Toshiba researchers in Cambridge, UK, have developed a quantum dot instrument capable of detecting a single photon. Unlike current devices, such as the photomultiplier tube and the avalanche photodiode, it does not rely on avalanche amplication of the signal and is therefore less prone to noise.
Inside a sandwich of gallium arsenide and aluminium gallium arsenide is a layer of quantum dots, each just a few nanometres in diameter (May p9). When a photon hits the device, an electron escapes from one of the quantum dots and is detected by a change in resistance in a conducting layer just a few nanometres above the dots. So far the device only operates at 4 K, but the researchers' next target is 77 K, and ultimately room temperature operation.