Sensitive sensor spots single photons

By Chhavi Sachdev, 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 heat.

The device could spot defects in computer chips, improve communications between planets, and even enable snoop-proof secret messages.

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 a photon.

"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 pick up.

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 quantum cryptographic 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.

Timeline:  Now
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.


October 31, 2001

Page One

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Sensitive sensor spots single photons

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