Sensors that detect and count single photons, the smallest quantities of light, with 88 percent efficiency have been demonstrated by physicists at the National Institute of Standard and Technology (NIST). This record efficiency is an important step toward making reliable single photon detectors for use in practical quantum cryptography systems, the most secure method known for ensuring the privacy of a communications channel.
The four yellow squares in the center of this micrograph are NIST single photon detectors. The top two detectors are 25 by 25 micrometers. The bottom two detectors are 50 by 50 micrometers. The detectors operate with a record 88 percent efficiency. Credit: NIST
Described in the June issue of Physical Review A, Rapid Communications,* the NIST detectors are composed of a small square of tungsten film, 25 by 25 micrometers and 20 nanometers thick, chilled to about 110 milliKelvin, the transition temperature between normal conductivity and superconductivity. When a fiber-optic line delivers a photon to the tungsten film, the temperature rises and results in an increase in electrical resistance. The change in temperature is proportional to the photon energy, allowing the sensor to determine the number of photons in a pulse of monochromatic light.
This type of detector typically has limited efficiency because some photons are reflected from the front surface and others are transmitted all the way through the tungsten. NIST scientists more than quadrupled the detection efficiency over the past two years by depositing the tungsten over a metallic mirror and topping it with an anti-reflective coating, to enable absorption of more light in the tungsten layer.
Laura Ost | EurekAlert!
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