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

Yanfeng Li1,2,3,*, Manman Wang1,2,3,*, Chuanyu Zeng1,2,3, Hanqing Liu4,5, Haiqiao Ni4,5, Zhichuan Niu4,5, and Chengyong Hu1,†

  • *These authors contributed equally to this work.
  • †Contact author: hucy@baqis.ac.cn
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Phys. Rev. Research 7, 043128 – Published 3 November, 2025

DOI: https://doi.org/10.1103/qsyr-ddw4

Abstract

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.

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Phys. Rev. Research 7, 043128– Published 3 November, 2025
  • Received 24 June 2025
  • Accepted 9 October 2025
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