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Photonics

Phase-Matched Design for Efficient Entangled Photon Pair Generation in 3R-MoS2 Waveguides

Shicheng Yu, Xiaojie Zhang, Lei Xia, Liang Zhai

Published Nov 8, 2025• 0 citations
Publisher

Abstract

Entangled photons are essential for photonic quantum technologies. Their generation typically relies on spontaneous parametric down-conversion, but conventional nonlinear crystals are bulky and hard to integrate on chips. Rhombohedral-stacked MoS2 combines a high refractive index, large second-order nonlinearity, and flexibility for heterogeneous integration, making it a promising platform for integrated quantum photonics. However, the typical thin-film form of 3R-MoS2 restricts the effective nonlinear interaction length, limiting entanglement generation efficiency in practical devices. To overcome this, phase-matching strategies in integrated waveguides are required but have so far remained undeveloped. Here, we introduce a waveguide-integrated 3R-MoS2 platform with periodic grooves to achieve quasi-phase matching, enhancing down-conversion efficiency. Leveraging χ(2) tensor symmetries and orthogonal waveguide modes, the design efficiently generates entangled photons, providing a compact, scalable route toward 2D-material-based integrated quantum photonic circuits.

DOI
10.3390/photonics12111100

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Entangled photons are essential for photonic quantum technologies. Their generation typically relies on spontaneous parametric down-conversion, but conventional nonlinear crystals are bulky and hard to integrate on chips. Rhombohedral-stacked MoS2 combines a high refractive index, large second-order nonlinearity, and flexibility for heterogeneous integration, making it a promising platform for integrated quantum photonics. However, the typical thin-film form of 3R-MoS2 restricts the effective nonlinear interaction length, limiting entanglement generation efficiency in practical devices. To overcome this, phase-matching strategies in integrated waveguides are required but have so far remained undeveloped. Here, we introduce a waveguide-integrated 3R-MoS2 platform with periodic grooves to achieve quasi-phase matching, enhancing down-conversion efficiency. Leveraging χ(2) tensor symmetries and orthogonal waveguide modes, the design efficiently generates entangled photons, providing a compact, scalable route toward 2D-material-based integrated quantum photonic circuits.
Transition metal dichalcogenides (TMDCs) thinned-down to the monolayer level allow the realisation of very broad-band, phase-matching free nonlinear devices. A particularly interesting application would be the generation of entangled photon-pairs, which form the backbone of many recent implementations in quantum optics. However, the limited absolute efficiency of monolayers has so far only allowed seeded parametric amplification [1], experimental evidence for the generation of entangled photon-pairs through spontaneous parametric down-conversion (SPDC) remained elusive up to now [2]. A promising route for the realization of broad-band photon-pair sources in van der Waals materials are moderately thicker, yet still phase-matching free layer stacks with preserved noncentrosymmetry [3], [4]. Here, we close the gap of a missing entangled photon-pair source in TMDCs and demonstrate, for the first time to our knowledge, pair generation via SPDC in 3R-MoS2.
Abstract Ultracompact entangled photon sources are pivotal to miniaturized quantum photonic devices. Van der Waals (vdW) nonlinear crystals promise efficient photon-pair generation and on-chip monolithic integration with nanophotonic circuitry. However, it remains challenging to generate maximally entangled Bell states of photon pairs with high purity, generation rate, and fidelity required for practical applications. Here, we realize a polarization-entangled photon-pair source based on spontaneous parametric down conversion in an ultrathin rhombohedral tungsten disulfide (3R-WS 2 ) crystal. This vdW entangled photonic source exhibits a high photon-pair purity with a coincidence-to-accidental ratio of above 800, a generation rate of 31 Hz, and two maximally polarization-entangled Bell states with fidelities exceeding 0.93 and entanglement degree over 0.97. These results stem from scalable optical nonlinearity, enhanced second-order susceptibility by electronic transitions, and a well-defined symmetry-enabled selection rule inherent in 3R-WS 2 . Our polarization entangled photon source can be integrated with photonic structures for generating more complex entangled states, thus paving an avenue for advanced quantum photonic systems toward computation and metrology.
Efficient generation of entangled photons typically relies on spontaneous parametric down-conversion (SPDC) in phase-matched macroscopic nonlinear media. However, generating entanglement under phase-matching constraints requires additional bulk optics or interferometers. In contrast, ultrathin van der Waals semiconductors - such as transition metal dichalcogenides (TMDs) - exhibit strong enough optical nonlinearities for SPDC to be observed from subwavelength-thick media, thereby bypassing conventional phase-matching constraints. In this microscopic domain, the intrinsic crystal symmetry governs the nonlinear optical response, enabling the native generation of polarization-entangled photon pairs. However, generating these states efficiently has been fundamentally restricted by the material's coherence length (LcL_cLc​), which limits the attainable conversion efficiency. Here, we investigate periodically-poled TMDs (PPTMDs) designed to scale up this interaction via quasi-phase matching. We demonstrate that mechanically flipping the sign of the nonlinearity at precise intervals of LcL_cLc​ introduces quasi-phase matching, that scales the pair-production rate while preserving the pristine, symmetry-generated polarization entanglement, with fidelities exceeding 99%. Backed by a rigorous theoretical model, our work clarifies the interplay between crystal symmetry and propagation effects in thin nonlinear media, providing a new avenue for engineering quantum light in nanophotonic systems.
Advanced photonic quantum technologies—from quantum key distribution to quantum computing—require on-chip sources of entangled photons that are both efficient and readily scalable. This theoretical study demonstrates the generation of polarization-entangled Bell states in structurally simple waveguides by exploiting the intrinsic properties of nonlinear crystals. Elaborate phase-matching strategies that commonly involve the spatial modulation of a waveguide’s linear or nonlinear optical properties are thereby circumvented. This study begins with the derivation of general criteria for the second-order susceptibility tensor that enable the generation of cross-polarized photon pairs via spontaneous parametric down-conversion in single-material waveguides. Based on these criteria, all birefringent, non-centrosymmetric crystal classes are then categorized regarding their suitability. Finally, using coupled mode theory, a numerical analysis is performed of cuboid waveguides made from two materials that are highly relevant to integrated photonics: lithium niobate, a well-established platform, and barium titanate, an emerging alternative. Barium titanate is found to consistently outperform lithium niobate by providing a higher nonlinear efficiency and high concurrence over a significantly broader spectral range. These findings outline a practical route towards highly efficient, fabrication-friendly, and scalable sources of polarization-entangled photons for integrated quantum photonic circuits.
I will discuss our recent results demonstrating exceptional nonlinear optical frequency conversion in transition metal dichalcogenides (TMDs). By fabricating nano-photonic elements such as waveguides, metasurfaces, and periodically poled stacks, we achieve macroscopic frequency conversion efficiencies over microscopic distances, 10 to 100× smaller than current systems with similar performances. Further, we report the broadband generation of entangled photon pairs at telecom wavelength via quasi-phase-matched spontaneous parametric down-conversion, showing the highest coincidence-to-accidental-ratio (CAR) ever demonstrated in a vdW material. This work opens the new and unexplored field of phase-matched nonlinear optics with microscopic van der Waals crystals, unlocking applications that require simple, ultra-compact technologies such as on-chip entangled photon-pair sources for integrated quantum circuitry and sensing.