Counter-propagating Entangled Photon Pairs from a Monolayer

TL;DR

This paper demonstrates the first experimental observation of polarization-entangled photon pairs emitted from a monolayer material, combining theoretical modeling with experimental validation of counter-propagating SPDC in atomically thin films.

Contribution

It provides the first experimental validation of counter-propagating SPDC in a monolayer, showing high-fidelity Bell states and broadband emission, advancing scalable quantum light sources.

Findings

Confirmed theoretical predictions of symmetric broadband emission.

Achieved high-fidelity Bell states in counter-propagating configuration.

Demonstrated polarization-entangled photon pairs from a monolayer.

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.