Portland, Ore. -- Using a superconducting detector, the National Institute of Standards and Technology has set a new world's distance record in quantum-key distribution--an uncrackable encryption technology that ensures absolute security by harnessing the quantum-physics principle that observations affect outcome. Until last week, Toshiba Research Europe Ltd. and Cambridge Research Laboratory held the record for the most sensitive detector with a prototype system that stretched 75 miles. Now NIST has upped the ante by more than 50 percent, reporting last week that it had transmitted over nearly 115 miles.
"With our superconducting ultralow-noise transition-edge sensor, we have dramatically extended the transmission distances for sending secret encryption keys," said Carl Williams, coordinator of the Quantum Information Program at NIST.
Audrius Berzanskis, vice president of security engineering at MagiQ Technologies Inc. (New York), called NIST's single-photon detection technique a boon for "everybody in the business" because "it's three times more efficient than other approaches to date, and it's much faster, allowing encryption keys to be changed more quickly than was possible before." MagiQ offers a commercial quantum-key distribution/encryption system, the Navajo Security Gateway.
Quantum encryption narrows an event down to the polarization of one photon, which cannot be observed without changing its statistics, thus making eavesdropping impossible. Quantum encryp- tion schemes send only the key itself one photon at a time using an ultrasensitive detector, slowing the quantum-key distribution rate to kilobits/second (encrypted information travels at normal data rates).
The more sensitive the detector, the longer the distance a single photon can travel down optical cables without a repeater. NIST's achievement rests on a cryogenic detector that uses superconductivity to sense individual photons. The superconducting detector was invented at NIST's Quantum Testbed facility (Boulder, Colo.) by researcher Sae Woo Nam, who "adapted his design from detectors he designed at Stanford University to measure light coming from very distant objects in space," Williams said.
The ultralow-noise detector is called a transition-edge sensor because it works by cooling a tungsten film to the point where it begins to superconduct: 100 microkelvin. The sensor is ultrasensitive; when biased at the superconducting transition temperature, even a single photon impacting the tungsten film raises its temperature enough to cause a large surge in the tungsten's resistance.
Williams said NIST was sponsoring not only Nam's work, but also several other teams developing ultrasensitive detectors and true single-photon emitters.