sensor spots single photons
Technology Research News
Stars and faulty transistors both emit
infrared radiation, or heat, and knowing exactly how much heat distant
stars and tiny transistors give off is useful. Researchers at the Moscow
State Pedagogical University and the University of Rochester have developed
a device that detects minuscule amounts.
The researchers' single-photon detector counts both infrared and visible
light photons, whether they are emanating from cooling stars or from a
misfiring transistor in your PC. It is more sensitive than current semiconductor
detectors, which are prone to giving 'dark counts,' or seeing photon
flashes when there aren't any, and which cannot see photons of infrared
The device could spot defects in computer
chips, improve communications between planets, and even enable snoop-proof
The device consists of very narrow, ultrathin superconducting strips of
niobium nitride that contain a hotspot of excited, warm electrons that
switch from a superconducting to a resistive state when they encounter
"The absorber strip is... initially operating in the superconductive state,
but very close to being in the resistive state. When a single photon is
absorbed into the superconducting strip, it is driven into the resistive
state and generates a measurable voltage that can be readily displayed
on a fast oscilloscope," said Carlo Williams, a member of the research
team, now a development scientist at Corning, Inc.
The device takes 30 picoseconds, or trillionths of a second, to catch
a photon and emit the resulting voltage, which translates to a speed of
over 10 Gigahertz, or10 billion distinct photons every second, said Roman
Sobolewski, a professor of electrical and computer engineering at the
University of Rochester.
It counts photons in the wavelength range of 0.4 to 3 microns, which covers
visible light and heat down to the mid-infrared range, Sobolewski said.
The detector could be used to evaluate working integrated circuits to
weed out those that leak electroluminescence, or electrical energy that
is converted into light, said Williams. "This could increase the yield
of manufactured integrated circuits with the minimum defects."
The photon detector is accurate even when the photon emission rate is
less than 10 photons per second, said Sobolewski. A candle flame emits
a hundred thousand trillion photons per second. Because the detector is
so sensitive, it could be used to measure ultraweak electroluminescence
from tiny nano circuits, which is hard for current photo detectors to
It could also be used in fast, free-space optical communication systems
for planetary exploration and earth-orbiting missions, said Sobolewski.
Light signals transmitted between earth and space are weakened by dust
and particles in the atmosphere and space; the highly sensitive single-photon
detector could boost the capacity of interplanetary communications.
The single-photon detector is also a prime candidate for making high-speed
devices, Sobolewski said.
Quantum cryptography promises perfectly secure communications because
anyone who eavesdrops on a message inevitably alters the stream of photons,
leading to the detection of the snooping. A number that is sent undetected
can be used as a key to encrypt a subsequent message and only the holder
of the key can decrypt the message. Because the detector can sense many
photons per second it would allow for high data rates, according to Sobolewski.
The device could be used in quantum cryptography as well as fundamental
tests of quantum physics and quantum computation,
which all work with single particles of matter, including photons, said
Emanuel Knill, a mathematician at the Los Alamos National Laboratory.
There is still a lot of work to be done, however. Quantum computation
requires that a single photon can be detected nearly 100% of the time,
said Knill. The best photodetectors today can achieve detection probabilities
above 90%, but at speeds much slower than the Moscow device. The 20% efficiency
of the device shows promise given the very high repetition rates, he said.
"The high repetition rates with low dark counts are useful in quantum
information processing, particularly for increasing the bit rates in quantum
cryptography. If the efficiency can be substantially improved, their device
could be a powerful tool for quantum information processing," said Knill.
"One of the tasks of this continuing project is to improve this number,"
said Williams. The device could be in practical use soon, Sobolewski said.
The researchers published their work in the journal Applied Physics Letters.
The research was funded by Schlumberger Semiconductor Solutions; The Office
of Naval Research (ONR), NATO, and The U.S. Civilian Research and Development
Foundation for the Independent States of the Former Soviet Republic.
Funding: Corporate; Government
TRN Categories: Optical Computing; Optoelectronics and Photonics
Story Type: News
Related Elements: Technical papers, "Picosecond Superconducting
Single-Photon Optical Detector," Applied Physics Letters, August 6, 2001.
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