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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Spectrally Tunable Generation of Entangled Photon Pairs from a Tilted Nonlinear Metasurface

Shaun Lung, Jinyong Ma, Maximilian A. Weissflog, Jihua Zhang, Sina Saravi, Thomas Pertsch, Frank Setzpfandt, Andrey A. Sukhorukov

Published Jun 23, 2025• 0 citations
Publisher

Abstract

Entangled photon pairs generated by spontaneous parametric down-conversion (SPDC) are the cornerstone of many quantum optical applications. Nonlinear metasurfaces have recently been introduced as a new class of compact SPDC sources [1], and the next step is to enable active tunability of their resulting quantum state. Here, we demonstrate the generation of narrow-band yet frequency tunable, spectrally entangled photon pairs from a nonlinear metasurface. For this, we use guided-mode resonances (GMR) with high angular dispersion. In previous works that use Mie- or BIC-type resonances, for a fixed pump wavelength, the demonstrated SPDC spectrum remained fixed by the respective resonance wavelength [1], [2]. Here, we theoretically and experimentally show that in a metasurface supporting GMRs, the non-degeneracy of the generated photon pairs can be tuned by tilting the metasurface while the pump wavelength can remain constant, see the conceptual rendering in Fig. 1(a).

DOI
10.1109/cleo/europe-eqec65582.2025.11110105

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Entangled photon pairs generated by spontaneous parametric down-conversion (SPDC) are the cornerstone of many quantum optical applications. Nonlinear metasurfaces have recently been introduced as a new class of compact SPDC sources [1], and the next step is to enable active tunability of their resulting quantum state. Here, we demonstrate the generation of narrow-band yet frequency tunable, spectrally entangled photon pairs from a nonlinear metasurface. For this, we use guided-mode resonances (GMR) with high angular dispersion. In previous works that use Mie- or BIC-type resonances, for a fixed pump wavelength, the demonstrated SPDC spectrum remained fixed by the respective resonance wavelength [1], [2]. Here, we theoretically and experimentally show that in a metasurface supporting GMRs, the non-degeneracy of the generated photon pairs can be tuned by tilting the metasurface while the pump wavelength can remain constant, see the conceptual rendering in Fig. 1(a).
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.
Entangled photon-pairs generated through spontaneous parametric down-conversion (SPDC) have evolved into an indispensable resource for various quantum communication, imaging, sensing and computing schemes. While photon-pair sources are so far mostly based on bulk-optical nonlinear components, resonant nonlinear metasurfaces with sub-wavelength thickness have recently been demonstrated as a new platform for photon-pair generation [1]–[3]. Contrary to bulk and integrated nonlinear structures where phase matching steers the pair generation efficiency in different directions and wavelengths, nanostructures can control these properties via their optical resonances. However, the tunability of spatial photon emission in multiple directions remained an open challenge.
Tunable photonic quantum entanglement promises to advance information processing beyond classical approaches, facilitating applications in quantum communications, imaging and computing. A practical approach to preparing quantum entanglement at room temperature is to engineer photon-pair generation through spontaneous parametric down-conversion (SPDC). Nonlinear flat-optics devices, incorporating subwavelength-thick nonlinear materials, are emerging as new platforms for generating photon pairs at the nanoscale. In particular, nanostructured metasurfaces supporting optical resonances can enhance and tailor SPDC processes, allowing for the generation of spectral, polarization, and spatial entanglement [1]. However, the tunability of polarization entanglement is subject to the symmetry of nonlinear tensors of materials, where the pump photon polarization serves as the only tuning parameter [2].
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.
Nonlinear metasurfaces have recently been established as a new platform for generating photon pairs via spontaneous parametric down-conversion. While for classical harmonic generation in metasurfaces a high level of control over all degrees of freedom of light has been reached, this capability is yet to be developed for photon-pair generation. In this work, we theoretically and experimentally demonstrate for the first time precise control of the emission angle of photon pairs generated from a nonlinear metasurface. Our measurements show angularly tunable pair generation with high coincidence-to-accidental ratio for both co- and counterpropagating emission. The underlying principle is the transverse phase matching of guided-mode resonances with strong angular dispersion in a nonlinear metasurface consisting of a silicon dioxide grating on a nonlinear lithium niobate guiding layer. We provide a straightforward design strategy for photon-pair generation in such a device and find very good agreement between the calculations and experimental results. Here, we use all-optical emission angle tuning by means of the pump wavelength; however, the principle could be extended to modulation via the electro-optic effect in lithium niobate. In sum, this work provides an important addition to the toolset of subwavelength thickness photon-pair sources.
The efficient and controllable generation of entangled photon pairs is a cornerstone for advancing quantum technologies, including quantum computing, communication, and sensing. Traditional methods, primarily relying on spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (SFWM) in bulk nonlinear crystals or waveguides, often suffer from low efficiency, bulky form factors, and limited control over the spectral and spatial properties of the generated photons. This paper explores a transformative approach leveraging resonant Bound States in the Continuum (BICs) within nanophotonic structures to overcome these limitations. BICs are extraordinary optical resonances characterized by theoretically infinite quality (Q) factors, enabling extreme light localization and immense field enhancement within subwavelength volumes. By integrating nonlinear optical materials into BIC-supporting structures, such as photonic crystal slabs or metasurfaces, the local electric field strength experienced by the nonlinear medium can be dramatically increased, boosting the efficiency of entangled photon generation. Furthermore, the sharp, tailor-able spectral response of BICs offers unprecedented control over the joint spectral amplitude of the photon pairs, promising high degrees of spectral purity and entanglement fidelity. This work outlines the theoretical underpinnings, potential device architectures, and anticipated advantages of BIC-enhanced entangled photon sources, paving the way for compact, high-brightness, and spectrally engineered quantum light sources critical for next-generation quantum technologies.