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arXiv · 2508.06860

Counter-propagating Entangled Photon Pairs from a Monolayer

Zhuoyuan Lu·Jiri Janousek·Syed M. Assad·Shuyao Qiu·Mayank Joshi·Yecheng Hu·Alex Y Song·Chuanyu Wang·Manuka Suriyage·Jie Zhao·Ping Koy Lam·Yuerui Lu
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Abstract

Non-phase-matched spontaneous parametric down-conversion (SPDC) in atomically thin materials provides new degrees of freedom and enhanced quantum information capacity compared to conventional phase-matched sources. These systems emerged as promising platforms for quantum computing, communication, and imaging, with the potential to support higher-order nonlinear processes. However, direct observation of photon-pair emission from a monolayer has remained experimentally challenging. In this work, we theoretically modeled SPDC emission across the full angular space from a monolayer GaSe film and experimentally validated the model through measurements of both co- and counter-propagating photon pairs. We demonstrated two-photon quantum correlations in the telecom C-band from the thinnest SPDC source reported to date. The spatially symmetric, broadband emission predicted by theory was confirmed experimentally. Furthermore, we observed high-fidelity Bell states in the counter-propagating configuration, marking the first realization of polarization-entangled photon pairs from a monolayer. Our results revealed the emission characteristics of SPDC in the deeply subwavelength, non-phase-matched regime, and introduced atomically thin, counterpropagating SPDC as a scalable and integrable platform for programmable quantum state generation, extendable via moir\'e superlattice engineering.

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Zhuoyuan Lu, Jiri Janousek, Syed M. Assad, Shuyao Qiu, Mayank Joshi, Yecheng Hu, Alex Y Song, Chuanyu Wang, Manuka Suriyage, Jie Zhao, Ping Koy Lam, Yuerui Lu. 2025-08-09. Counter-propagating Entangled Photon Pairs from a Monolayer. https://arxiv.org/abs/2508.06860

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