A photon trap. The heart of the detector, which has been around for a couple of years, is a wire 100 nanometers wide that meanders like coils on a refrigerator to increase the area of detection. The wire is cooled to just above absolute zero, at which temperature it becomes a superconductor. When a photon hits the wire and is absorbed, the wire heats up just enough to stop superconducting, creating a detectable jump in resistance.
In the new design, the photons that slip past or reflect off the wire bounce around in the photon trap, giving them more chances to be absorbed by the wire. The trap, with a little help from an antireflective coating, approximately tripled the efficiency of previous detection efforts.
Drop-off in the number of photons limits the range of quantum cryptography to 100-150 kilometers. But, according to Michael LaGasse, vice president of engineering at the Somerville, MA, labs of MagiQ, which is already commercializing quantum cryptography, a detector as efficient as Berggren's could double or triple these distances. Because of cost considerations, however, LaGasse says the new detectors are likely to find only niche markets, such as in military applications.