We propose a general scheme to investigate photon triplet generation (PTG) via third-order spontaneous parametric downconversion (TOSPDC) in χ (3) nonlinear structures. Our approach leverages the quantum-classical correspondence between TOSPDC and its reverse classical process, three-wave sum-frequency generation (TSFG), to efficiently estimate the PTG rate. We apply this framework to nonlinear metasurfaces supporting quasi-bound states in the continuum (qBICs) in the optical range. From numerical analysis of noncollinear TSFG with degenerate input waves at qBIC wavelengths, we predict wavelength-tunable three-photon emission with spatio-angular correlations. These findings establish a novel method for modeling TOSPDC and also highlight the potential of nonlinear resonant metasurfaces as compact free-space photon triplet sources with quantum state control.
We propose a general scheme to investigate photon triplet generation (PTG) via third-order spontaneous parametric downconversion (TOSPDC) in χ (3) nonlinear structures. Our approach leverages the quantum-classical correspondence between TOSPDC and its reverse classical process, three-wave sum-frequency generation (TSFG), to efficiently estimate the PTG rate. We apply this framework to nonlinear metasurfaces supporting quasi-bound states in the continuum (qBICs) in the optical range. From numerical analysis of noncollinear TSFG with degenerate input waves at qBIC wavelengths, we predict wavelength-tunable three-photon emission with spatio-angular correlations. These findings establish a novel method for modeling TOSPDC and also highlight the potential of nonlinear resonant metasurfaces as compact free-space photon triplet sources with quantum state control.
We study the third-order spontaneous parametric down-conversion (TOSPDC) process, as a means to generate entangled photon triplets. Specifically, we consider thin optical fibers as the nonlinear medium to be used as the basis for TOSPDC in configurations where phase matching is attained through the use of more than one fiber transverse modes. Our analysis in this paper, which follows from our earlier paper [Opt. Lett. 36, 190--192 (2011)], aims to supply experimentalists with the details required in order to design a TOSPDC photon-triplet source. Specifically, our analysis focuses on the photon triplet state, on the rate of emission, and on the TOSPDC phase-matching characteristics for the cases of frequency-degenerate and frequency nondegenerate TOSPDC.
Nonlinear metasurfaces have emerged as a versatile platform for photon-pair generation through spontaneous parametric down-conversion (SPDC). However, the angular dispersion of optical resonances restricts the emission range and momentum-space uniformity of the generated photon pairs, thereby hindering practical applications in quantum sensing and imaging. Here, we propose and numerically investigate an all-dielectric InGaP metasurface that supports angle-robust photon-pair generation enabled by tailored multipolar flatband resonances. By introducing elliptical nanopores into nonlinear nanodisks, we engineer the far-field coupling and polarization response of weakly dispersive flatband modes while preserving strong field confinement. As a result, the tailored structure exhibits a more than one-order-of-magnitude enhancement in second-harmonic generation efficiency compared with the unperforated design. Based on the quantum-classical correspondence between SPDC and sum-frequency generation (SFG), we further reveal a pronounced enhancement of the nonlinear response and demonstrate stable photon-pair generation with rates up to approximately 2×10 5 Hz over an angular range of ±12°. In addition, the generated photon pairs remain robust over a broad momentum range and show tunable polarization properties under different pump wavelengths. These results provide a practical route toward compact and efficient quantum light sources in nonlinear metasurfaces, and highlight the potential of flatband engineering for applications in quantum imaging and integrated quantum photonics.
Metasurfaces are highly effective at manipulating classical light in the linear regime; however, effectively controlling the polarization of nonclassical light generated from nonlinear resonant metasurfaces remains a challenge. Here, we present a solution by achieving polarization engineering of frequency-nondegenerate biphotons emitted via spontaneous parametric down-conversion in GaAs metasurfaces, utilizing quasi-bound states in the continuum (qBIC) resonances to enhance biphoton generation. Through comprehensive polarization tomography, we demonstrate that the emitted photons' polarization directly reflects the qBIC mode's far-field properties. Furthermore, we show that both the type of qBIC mode and the symmetry of the meta-atoms can be tailored to control each single-photon polarization state, and that the subsequent two-photon polarization states are nearly separable, offering potential applications in the heralded generation of single photons with adjustable polarization. This work provides a significant step toward utilizing metasurfaces to generate quantum light and engineer their polarization, a critical aspect for future 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.
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.
Quantum state engineering, the cornerstone of quantum photonic technologies, mainly relies on spontaneous parametric downconversion and four-wave mixing, where one or two pump photons spontaneously decay into a photon pair. Both of these nonlinear effects require momentum conservation for the participating photons, which strongly limits the versatility of the resulting quantum states. Nonlinear metasurfaces have subwavelength thickness and allow the relaxation of this constraint; when combined with resonances, they greatly expand the possibilities of quantum state engineering. Here, we generated entangled photons via spontaneous parametric downconversion in semiconductor metasurfaces with high-quality factor, quasi-bound state in the continuum resonances. By enhancing the quantum vacuum field, our metasurfaces boost the emission of nondegenerate entangled photons within multiple narrow resonance bands and over a wide spectral range. A single resonance or several resonances in the same sample, pumped at multiple wavelengths, can generate multifrequency quantum states, including cluster states. These features reveal metasurfaces as versatile sources of complex states for quantum information.