Quantum dots detect single photons

31 Oct 2007

Quantum computing applications could benefit from research into single-photon detectors based on semiconductor quantum dots.

A semiconductor detector based on quantum dots is helping researchers at the National Institute of Standards and Technology (NIST) to detect photons quickly and efficiently. What's more, they claim the detector is not only sensitive to single photons but can also directly measure the number of detected photons (Nature Photonics 1 585).

"Our major achievement has been in efficiency and photon-number resolution," Eric Gansen, a researcher at NIST, told optics.org. "Historically, quantum-dot transistor detectors have exhibited low detection efficiency. Our device exhibits single-shot single-photon sensitivity, a linear response and low dark counts."

The ability of detectors to determine the number of photons is important for applications such as linear optics, quantum computing and for characterizing non-classical states of light.

"Our devices are attractive single-photon detectors for a number of reasons," said Gansen. "They have photon-number resolving capabilities - an attribute not exhibited by commercially available single-element APDs. Their compact semiconductor composition makes them compatible with traditional integrated circuitry. Finally, they can potentially operate at temperatures higher than other single-photon detectors."

The team's device is essentially a quantum dot, optically-gated, field-effect transistor. When the active area of the detector is illuminated, photo-generated carriers trapped by the quantum dots screen the gate field, causing a change in channel current that is proportional to the number of confined carriers.

"We tailored the semiconductor layers and electric fields to efficiently detect photons absorbed in the absorption layer of the device," explained Gansen. "When a reverse bias is applied to the gate, electrons and holes in the absorption layer are separated by the internal electric field. The holes are directed towards the quantum dots, where they are trapped, while the electrons are swept in the opposite direction."

The amount of current flowing in the channel depends on the number of holes trapped by the quantum dots. By measuring the channel response, Gansen and his colleagues can count the number of detected photons. "We characterized the device by illuminating it with a train of laser pulses and by monitoring the change in the transistor's channel current caused by each pulse," explained Gansen. "Discrete numbers of detected photons produced well-resolved changes in the channel current."

The team plans to operate its devices at increased speeds and higher temperatures and is trying to reduce the noise in the detection circuitry. "We want to focus on increasing the overall detection efficiency of the devices by incorporating the structures in resonant cavities," said Gansen. "We will also tailor the material composition in order to detect telecommunication wavelengths."