Bits of uncertainty: quantum security.

Bits of uncertainty: Quantum security

The trouble with sending a secret message is that the recipient must have a key for deciphering it. This means the two parties must initially either meet in person or risk sending the key by some less secure communications channel, and that invites interception. Inspired by an idea first proposed nearly a decade ago, a group of researchers has now designed and constructed a device that uses the uncertainty principle of quantum physics to provide a safe but public means for transmitting vital, secret information.

The device uses extremely faint flashes of light -- only one photon per flash -- to carry messages. Each photon has a certain linear polarization (whether the electric field associated with the light is oscillating horizontally or vertically) and a certain circular polarization (whether the electric field is rotating in a right-handed or left-handed sense about its direction of travel). According to the uncertainty principle, there's no way to measure a photon's linear and circular polarizations simultaneously. Measuring one disturbs the other.

A sender can use the polarizations of individual photons to send a sequence of signals to the receiver, randomly choosing whether to encode a bit of information as a specific linear or circular polarization. For each photon detected, the receiver chooses randomly which type of polarization to measure. About half the polarization measurements would match the values the sender transmitted. By ascertaining which photons were correctly measured, the sender and receiver could derive a code, known only to them, which would serve as a key for encrypting and deciphering messages.

Because any measurement attempted by a third party would unpredictably alter a photon's polarization, an eavesdropper couldn't intercept the transmission without irrevocably scrambling the message and alerting both the sender and receiver to the surreptitious surveillance. To check for eavesdropping, the receiver would simply compare notes with the sender, ascertaining what the results for a number of selected measurements should have been. Statistical deviations from the expected results would signal an eavesdropper's presence.

This so-called "quantum public key distribution" system is the first communications system ever built to depend on the uncertainty principle to ensure secrecy, say its inventors, Charles H. Bennett of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y., and Gilles Brassard of the University of Montreal. "The system relies on the uncertainty principle to enable its users to detect eavesdropping on the quantum channel, even by an opponent with superior technology, and reject the compromised transmissions."

After playing with the idea for several years, Bennett and a colleague constructed a working model of the system last summer. The device consists of tiny diode lasers for generating faint light flashes and detectors fro picking up the signals. The entire appratus sits within a light-tight box about 13 inches long. A computer program controls the apparatus, tallies the signals sent, received and intercepted, and displays the results.

Because it is relatively slow and can be used only for communicating random bits, the apparatus is best suited for transmitting cryptographic keys. Once the two users establish a key, they can exchange secret messages by way of a faster, conventional communications channel.

However, the device's present size severely limits its usefulness. Bennett, who described his demonstration model at last week's Eurocrypt conference in Aarhus, Denmark, now plans to build an improved device using an optical-fiber cable for transmitting light pulses over distances up to 500 meters. Going to greater lengths is tricky because the light pulses must necessarily be weak, which means they travel only a limited distance along optical fibers before fading away.