Generation of a near-ideal indistinguishable two-photon state by incoherent light

High-quality quantum states lie at the heart of advanced quantum information processing. The degree of photon indistinguishability is critical for applications from photonic quantum computation to precision metrology. The two-photon Hong–Ou–Mandel (HOM) interference effect provides a rigorous quantification method, with its visibility serving as the ultimate benchmark for source quality. Generally, coherent pumping is widely regarded as indispensable for the preparation of quantum sources. As a result, incoherent light sources have seen limited applications in the current quantum technologies. We generate an indistinguishable two-photon state by incoherent light generated by frequency doubling of amplified spontaneous emission light. The theoretical analysis indicates that phase randomization of the pumping does not affect the coincidence visibility in two-photon intensity interference. Moreover, temporal incoherence further enhances the symmetry of the generated spectrum in second-harmonic generation. In the experiment, the incoherently pumped photon sources exhibit a heralding efficiency of ∼60% and a coincidence-to-accidental ratio exceeding 15,000. The observed HOM interference fringes show the visibility of 99.1% without any spectrum filtering, confirming the near-ideal indistinguishability of the photons. Our study reveals the role of temporal coherence in second-order nonlinear interactions. It provides a potential approach to use an easily accessible incoherent light for engineering high-quality quantum sources.