ABSTRACT The design of nonlinear metasurfaces for quantum applications has traditionally focused on classical figures of merit such as tuning resonances at the pump and target frequencies. While these approaches can enhance nonlinear efficiency, they do not guarantee the precise phase control needed for generating pure quantum state. In this work, we present a hybrid quantum‐classical inverse design framework that directly incorporates quantum metrics into the optimization process. By leveraging an advanced multiobjective optimization, we optimized both entanglement fidelity and the spontaneous parametric down conversion (SPDC) rate, ensuring that high brightness and quantum purity are achieved simultaneously. In general, classical methods that rely on bound states in the continuum (BIC) modes or sharp resonances, can improve the SPDC rate but are often sensitive and difficult to control across the polarization channels leading to low quantum purity, uncontrolled correlations, and reduced entanglement fidelity. In contrast, our approach demonstrates that reliable, easy‐to‐fabricate metasurfaces can achieve superior quantum performance without these constraints. When applied to a [100] AlGaAs nanohole metasurface, our numerical simulations predict a Bell‐state fidelity of 0.989 combined with a collected SPDC rate of 55 Hz/mW, integrated over a wide angular emission range. Our optimized design represents a robust architecture for compact, high‐purity sources of entangled photon pairs.
ABSTRACT The design of nonlinear metasurfaces for quantum applications has traditionally focused on classical figures of merit such as tuning resonances at the pump and target frequencies. While these approaches can enhance nonlinear efficiency, they do not guarantee the precise phase control needed for generating pure quantum state. In this work, we present a hybrid quantum‐classical inverse design framework that directly incorporates quantum metrics into the optimization process. By leveraging an advanced multiobjective optimization, we optimized both entanglement fidelity and the spontaneous parametric down conversion (SPDC) rate, ensuring that high brightness and quantum purity are achieved simultaneously. In general, classical methods that rely on bound states in the continuum (BIC) modes or sharp resonances, can improve the SPDC rate but are often sensitive and difficult to control across the polarization channels leading to low quantum purity, uncontrolled correlations, and reduced entanglement fidelity. In contrast, our approach demonstrates that reliable, easy‐to‐fabricate metasurfaces can achieve superior quantum performance without these constraints. When applied to a [100] AlGaAs nanohole metasurface, our numerical simulations predict a Bell‐state fidelity of 0.989 combined with a collected SPDC rate of 55 Hz/mW, integrated over a wide angular emission range. Our optimized design represents a robust architecture for compact, high‐purity sources of entangled photon pairs.
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.
Spontaneous parametric down-conversion (SPDC) is a fundamental quantum optical technique for generating entangled photon pairs, which serves as the foundation for various quantum applications, including quantum cryptography, quantum simulation, quantum metrology, and the exploration of fundamental physical laws in quantum mechanics. Nevertheless, SPDC is plagued by low efficiency due to the extremely weak nature of this spontaneous quantum nonlinear process. In this work, we design a simple dielectric metasurface structure composed of lithium niobate (LN) defect disks that can support dual quasi-bound states in the continuum (quasi-BICs) in the near-infrared range. By aligning the high-Q quasi-BIC resonances with the generated signal and idler frequencies, our numerical simulation results demonstrate that the local electric field in the nonlinear medium reaches a substantial enhancement of up to two orders of magnitude, resulting in a significant improvement in the efficiency of the classical nonlinear optical sum frequency generation (SFG) and its inverse SPDC quantum process. Our findings pave the way for the development of high-quality compact nanosources of entangled photon pairs with potential applications spanning across cutting-edge fields such as quantum communications, computing, and imaging that can potentially revolutionize the field of quantum technology.
Recently, metasurface has gained a considerable attention in the field of quantum photonics owing to its ability to generate and shape quantum state of light in unprecedented way. Yet, optimizing the metasurface for quantum applications demands precise mode coupling which necessitates and advanced optimization strategy. In this work, we exploit an advanced statistical learning optimization strategy to design a metasurface for counter-propagating entangled photon pair generation. We consider a four-nanohole unit cell with different diameters. This structure enables effective interaction and improves the nonlinear field overlap initiating efficient spontaneous parametric dawn conversion. The optimized design yields 2.02×10^5 pair/s photon pair generation rate, and 5.7x10^-1 photon pair conversion efficiency with exceptional entanglement fidelity rate as 0.9969. Compared to existing designs, our structure offers both superior efficiency and ease of fabrication, making it highly suitable for a wide range of quantum photonic applications.
Here we demonstrate the efficient photon-pair generation via spontaneous parametric down conversion from a semiconductor metasurface supporting dual quasibound states in the continuum (quasi-BICs). In a simple metasurface composed of AlGaAs elliptical nanocyclinders, the two high-Q quasi-BIC resonances that coincide with the generated signal and idler frequencies significantly boost the local electric field. This leads to a substantial enhancement in the reverse classical nonlinear process of sum-frequency generation and subsequently the remarkably high generation rate of photon pairs under the quantum-classical correspondence principle. Within a narrowband wavelength regime around the quasi-BIC resonances, the rate of pair production is enhanced up to $\ensuremath{\sim}{10}^{4}$ Hz, two orders of magnitude larger than that in the Mie resonant AlGaAs nanoantennas. Moreover, the photon pair emission is mainly concentrated in the normal direction with respect to the metasurface, and shows a tunable rate with the Q factor by engineering the rotation angle of nanocylinders. The presented work enables nanoscale sources of high-quality entangled photons which will find applications in advanced quantum imaging and communications.
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.
We present theoretical and experimental advances on photon-pair generation through spontaneous parametric down-conversion (SPDC) in nonlinear nanostructures and metasurfaces, underpinning quantum entanglement engineering at sub-wavelength scale for photon shaping with tailored polarization and spatial correlations. We derive explicit theoretical expressions for the measurable photon counts and quantum correlations, incorporating material dispersion and absorption characteristics [Phys. Rev. Lett. 117, 123901 (2016)], and demonstrate experimentally the generation of quantum heralded photon pairs from a single AlGaAs nanodisk [Optica 6, 1416 (2019)]. We further reveal the opportunities to enhance the generation rate through high-Q resonances including bound states in the continuum [doi:10.1117/12.2539888 (2019)]. We also outline the applications of metasurfaces for the manipulation and measurement of multi-photon quantum states for free-space quantum imaging and communications.