Phys. Rev. Lett. 108, 093602 (2012)

Clemens Matthiesen and co-workers from the University of Cambridge and the University of Rochester have reported the emission of highly coherent (sub-natural linewidth) single photons from InAs quantum dots. The researchers used a confocal microscope to collect optically excited resonant fluorescent spectra at a temperature of 4 K. The InAs quantum dots were embedded in a Schottky diode heterostructure, which allowed their excitonic energy levels to be tuned through the d.c. Stark effect. To combine the processes of absorption and emission into a single coherent event, the researchers decreased the excitation power so that the Rabi frequency was less than a fifth of the spontaneous emission rate. Strong antibunching in the intensity-correlation measurement suggests that the generated photons were non-classical in nature. The researchers measured a coherence time of 22 ns — 30 times longer than the lifetime of the quantum dot transition — through field-correlation measurements using a Michelson interferometer. The single photons exhibited a linewidth of 7 MHz, which is a metric inherited from the excitation laser.