Phys. Rev. X 4, 021004 (2014)

Highly sensitive coherent reflectivity measurements have enabled a French team to monitor in real-time single-charge 'jumps' in a semiconductor quantum dot in a cavity. Loic Lanco and co-workers from the Laboratoire de Photonique et Nanostructures in Marcoussis and the Université Paris Diderot say that the strongly enhanced light–matter coupling in the cavity and the use of an almost shot-noise-limited detection arrangement has allowed them to perform measurements on the microsecond timescale — five orders of magnitude faster than previous efforts. The researchers say that the charge transitions in a cryogenically cooled InGaAs quantum-dot micropillar can be monitored with a temporal error of less than 0.2% and that the transitions correspond to the capture and subsequent release of a charge carrier. Light from a continuous-wave single-mode laser with a tunable photon energy was intensity modulated at a rate of 2 MHz and focused onto the micropillar, where it was reflected. The incident and reflected powers were detected by a pair of silicon avalanche photodiodes connected to lock-in amplifiers. The high speed of the technique may make it useful for monitoring other rapid single-quantum events, such as spin flips of a single electron or hole.