Scientists at the National
Institute of Standards and Technology (NIST) have developed* the world's
most efficient single photon detector, which is able to count individual particles
of light traveling through fiber optic cables with roughly 99 percent efficiency.
The team's efforts could bring improvements to secure electronic communication,
advanced quantum computation and the measurement of optical power.
Using essentially the same technology that permitted them to achieve 88 percent
detection efficiency five years ago,** the team has enhanced its ability to
detect photons largely by improving the alignment of the detector and the optical
fibers that guide photons into it. 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.
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,” says 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.”
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 defense against
The team has optimized the detection for 810 nanometers—an infrared wavelength—and
it still has high efficiency at other wavelengths that are interesting for fiber
optic communications, 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 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 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.”
* 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. Presented at the SPIE Symposium on SPIE Defense, Security, and Sensing,
Orlando World Center Marriott Resort and Convention Center, Crystal J1 Ballroom,
3 p.m. April 7, 2010.
** See “NIST Photon Detectors Have Record Efficiency” in NIST Tech
Beat, June 2, 2005, at www.nist.gov/public_affairs/techbeat/tb2005_0602.htm#photon.