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19 Apr 2010
A single photon detector developed by NIST has a reported efficiency of 99%.
Scientists at the National Institute of Standards and Technology (NIST) in the US say that they have developed the world's most efficient single photon detector. The device is said to be able to count individual photons travelling through fibre optic cables with roughly 99% efficiency. The team's efforts could bring improvements to secure electronic communication, advanced quantum computation and the measurement of optical power.
According to team member Sae Woo Nam, the advantage of this type of single photon detector is that the new detector design not only measures lower levels of light than have ever been possible, but does so with great accuracy.
"When these detectors indicate they've spotted a photon, they're trustworthy. They don't give false positives," said Nam, a physicist in NIST's optoelectronics division. "Other types of detectors have really high gain so they can measure a single photon, but their noise levels are such that occasionally a noise glitch is mistakenly identified as a photon. This causes an error in the measurement. Reducing these errors is really important for those who are doing calculations or communications."
The basic principle of the detector is to use a superconductor as an ultra-sensitive thermometer. Each individual photon hitting the detector raises its temperature – and increases electrical resistance – by a minute amount, which the instrument registers as the presence of a photon.
Using essentially the same technology, the team achieved a detection efficiency of 88% five years ago. The researchers have now enhanced the ability to detect photons largely by improving the alignment of the detector and the optical fibres that guide photons into it.
The ability to count individual photons is valuable to designers of certain types of quantum computers as well as scientists engaged in quantum optical experiments, which concern exotic states of light that cannot be described by classical physics. But one of the most promising potential applications of a high-efficiency photon detector is a way to secure long-distance data transmission against unwanted interception. A detector that could recognize that a photon forming part of a transmission was missing would be a substantial defence against information theft.
The team's detector is optimized for 810 nm but is said to have a high efficiency at other wavelengths that are of interest for fibre-optic communication, as well as the quantum optics community. Ironically, the detector is so efficient that it outstrips current technology's ability to determine its precise efficiency.
"We can't be sure from direct measurement that we've achieved 99% efficiency because the metrology is not in place to determine how close we are – there's no well established technique," commented Nam. "What is great about our latest progress is that we measure nearly the same detection efficiency for every device we build, package and test. It's the reproducibility that gives us confidence."
The team is currently working to develop evaluation techniques that can measure up to the detector's abilities, and Nam says the team's creation could also help evaluate other light-gathering devices.
"NIST offers a standardized service for measuring the efficiency of photodetectors and optical power meters," he says. "We're trying to develop a calibration technique that extends to ultra-low levels of light. It should be valuable for anyone looking at single photons."
• The NIST team presented details of this work at the SPIE Symposium on SPIE Defense, Security, and Sensing, which was held in Orlando earlier this month. Paper title: "Superconducting transition-edge sensors optimized for high-efficiency photon-number resolving detectors". Authors: A E Lita, B Calkins, L A Pellouchoud, A J Miller and S Nam.
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