Researchers from the National Institute of Standards and Technology (NIST) and Boston University have demonstrated a detector that counts single pulses of light, while simultaneously reducing false or "dark counts" to virtually zero.
Reported in the July 28, 2003, issue of Applied Physics Letters*, the advance provides a key technology needed for future development of secure quantum communications and cryptography.
Quantum communications and cryptography is a codemaker's Holy Grail. The idea is to use a rapid series of light pulses (photons) in one of two different states to transmit information in an unbreakable code.
The photon detector project is part of a multi-disciplinary NIST effort to develop the sophisticated measurement methods needed to make quantum communication and cryptography possible. Funding was provided by the Defense Advanced Research Projects Agency (DARPA) and the NIST Advanced Technology Program (ATP).
Most current photon detectors operate best with visible light, cannot reliably detect single photons and suffer from high dark counts due to random electronic noise. The new device operates with the wavelength of near-infrared light used for fiber optic communications and produces negligible dark counts. Instead of using light-sensitive materials, the NIST device uses a tungsten film coupled to a fiber optic communication line. The film is chilled to 120 milliKelvin, at its transition temperature between normal conductivity and superconductivity. When the fiber optic line delivers a photon to the tungsten film, the temperature rises and the apparatus detects it as an increase in electrical resistance.
The device detects about 20,000 photons per second and works with an efficiency of about 20 percent. With planned improvements, the research team hopes to increase efficiencies to greater than 80 percent.
* Miller, A.J., Nam, S.W., Martinis, J.M. and Sergienko, A.V. Demonstration of a low-noise near-infrared photon counter with multi-photon discrimination, Applied Physics Letters (July 28, 2003), Vol. 83, No. 4, pp. 791-793.
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