Indistinguishable Photons from a Two-Photon Cascade.

Decay of a four-level diamond scheme via a cascade is a potential source of entangled photon pairs. A solid-state implementation is the biexciton cascade in a semiconductor quantum dot. While high entanglement fidelities have been demonstrated, the two photons, XX and X, are temporally correlated, typically resulting in poor photon coherence. Here, we demonstrate a high two-photon interference visibility (a measure of the photon coherence) for both XX (90±2%) and X (80±6%) photons. This is achieved by Purcell enhancing the biexciton transition in a low-noise device. We find that the interference visibility follows the well-known quantum optics result upon tuning the XX:X lifetime ratio over 2 orders of magnitude. In addition, we show that the XX purity, 98%, is limited not by quantum optics but by the phonons in the semiconductor matrix via cavity feeding.