Efficient Detector for Single Photons

In a single-photon detector, individual photons from a light source produce detectable electronic signals as well as pulses of electronic noise that are correlated with the original signal. NIST’s newly patented detection system reduces this noise and increases the detector’s efficiency, improving the ability to detect single photons. (Source: Bienfang / NIST)

In a single-photon detector, individual photons from a light source produce detectable electronic signals as well as pulses of electronic noise that are correlated with the original signal. NIST’s newly patented detection system reduces this noise and increases the detector’s efficiency, improving the ability to detect single photons. (Source: Bienfang / NIST)

Individual photons of light now can be detected far more effi­ciently using a device patented by a team including the National Institute of Standards and Techno­logy NIST, whose scientists have overcome long­standing limi­tations with one of the most commonly used type of single-photon detectors. Their invention could allow higher rates of trans­mission of encrypted electronic information and improved detection of greenhouse gases in the atmo­sphere.

Light is widely used for communi­cations, carrying phone conver­sations and video signals through fiber-optic cables around the world in pulses composed of many photons. Single photons are the weakest signal that can be transmitted, and they too have a wide range of appli­cations, including observing how single molecules or atoms behave, imple­menting an en­cryption technique called quantum key distri­bution, and creating high-reso­lution maps. However, single photons of light typi­cally have very little energy, making them difficult to detect.

A common type of detector — based on indium-gallium-arsenide semi­conductors — has been catching indi­vidual photons for years, and it is widely used in quantum crypto­graphy research because it can detect photons at the particular wave­lengths that travel through fiber. Unfor­tunately, when the detector receives a photon and outputs a signal, sometimes an echo of electronic noise is induced within the detector. Tradi­tionally, to reduce the chances of this happening, the detector must be disabled for some time after each detec­tion, limiting how often it can detect photons. Recent research has shown that the noisy echo can also be suppressed by acti­vating the detector only for very short times, effec­tively creating a “gate” in the detector that only opens briefly to accept signals. Unfor­tunately, this also means that the photons have to arrive only during those short intervals.

The team, which also includes scientists working at the California Institute of Techno­logy and the Univer­sity of Maryland, has patented a method to detect the photons that arrive when the gates are either open or closed. The NIST team had developed a highly sensi­tive way to read tiny signals from the detector, a method that is based on electronic inter­ferometry, or the combining of waves such that they cancel each other out. The approach allows readout of tiny signals even when the voltage pulses that open the gate are large, and the team found that these large pulses allow the detector to be operated in a new way. The pulses turn on the detector during the gate as usual. But in between gate openings the pulses turn the detector off so well that signals produced by absorbing a photon can linger for a while in the device. Then the next time the gate opens, these lin­gering signals can be ampli­fied and read out.

The new detector can count indi­vidual photons at a very high maximum rate — several hundred million per second — and at higher than normal effi­ciency, while main­taining low noise. Its efficiency is at least 50 percent for photons in the near infrared, the standard wavelength range used in tele­communi­cations. Commercial detectors operate with only 20 to 30 percent efficiency. The added ability to detect photons that arrive when the gate is closed increases the detector’s efficiency, an improve­ment that would be parti­cularly beneficial in appli­cations in which photons could arrive at any moment, such as atmo­spheric scanning and topo­graphic mapping.

“Single-photon detectors are useful for sensing the presence of some green­house gases in the atmo­sphere by sending a laser pulse into the air and seeing when photons come back,” said NIST physicist Josh Bienfang. “Fast-gated detectors are great for this because they can count efficiently and at high rates, but they are only sen­sitive during the gates because, of course, the returned photons can arrive at any time. Our new method can address this short­coming.” (Source: NIST)

Reference: J. C. Bienfang et al.: Photon detector and process for detecting a single photon, online July 26 2016, U.S. Patent No. 9,401,448

Link: Quantum Optics Group, National Institute of Standards and Technology NIST, Gaithersburg, US

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