Using essentially the same technology that enabled them to achieve 88 percent detection efficiency five years ago, the team has significantly improved its photon detection capabilities by refining the detector's alignment and the optical fibers that guide the photons within it. The detector's basic principle is to use a superconductor as a highly sensitive thermometer. Each individual photon that strikes the detector raises the temperature and increases the electrical resistance by a certain amount, which the instrument registers as the presence of a photon.

According to team member Sae Woo Nam, the advantage of this unique type of photon detector is that the new detector design not only measures the lowest levels of light ever possible, but does so with remarkable accuracy.

"When these detectors indicate that they have detected a photon, they are reliable. They don't give false positives," says Nam, a physicist at NIST in the Optoelectronics Division. "Other types of detectors have very high gain to measure a single photon, but their noise levels are such that occasionally a glitch is mistakenly identified as a photon. This causes a measurement error. Reducing these errors is really important for those doing the calculations or communications."

The ability to count individual photons is very valuable to designers of certain types of quantum computers, as well as scientists working on quantum optical experiments, which deal with 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 securing long-distance data transmission by preventing unwanted interception. A detector that can identify when a photon that was part of a transmission has disappeared would be a very important defense against information theft.
The team has optimized detection for 810 nanometers, an infrared wavelength, and it still has high efficiency at other wavelengths that are attractive for fiber optic communications, as well as for the quantum optics community. Ironically, the detector is so efficient that it surpasses the ability of current technology to determine its precise effectiveness.

"We can't be sure with direct measurement that we've reached 99 percent efficiency because current metrology isn't able to determine that we're at that percentage," says Nam. "The important thing about our latest breakthrough is that we can measure the detection efficiency almost the same for every device we build."
The team is working to develop evaluation techniques that can measure the detector's capabilities, and Viet notes that the team's creation could also help evaluate other light-harvesting devices.