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Digest - IEEE Antennas and Propagation Society. International Symposium

Chiral Single Photon Emission with Bound States in the Continuum Plasmonic Metasurfaces

Sky Semone, Christos Argyropoulos

Published Jul 13, 2025• 0 citations
Publisher

Abstract

Single-photon sources can be realized by spontaneous parametric down-conversion (SPDC), where a pair of entangled single photons is generated. However, SPDC is an inherently weak process and lacks control in the polarization of the generated photons. Increasing the efficiency and controlling the polarization of SPDC-based sources is critical for their practical use in quantum communications and computing. Here, we demonstrate a new chiral plasmonic metasurface leveraging a resonant quasi-bound state in the continuum (qBIC) response in reflection that significantly enhances single-photon pair generation rates and circularly polarizes the produced non-classical (quantum) light. The ultrathin design consists of a lithium niobate$\left(\text{LiNbO}_{3}\right)$crystal overlaid by a chiral silver grating made of tilted nanorods responsible for the qBIC chiral response. When excited by sources of polarity and wavelength matching to the metasurface resonant conditions, a significant enhancement of the electric field can occur within the$\text{LiNbO}_{3}$layer. With two incident circular polarized sources, the nonlinear sum frequency generation (SFG) efficiency is increased drastically over the$\text{LiNbO}_{3}$layer alone leading to a substantially boosted rate of single photon-pairs created by SPDC due to the quantum-classical correspondence principle. The design presented here is ultrathin, relies on efficient free space coupling of chiral radiation, and operates in room temperature, resulting in a new quantum source of chiral single-photon pairs that can be easily integrated into a wide range of applications in quantum photonics.

DOI
10.1109/ap-s/cnc-usnc-ursi55537.2025.11266417

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Single-photon sources can be realized by spontaneous parametric down-conversion (SPDC), where a pair of entangled single photons is generated. However, SPDC is an inherently weak process and lacks control in the polarization of the generated photons. Increasing the efficiency and controlling the polarization of SPDC-based sources is critical for their practical use in quantum communications and computing. Here, we demonstrate a new chiral plasmonic metasurface leveraging a resonant quasi-bound state in the continuum (qBIC) response in reflection that significantly enhances single-photon pair generation rates and circularly polarizes the produced non-classical (quantum) light. The ultrathin design consists of a lithium niobate(LiNbO3)\left(\text{LiNbO}_{3}\right)(LiNbO3​)crystal overlaid by a chiral silver grating made of tilted nanorods responsible for the qBIC chiral response. When excited by sources of polarity and wavelength matching to the metasurface resonant conditions, a significant enhancement of the electric field can occur within theLiNbO3\text{LiNbO}_{3}LiNbO3​layer. With two incident circular polarized sources, the nonlinear sum frequency generation (SFG) efficiency is increased drastically over theLiNbO3\text{LiNbO}_{3}LiNbO3​layer alone leading to a substantially boosted rate of single photon-pairs created by SPDC due to the quantum-classical correspondence principle. The design presented here is ultrathin, relies on efficient free space coupling of chiral radiation, and operates in room temperature, resulting in a new quantum source of chiral single-photon pairs that can be easily integrated into a wide range of applications in quantum photonics.
Spontaneous parametric down conversion (SPDC) has proven to be a robust and prominent method for creating non-classical light sources of entangled single-photon pairs. However, such sources suffer from low efficiency due to the inherent weakness of the SPDC process. Moreover, there is no control of polarization in the generated photons. Circularly polarized SPDC-based sources with an efficient chiral response are currently missing, which severely precludes the realization of a wide range of practical applications in quantum computing and communications. The current work solves these inherent problems by demonstrating a novel chiral plasmonic metasurface that leverages a strong quasi-bound state in the continuum (qBIC) resonance, which significantly increases the SPDC photon generation efficiency and realizes circularly polarized (chiral) single-photon pairs. The presented metasurface design is ultrathin, supports efficient free space outcoupling of chiral classical and quantum radiation, and operates at room temperature. Hence, it offers the potential to realize a compact and integrable quantum source of circularly polarized entangled single-photon pairs.
) crystal to realize a new scalable, ultrathin, and efficient SPDC source. By coinciding fundamental and higher order resonances of the metasurface with the generated signal and idler frequencies, respectively, the electric field in the nonlinear media is significantly boosted. This leads to a substantial enhancement in the SPDC process which, subsequently, by using the quantum-classical correspondence principle, translates to very high photon-pair generation rates. The emitted radiation is highly directional and perpendicular to the metasurface in contrast to relevant dielectric structures. The incorporation of circular polarized excitation further increases the photon-pair generation efficiency. The presented work will lead to the design of new efficient ultrathin SPDC single-photon nanophotonic sources working at room temperature that are expected to be critical components in free-space quantum optical communications. In a more general context, our findings can have various applications in the emerging field of quantum plasmonics.
The Spontaneous Parametric Down-Conversion (SPDC) process is a fundamental method for the generation of entangled and correlated single-photon pairs. This research investigates the use of a plasmonic metasurface consisting of metallic nanostripes deposited on a bulk lithium niobate (LiNbO3) crystal to significantly increase efficiency of photon pair generation. The geometric parameters of the metasurface features can be set to create resonances with the signal and idler frequencies, boosting the electric field in the LiNbO3crystal and enhancing the SPDC process. The photon pair generation rate is found by invoking the quantum-classical correspondence principle and computing the rate for the classical sum frequency generation (SFG) process. Efficient room temperature SPDC single-photon nanophotonic sources show promise for developing free space quantum optical communications and have multiple applications across the field of quantum optics.
Spontaneous Parametric Down-Conversion (SPDC) is a quantum optical process where a photon spontaneously splits into a pair of lower energy entangled single-photons. Here, we demonstrate that the quantum effect of SPDC can be substantially boosted at room temperature by utilizing nonlinear plasmonic metasurfaces. The presented plasmonic metasurface design is composed of silver nanostripes grown on a bulk lithium niobate (LiNbO3) crystal terminated by a silver substrate. The quantum-classical correspondence principle is used to compute the SPDC entangled photon pair generation rate by the classical sum frequency generation (SFG) nonlinear optical process. The generated signal and idler frequencies are matched to the metasurface fundamental and higher order resonances leading to substantially boosted SFG efficiency and, consequently, significantly enhanced SPDC generation rate. The quantum SPDC process is at the heart of many quantum optical emerging apparatuses, since efficient single-photon generation at room temperature is required in a plethora of applications in quantum communications, cryptography, and sensing.
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.
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-QQQ 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 QQQ 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.