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.
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.
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)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 theLiNbO3layer. With two incident circular polarized sources, the nonlinear sum frequency generation (SFG) efficiency is increased drastically over theLiNbO3layer 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.
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.
) 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.
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.
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.