Indistinguishable photons from a single-photon device

Single-photon sources have recently been demonstrated using a variety of devices, including molecules(1-3), mesoscopic quantum wells(4), colour centres(5), trapped ions(6) and semiconductor quantum dots(7-11). Compared with a Poisson-distributed source of the same intensity, these sources rarely emit two or more photons in the same pulse. Numerous applications for single-photon sources have been proposed in the field of quantum information, but most-including linear-optical quantum computation(12)-also require consecutive photons to have identical wave packets. For a source based on a single quantum emitter, the emitter must therefore be excited in a rapid or deterministic way, and interact little with its surrounding environment. Here we test the indistinguishability of photons emitted by a semiconductor quantum dot in a microcavity through a Hong-Ou-Mandel-type two-photon interference experiment(13,14). We find that consecutive photons are largely indistinguishable, with a mean wave-packet overlap as large as 0.81, making this source useful in a variety of experiments in quantum optics and quantum information.