Detecting single photons with confidence is not easy: National Institute of Standards and Technology (NIST) has developed "the world’s most efficient single photon detector", which is able to count individual photons with roughly 99 percent efficiency. The basic principle of the detector is to use a superconductor as an ultra-sensitive thermometer. Each individual photon hitting the detector raises the temperature—and increases electrical resistance—by a minute amount, which the instrument registers as the presence of a photon.
“When these detectors indicate they’ve spotted a photon, they’re trustworthy. They don’t give false positives,” says Sae Woo Nam, a physicist with 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.”
“We can’t be sure from direct measurement that we’ve achieved 99 percent efficiency because the metrology is not in place to determine how close we are—there’s no well-established technique,” Nam says. “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 work was presented at the SPIE Symposium on Defense, Security, and Sensing in Orlando World Center Marriott Resort and Convention Center on April 7, 2010: A.E. Lita, B. Calkins, L.A. Pellouchoud, A.J. Miller and S. Nam. Superconducting transition-edge sensors optimized for high-efficiency photon-number resolving detectors.