Quantum crypto scheme goes one-way

Quantum cryptography is theoretically capable of enabling perfectly secure communications: if only one photon is used to transmit each bit of a secret, one-time key that can be used to decrypt a message, there is no way an eavesdropper can gain access to the key without the sender and receiver realizing that the key has been compromised.

Data is usually encoded in the cycle, or phase, of each photon. One problem is that heat and vibrations alter optical fiber, which can cause a photon's phase to change, or drift.

Most quantum cryptography systems send signals on a round trip to automatically compensate for changes in the fiber. Existing one-way methods must be recalibrated every few minutes. However, sending signals round-trip leaves the communications vulnerable to some eavesdropping techniques.

Researchers from Toshiba Research have demonstrated a one-way quantum key distribution system that automatically compensates for phase drift.

The system sends a strong reference light pulse 24 nanoseconds after each signal photon. The receiver analyzes the reference pulse and uses the information to calibrate the transmitter. A nanosecond is one billionth of a second.

The researchers' prototype showed an error rate of 0.87 percent and transmitted secret key bits 99.6 percent of the time during a 19-hour session over a 20-kilometer telecom fiber. The tests show that the method is suitable for practical applications, according to the researchers.

The method could be ready for practical use in two years, according to the researchers. The work appeared in the January 24, 2005 issue of Optics Express.