Quantum correlations of spontaneous two-photon emission from a quantum dot

Spontaneous two-photon emission (STPE) is a second-order quantum radiation process with implications in astrophysics¹, atomic physics² and quantum technology³. In particular, on-demand STPE from single quantum emitters has long been predicted to revolutionize photonic quantum science and technology^4,5. Here we report STPE with brightness comparable to that of competing single-photon radiation from a single semiconductor quantum dot deterministically coupled to a high-quality micropillar cavity. This is because of strong vacuum fluctuations in the microcavity, which drive a biexciton directly to the ground state. We show the quantum nature associated with STPE in the cavity quantum electrodynamics regime using photon statistics measurements. Furthermore, STPE is exploited to build unconventional entangled quantum light sources that can simultaneously achieve near-unity entanglement fidelity for spontaneous parametric down-conversion sources and on-demand photon emission for atomic quantum emitters. Our work provides insights into the two-photon process in the quantum regime, which could empower photonic quantum technology with nonlinear quantum radiation.