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Advanced Optical Materials

Enhanced and Switchable Generation of Two Polarization Bell States from a Single Metasurface

Chaoxin Shi, Qingqing Mo, Chunxiong Bao, Jianjun Zhang, Jihua Zhang

Published Sep 19, 2025• 2 citations
Publisher

Abstract

Abstract Polarization Bell states are essential resources for various quantum technologies. The most common approach for generating polarization Bell states is through the spontaneous parametric down‐conversion (SPDC) in nonlinear crystals, or more recently, in ultrathin nonlinear metasurfaces. However, both platforms typically require multiplexing two SPDC processes, each producing one of the two orthogonal polarization states whose superposition results in the Bell states. Here, a Bell state source is proposed from a nonlinear metasurface by integrating a 2D and symmetrical SiO 2 nano‐grating on a LiNbO 3 film. This metasurface supports multiple high‐Q guided mode resonances and enables enhanced generation of polarization Bell states without the need of multiplexing. Furthermore, the same metasurface enables switchable generation of two distinct Bell states with high fidelity simply by switching the pump wavelength‐a capability not achievable with conventional crystal sources. This work paves an effective way toward ultrathin, multifunctional entangled photon sources, which are essential for the development of miniaturized, scalable quantum photonic systems.

DOI
10.1002/adom.202500652

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Abstract Polarization Bell states are essential resources for various quantum technologies. The most common approach for generating polarization Bell states is through the spontaneous parametric down‐conversion (SPDC) in nonlinear crystals, or more recently, in ultrathin nonlinear metasurfaces. However, both platforms typically require multiplexing two SPDC processes, each producing one of the two orthogonal polarization states whose superposition results in the Bell states. Here, a Bell state source is proposed from a nonlinear metasurface by integrating a 2D and symmetrical SiO 2 nano‐grating on a LiNbO 3 film. This metasurface supports multiple high‐Q guided mode resonances and enables enhanced generation of polarization Bell states without the need of multiplexing. Furthermore, the same metasurface enables switchable generation of two distinct Bell states with high fidelity simply by switching the pump wavelength‐a capability not achievable with conventional crystal sources. This work paves an effective way toward ultrathin, multifunctional entangled photon sources, which are essential for the development of miniaturized, scalable quantum photonic systems.
Flat optics has emerged as a promising platform for ultrathin polarization-entangled sources via spontaneous parametric downconversion in subwavelength-thick nonlinear films or metasurfaces. However, previous schemes typically required multiplexing two films or two meta-structures with different orientations. Here, we propose a nonlinear metasurface composed of a single silica grating atop a lithium niobate film to generate polarization-entangled photon pairs. By engineering the angular dispersion of metasurface resonances, adjusting the pump frequency, and leveraging the polarization correlated transverse phase matching, the |HH〉 and |VV〉 states are simultaneously enhanced by over 2000 times, while the |HV〉 and |VH〉 states remain similar when compared to the pure film. Therefore, the degree of polarization entanglement is significantly improved. Furthermore, by tuning the pump polarization and film orientation, and applying spatial filtering, the concurrence of the two-photon state is optimized to near one, meaning maximal entanglement. Such an ultrathin and spatial-separated-entangled photon-pair source will be useful in realizing miniaturized quantum photonic systems for various applications.
Photon-based polarization entanglement in the form of Bell states and qutrits is an essential tool for understanding complex quantum phenomena and implementing various quantum technologies such as free-space quantum communication and quantum imaging. Polarization-entangled photons are conventionally generated in bulky nonlinear crystals through spontaneous parametric down-conversion (SPDC). In this approach, the photon polarization is intrinsically defined by the nonlinear tensor of the material, restricting the versatility of the generated quantum states. The recent developments of dielectric nanostructures with sub-wavelength thickness called metasurfaces [1] enable miniaturization of quantum light sources, while dramatically enhancing the photon-pair generation and allowing the preparation of complex quantum states with spectral [2] and spatial [3] entanglement. However, optically reconfigurable generation of biphoton polarization states with metasurfaces remained unexplored.
Complex polarization states of photon pairs are indispensable in various quantum technologies. Conventional methods for preparing desired two-photon polarization states are realized through bulky nonlinear crystals, which can restrict the versatility and tunability of the generated quantum states due to the fixed crystal nonlinear susceptibility. Here we present a solution using a nonlinear metasurface incorporating multiplexed silica metagratings on a lithium niobate film of 300 nm thickness. We fabricate two orthogonal metagratings on a single substrate with an identical resonant wavelength, thereby enabling the spectral indistinguishability of the emitted photons, and we demonstrate in experiments that the two-photon polarization states can be shaped by the metagrating orientation. Leveraging this essential property, we formulate a theoretical approach for generating arbitrary polarization-entangled qutrit states by combining three metagratings on a single metasurface, allowing the encoding of the desired quantum states or information. Our findings enable miniaturized optically controlled quantum devices by using ultrathin metasurfaces as polarization-entangled photon sources.
The ability to generate quantum entangled photon pairs through spontaneous parametric down conversion (SPDC) is playing a pivotal role in many applications in quantum technologies, including quantum communications, quantum computation, and quantum imaging. Metasurfaces, two-dimensional arrays of nanostructures with subwavelength thickness, have recently shown extraordinary capability in manipulating classical and quantum states of light and realizing miniaturized SPDC sources. Whereas previous studies have primarily focused on periodic metasurfaces, we uncover the potential of spatially modulated nanopatterns for further SPDC enhancement and state engineering. Specifically, we propose a nonlinear metasurface featuring a lateral guided-mode cavity formed by two distributed Bragg reflectors. With the quasi-normal mode theory, we predict an SPDC rate up to 157 Hz/mW, which represents about 30 times rate enhancement when compared to the metasurface without a cavity. Furthermore, the cavity supports a continuous set of high-Q resonances across a broad bandwidth, allowing the all-optical tuning of the photon-pair emission directions transverse to the cavity by controlling the pump wavelength, which can find applications in quantum imaging.
Two-photon interference, a quantum phenomenon arising from the principle of indistinguishability, is a powerful tool for quantum state engineering and plays a fundamental role in various quantum technologies. These technologies demand robust and efficient sources of quantum light, as well as scalable, integrable and multifunctional platforms. In this regard, quantum optical metasurfaces (QOMs) are emerging as promising platforms for quantum light generation, namely biphotons via spontaneous parametric down-conversion (SPDC), and its engineering. Due to the relaxation of phase matching, SPDC in QOMs allows different channels of biphoton generation, such as those supported by overlapping resonances, to occur simultaneously. In previously reported QOMs, however, SPDC was too weak to observe such effects. Here we develop QOMs based on [110]-oriented GaAs that provide more than an order of magnitude enhancement in SPDC rate, after accounting for the spectral bandwidth, compared to any other QOMs studied to date. This boosted efficiency allows the QOMs support the simultaneous generation of SPDC from several spectrally overlapping optical modes. Using polarization components in the interferometer analyzer, we intentionally erase the distinguishability between the biphotons from a high-QQQ quasi-bound-state-in-the-continuum resonance and a low-QQQ Mie resonance, which results in the first-time observation of two-photon interference in the spectral domain in these types of devices. This quantum interference can considerably enrich the generation of entangled photons in metasurfaces. Their advanced multifunctionality, improved nonlinear response, ease of fabrication and compact footprint of [110]-GaAs QOMs position them as promising platforms to fulfill the requirements for photonic quantum technologies.