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arXiv:2308.16439 · Chinese Optics Letters

Photon pair generation from lithium niobate metasurface with tunable spatial entanglement

Jihua Zhang, Jinyong Ma, Dragomir N. Neshev, Andrey A. Sukhorukov

Published Nov 30, 2022• 18 citations
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Abstract

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.

DOI
10.3788/col202321.010005

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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.
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.
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.
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.
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.