that can send secure strings of digits to multiple users over a distance of kilometers.
Like earlier efforts, the scheme, designed by Paul Townsend of the British Telephone Laboratories in Ipswich, U.K., relies on Heisenberg's uncertainty principle--the German physicist's famous insistence that any measurement of a system changes its state. A person who receives a photon that may be linearly or circularly polarized, for example, has to choose the appropriate filter to determine the direction of polarization. The wrong kind of filter--a linear filter for a circularly polarized photon, or vice versa--not only won't elicit any information; it also will destroy the information contained in the photon.
In Townsend's scheme, Alice (as the sender is traditionally known) sends Bob (the receiver) a sequence of photons, randomly polarized either linearly, in the vertical or horizontal directions, or circularly, to the left or the right. Bob randomly views each photon through a linear or circular filter. At the end of the transmission, Bob calls Alice on the telephone and tells her which filter he used to view each photon. Alice then tells him which photons were viewed with the correct filter; he discards the rest of the data.
Now Alice and Bob have identical sequences of 0s and 1s (0 for a vertically- or left-polarized photon, 1 if it's horizontally- or right-polarized). By checking this string for errors, Bob and Alice can see whether anyone has tried to intercept the transmission: An eavesdropper's efforts to view the polarization of the photons will ruin the ones for which he used the wrong filter, causing mistakes in the transmission. "If you take the conservative view and assume that all the errors are caused by an eavesdropper," says Townsend, the message is discarded. An uncontaminated string of digits allows Bob to read a later message transmitted by conventional means.
With the help of beam splitters and supersensitive detectors, Townsend put the scheme to work in a full-sized fiber-optic network with one sender and three receivers; each path was more than 5 kilometers long. Such local cryptographic networks could have practical value for diplomatic and military installations, says Charles Bennett, a cryptographer at IBM's Thomas Watson Laboratory: "Say you had two different buildings; you could connect them with a fiber--you don't have to guard or inspect it." Richard Hughes, a physicist at Los Alamos National Laboratory, agrees. Quantum cryptography "is moving out of the physics lab," he says, and coming closer to covert operations.