Two-photon interference (TPI) underpins many photonic quantum technologies and is commonly regarded as a quantum effect requiring indistinguishable photons. Here, we demonstrate TPI between single photons with ultralong coherence time and an independent continuous-wave laser. The measurements reveal a sharp antibunching dip, governed by the single-photon correlation time, together with a broad Hong-Ou-Mandel dip whose half-width is set by the mutual coherence time of the two sources. We achieve a maximum TPI visibility of 72±2%, well above the 50% classical limit, thereby confirming the quantum nature of TPI. At the same time, we observe a broad visibility background and a beat visibility below 50%, signatures of classical interference. Both quantum and classical visibility features remain unchanged for frequency separations up to 104 times the linewidth of the single photons, showing no distinction between distinguishable and indistinguishable photons. By applying both general wave superposition theory and a quantum field approach, we derive the same cross-correlation functions that quantitatively reproduce the data. Our results show that quantum and classical features of TPI originate from the same underlying fourth-order interference process arising from second-order interference within the mutual coherence time. Photon indistinguishability is a sufficient condition for TPI to occur, but not a necessary one. This insight offers different perspectives for the development of quantum photonic technologies based on independent photons.

Paper link: https://journals.aps.org/prresearch/abstract/10.1103/qsyr-ddw4

Two-photon interference (TPI) is a fundamental phenomenon in quantum optics and plays a crucial role in optical quantum technologies. TPI is commonly considered as quantum interference with an upper bound of 100% for both the TPI and the beat visibility in contrast to its classical counterpart with a maximum visibility of 50%?. However, this is not always the case. Here, we report a simultaneous observation of quantum and classical TPI of single photons with ultralong coherence time, which is five orders of magnitude longer than the photon correlation time. We observe a TPI visibility of 94.3%?±?0.2% well above the 50% classical limit indicating the quantum feature, and a beat visibility of 50% reflecting the classical feature. In addition an anti-bunching central dip due to single-photon anti-correlation, we also observe two bunching side peaks in cross correlation curves for indistinguishable photons. Using the general wave superposition theory and the quantum field approach, we derive the same cross correlation functions, which fully reproduce and explain the experiments. Our results reveal that the quantum and classical features of TPI result from the same interference, as the fourth-order temporal interference arises from the second-order temporal interference between two photons on the timescale of the photon coherence time.

Paper link: https://pubs.aip.org/aip/apl/article-abstract/125/15/154002/3316378/Interference-of-single-photons-with-ultralong?redirectedFrom=fulltext