Quantum-dot photon detectors

Physicists at Toshiba Research Europe and the University of Cambridge have developed a device that can efficiently detect single photons. The device employs a layer of self-assembled quantum dots—acting as artificial atoms with quantized electron energy states—encased in a resonant tunneling diode that has two conducting gallium arsenide layers separated by an insulating aluminum arsenide layer. If the GaAs layers have the right voltage alignment, a current can tunnel between them; but the physicists purposely misaligned them a bit to prevent such tunneling. A photon striking the diode generates an electron or a hole that can be captured by a nearby quantum dot whose suddenly altered energy state restores the resonance, and the resulting tunneling current is detected. Thus far, the low-noise detection scheme has allowed the researchers to detect single photons with 12.5% quantum efficiency and 150-ns time resolution, but they expect improvements in both quantities. (J. C....