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12 October 2007


NIST demos first transistor-based single-photon detector

The first transistor-based detector that can count the number of individual photons absorbed has been demonstrated by researchers at the US National Institute of Standards and Technology (NIST), Los Alamos National Laboratory and Heriot-Watt University, UK (E.J. Gansen et al, Nature Photonics 1, 585 (2007), 1 October).

Most other types of single-photon detectors simply ‘click’ in response to any small number of photons. The new ‘QDOGFET’ detector (quantum dot optically gated field-effect transistor) is a modified FET consisting of AlGaAs and a GaAs absorbing layer containing about 1000 quantum dots, and can accurately count one, two or three photons at least 83% of the time.

The transistor could be integrated easily into electronics and may be able to operate at higher temperatures than other single-photon detectors - practical advantages for applications such as quantum key distribution (QKD) for ‘unbreakable’ encryption using single photons.

Counting requires a linear, stepwise response and low-noise operation. This capability is essential for advanced forms of precision optical metrology - a focus at NIST - and could be used both to detect photons and to evaluate single-photon sources for QKD. The new device also has the potential to be cooled electronically, at much higher temperatures than typical cryogenic photon detectors.

In the new QDOGFET detector, as each photon is absorbed, a positively charged hole is trapped by a quantum dot, while the corresponding electron is swept by the applied voltage into the channel. The amount of current flowing in the channel depends on the number of holes trapped by the quantum dots. By measuring the channel response, the detected photons can be counted. NIST measurements show that, on average, each trapped hole boosts the channel current by about 0.2nA. The detector has an internal quantum efficiency (percentage of absorbed photons that result in trapped holes) of 68±18%, which is a record high for this type of photon detector, NIST claims.




Picture: When light enters through the transmission window (see electron micrograph of top of device), it penetrates the GaAs absorbing layer and separates
electrons from the holes that they formerly occupied. Quantum dots (red dots) trap the positively charged holes, while electrons flow into the channel (green Xs). By measuring the channel current, the number of photons that are absorbed can be determined. (Courtesy of NIST.)



The QDOGFET currently detects single photons at wavelengths of about 800nm. By using different semiconductor materials, NIST hopes to make detectors that respond to the longer near-infrared wavelengths used in telecommunications. In addition, researchers hope to boost the external quantum efficiency (the percentage of photons hitting the detector that are actually detected) - currently below 10% - and operate the device at faster speeds.

The NIST research is supported in part by the US government's Disruptive Technology Office, formerly the Advanced Research and Development Activity (ARDA).

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