Quantum-Dot Photon Detectors

Physicists at Toshiba Research Europe and the University of Cambridge have developed a device that can efficiently detect single photons, an achievement that should assist researchers in a number of diagnostic fields, such as medical imaging, chemical analysis, and environmental monitoring. The device depends on a quantum dot, a tiny semiconductor island that, owing to its essentially zero-dimensional physical extent (a disk 30 nm wide and 8 nm tall), forces electrons to possess only certain discrete energies.

Indeed, quantum dots are sometimes referred to as artificial atoms because of their small size and quantized electron energy states. This quantum dot is encased inside another semiconductor structure called a resonant tunneling diode. In the diode two conducting gallium-arsenide layers are separated by an insulating aluminum-arsenide layer. If the GaAs layers have the right voltage alignment a current can tunnel from the one layer to the other. If misaligned, little current flows. Here’s where the quantum dot comes in. The layers can be purposely slightly misaligned in such a way that capture by the dot of a “hole” excited in the diode by an incident photon can re-align the two GaAs layers, allowing the tunneling current to resume. In other words, the arrival of a photon in the dot results in the switch-on of the diode.

This form of single-photon detection gets around the frequent false detections arising from the avalanche of electrons needed in the common amplified-photoelectron approach to photon detection. Right now, the device correctly detects single photons at a rate of 12%, but this should shortly rise to 65%, Toshiba physicist Andrew Shields (andrew.shields@crl.toshiba.co.uk, 44-1223-436900, www.QUANTUM.TOSHIBA.CO.UK) believes. At that level the dot-diode detector could speed up bit rates used in quantum cryptography and other forms of quantum information processing. (Blakesley et al., Physical Review Letters, 18 February 2004)