New Quantum Dot Transistor Counts Individual Photons

NIST's modified field-effect transistor can count single photons, or particles of light. When light enters through the transmission window (see electron micrograph of top of device), it penetrates the gallium arsenide absorbing layer and separates electrons from the holes 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, researchers can determine the number of photons absorbed. Credit: NIST