A low-noise receiver developed by a team of scientists at NEC Corp. and Japan Science and Technology Corp., in collaboration with the Telecommunications Advancement Organization of Japan, has enabled the transmission of single 1550-nm photons over 100 km of standard optical fiber. The work has potential in the development of quantum key distribution systems for high-security applications on metropolitan area networks, such as for data communications between government institutions and between central banks.Also known as quantum cryptography, quantum key distribution employs phenomena of quantum mechanics to generate perfectly secure code keys for binary encryption. Although research into the technique has accelerated over the past few years, and early systems --such as that from id Quantique of Geneva -- have appeared in the marketplace, a limitation for its deployment over long fiber optic spans has been a lack of detectors with the sensitivity to distinguish single photons from the background noise.Two potential solutions to this problem include the development of ultralow-noise single-photon detectors and the linking of the signals from two detectors. The researchers in Japan employed the latter tactic in their demonstration of transmission with nonentangled photons, using coupled, gated-mode avalanche photodiodes in a Mach-Zehnder interferometer configuration.In gated mode, the reverse bias on an avalanche photodiode is maintained at just below the breakdown threshold, and a further, pulsed reverse bias is synchronized with the expected arrival of a photon to briefly increase the voltage to above breakdown. This avoids the increased dark counts and the afterpulses of the Geiger mode, but the gate pulses produce spikes in the output signal that can obscure detection. Combining the signals from two nearly identical, similarly gated detectors in a 180° hybrid junction cancels these spikes, reducing the threshold for detection such that fiber scattering becomes the limiting factor in the system.Kazuo Nakamura, senior manager at NEC's Fundamental Research Laboratories in Tsukuba, Japan, explained that conventional distributed feedback laser diodes, heavily attenuated to produce an average photon number per pulse of 0.1, served as the source of 1550-nm single photons in the experiment. Phase modulators, biased at 500 kHz, encoded the photons, and 0.25-dB/km optical fiber was used for the 100-km span. The demonstration system operated at a rate of 7 bits per second, but the researchers expect to be able to increase this to 30 bits per second.Nakamura said that they hope to develop a prototype system for commercialization within one to two years. They also will investigate the use of entangled photons and quantum repeaters for systems that would exceed the 200-km maximum transmission distance allowed by the twin-detector setup if used with low-loss fiber.