Converting laser light into single photons with ultralong coherence time

Single photons with ultralong coherence time and high indistinguishability are key resources for interference-based photonic quantum technologies. While laser light exhibits excellent coherence, it cannot be attenuated to single photons through linear optics. Here, we demonstrate high-quality single photons coherently reflected from a single quantum dot (QD) coupled to a double-sided optical microcavity and driven by a continuous-wave single-frequency laser in the low-driving regime. Measuring the reflected light without cross-polarization filtering to suppress laser scattering, we observe a second-order correlation of g (2) (0)=0.030±0.002 and a two-photon interference visibility of V (0)=94.3%±0.2%, which remains robust for photon separations up to 8 km. Under weak driving, these photons exhibit an ultralong coherence time of 258±2µs—six orders of magnitude longer than the coherence time (115 ps) of spontaneously emitted photons—directly inherited from the driving laser. The observed anti-bunching arises from single-photon switching governed by the QD’s saturation nonlinearity and is further explained by quantum interference between the reflected driving field and incoherent scattering. By inheriting the laser’s ultralong first-order coherence and robust photon indistinguishability, coherently scattered single photons hold promise as valuable resources for quantum information science and photonic quantum technologies.