Interference of single photons with ultralong coherence time.

This article focuses on the simultaneous observation and theoretical explanation of quantum and classical two-photon interference (TPI) using single photons with ultralong coherence time generated from a single quantum dot (QD) coupled to an optical microcavity. The study reports a TPI visibility of 94.3% ± 0.2%, exceeding the classical limit of 50%, alongside a classical beat visibility of 50%, demonstrating that both quantum and classical features arise from the same fourth-order temporal interference linked to second-order temporal interference on the photon coherence timescale. Experimental results, including anti-bunching central dips and previously unreported bunching side peaks in cross-correlation measurements, are accurately reproduced by general wave superposition theory and quantum field approaches. The findings challenge the conventional view that photon indistinguishability is required for TPI and suggest that laser-converted single photons with inherited coherence could advance quantum information technologies such as long-distance quantum interference and quantum networking.