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Optics Letters

Polarization-entangled photon pairs from lithium niobate metasurfaces

Qingqing Mo, Chaoxin Shi, Xin-Ding Zhang, Jianjun Zhang, Jihua Zhang

Published Mar 11, 2025• 1 citation
Publisher

Abstract

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.

DOI
10.1364/ol.559126

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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.
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.
Metasurfaces consisting of nanoscale structures are underpinning new physical principles for the creation and shaping of quantum states of light. Multiphoton states that are entangled in spatial or angular domains are an essential resource for many quantum applications; however, their production traditionally relies on bulky nonlinear crystals. We predict and demonstrate experimentally the generation of spatially entangled photon pairs through spontaneous parametric down-conversion from a metasurface incorporating a nonlinear thin film of lithium niobate covered by a silica meta-grating. We measure the correlations of photon pairs and identify their spatial antibunching through violation of the classical Cauchy-Schwarz inequality, witnessing the presence of multimode entanglement. Simultaneously, the photon-pair rate is strongly enhanced by 450 times as compared to unpatterned films because of high-quality-factor resonances. These results pave the way to miniaturization of various quantum devices by incorporating ultrathin metasurfaces functioning as room temperature sources of quantum-entangled photons.
The two-photon state with spatial entanglement is an essential resource for testing fundamental laws of quantum mechanics and various quantum applications.Its creation typically relies on spontaneous parametric downconversion in bulky nonlinear crystals where the tunability of spatial entanglement is limited.Here, we predict that ultrathin nonlinear lithium niobate metasurfaces can generate and diversely tune spatially entangled photon pairs.The spatial properties of photons including the emission pattern, rate, and degree of spatial entanglement are analyzed theoretically with the coupled mode theory and Schmidt decomposition method.We show that by leveraging the strong angular dispersion of the metasurface, the degree of spatial entanglement quantified by the Schmidt number can be decreased or increased by changing the pump laser wavelength and a Gaussian beam size.This flexibility can facilitate diverse quantum applications of entangled photon states generated from nonlinear metasurfaces.
We predict and demonstrate experimentally, for the first time to our knowledge, the generation of spatially entangled photon pairs through spontaneous parametric down-conversion from a metasurface incorporating a nonlinear thin film of lithium niobate covered by a silica meta-grating. We identify the spatial entanglement of the photon pairs through violation of the classical Cauchy-Schwartz inequality. Simultaneously, the photon-pair rate is strongly enhanced by 450 times compared to unpatterned films.
All-dielectric optical metasurfaces are a workhorse in nano-optics, because of both their ability to manipulate light in different degrees of freedom and their excellent performance at light frequency conversion. Here, we demonstrate first-time generation of photon pairs via spontaneous parametric-down conversion in lithium niobate quantum optical metasurfaces with electric and magnetic Mie-like resonances at various wavelengths. By engineering the quantum optical metasurface, we tailor the photon-pair spectrum in a controlled way. Within a narrow bandwidth around the resonance, the rate of pair production is enhanced up to 2 orders of magnitude, compared to an unpatterned film of the same thickness and material. These results enable flat-optics sources of entangled photons-a new promising platform for quantum optics experiments.
All-dielectric optical metasurfaces are a workhorse in nano-optics due to both their ability to manipulate light in different degrees of freedom and their excellent performance at light frequency conversion. Here, we demonstrate first-time generation of photon pairs via spontaneous parametric-down conversion in lithium niobate quantum optical metasurfaces with electric and magnetic Mie-like resonances at various wavelengths. By engineering the quantum optical metasurface, we tailor the photon-pair spectrum in a controlled way. Within a narrow bandwidth around the resonance, the rate of pair production is enhanced up to two orders of magnitude compared to an unpatterned film of the same thickness and material. These results enable flat-optics sources of entangled photons -- a new promising platform for quantum optics experiments.