Two-photon interference between an independent laser and single photons with ultralong coherence time

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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mn>72</a:mn> <a:mo>±</a:mo> <a:mn>2</a:mn> <a:mo>%</a:mo> </a:mrow> </a:math> , well above the <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:mn>50</b:mn> <b:mo>%</b:mo> </b:mrow> </b:math> classical limit, thereby confirming the quantum nature of TPI. At the same time, we observe a broad visibility background and a beat visibility below <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mn>50</c:mn> <c:mo>%</c:mo> </c:mrow> </c:math> , signatures of classical interference. Both quantum and classical visibility features remain unchanged for frequency separations up to <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:msup> <d:mn>10</d:mn> <d:mn>4</d:mn> </d:msup> </d:math> 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.