Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Light Science & Applications

Fano interference of photon pairs from a metasurface

Jiho Noh, Tomás Santiago‐Cruz, Chloe F. Doiron, Hyunseung Jung, Jaeyeon Yu, Sadhvikas Addamane, Maria V. Chekhova, Igal Brener

Published Oct 17, 2025• 5 citations
Publisher

Abstract

Two-photon interference, a quantum phenomenon arising from the principle of indistinguishability, is a powerful tool for quantum state engineering and plays a fundamental role in various quantum technologies. These technologies demand robust and efficient sources of quantum light, as well as scalable, integrable, and multifunctional platforms. In this regard, quantum optical metasurfaces (QOMs) are emerging as promising platforms for the generation and engineering of quantum light, in particular pairs of entangled photons (biphotons) via spontaneous parametric down-conversion (SPDC). Due to the relaxation of the phase-matching condition, SPDC in QOMs allows different channels of biphoton generation, such as those supported by overlapping resonances, to occur simultaneously. In previously reported QOMs, however, SPDC was too weak to observe such effects. Here, we develop QOMs based on [110]-oriented GaAs that provide an order-of-magnitude enhancement in SPDC rate, after accounting for the spectral bandwidth, compared to any other QOMs studied to date. This boosted efficiency allows the QOMs to support the simultaneous generation of SPDC from several spectrally overlapping optical modes. Using a linear polarizer, we intentionally erase the distinguishability between the biphotons from a high-Q quasi-bound-state-in-the-continuum resonance and a low-Q Mie resonance, which results in the first-time observation of two-photon interference, shown in the form of a Fano contour, in the spectrum of biphotons. This quantum interference can enrich the generation of entangled photons in metasurfaces. Their advanced multifunctionality, improved nonlinear response, ease of fabrication, and compact footprint of [110]-GaAs QOMs position them as promising platforms to fulfill the requirements of photonic quantum technologies.

DOI
10.1038/s41377-025-01998-5

Related papers

Two-photon Interference of Biphotons Emitted by Overlapping Resonances in Metasurfaces

Jiho Noh, Tomás Santiago‐Cruz, Chloe F. Doiron +5 more

Two-photon interference, a quantum phenomenon arising from the principle of indistinguishability, is a powerful tool for quantum state engineering and plays a fundamental role in various quantum technologies. These technologies demand robust and efficient sources of quantum light, as well as scalable, integrable and multifunctional platforms. In this regard, quantum optical metasurfaces (QOMs) are emerging as promising platforms for quantum light generation, namely biphotons via spontaneous parametric down-conversion (SPDC), and its engineering. Due to the relaxation of phase matching, SPDC in QOMs allows different channels of biphoton generation, such as those supported by overlapping resonances, to occur simultaneously. In previously reported QOMs, however, SPDC was too weak to observe such effects. Here we develop QOMs based on [110]-oriented GaAs that provide more than an order of magnitude enhancement in SPDC rate, after accounting for the spectral bandwidth, compared to any other QOMs studied to date. This boosted efficiency allows the QOMs support the simultaneous generation of SPDC from several spectrally overlapping optical modes. Using polarization components in the interferometer analyzer, we intentionally erase the distinguishability between the biphotons from a high-$Q$ quasi-bound-state-in-the-continuum resonance and a low-$Q$ Mie resonance, which results in the first-time observation of two-photon interference in the spectral domain in these types of devices. This quantum interference can considerably enrich the generation of entangled photons in metasurfaces. Their advanced multifunctionality, improved nonlinear response, ease of fabrication and compact footprint of [110]-GaAs QOMs position them as promising platforms to fulfill the requirements for photonic quantum technologies.

arXiv:2501.11850•Jan 21, 2025•0 citations

Towards photon pair generation via spontaneous four-wave mixing in nonlinear metasurfaces

Son, Changjin

Entangled photons have acted as a workhorse in many quantum technologies, including cryptography, imaging, sensing, and computation. With the growing demands of quantum technology, the entangled photon pair sources are required to be compact and integrable. For the last few years, the field of quantum optics in the nanoscale has demonstrated photon pair generation in ’flat’ optics such as thin layers, Van der Waals crystals, micro-, nano-antennas, and metasurfaces. The word ’flat’ is used because such optical platforms have sub-wavelength thicknesses. Due to the short travel length inside the sources, they can possess multiple functions that their bulky counterparts cannot have. This doctoral thesis addresses photon pair generation in ’flat’ optics via second- and third-order nonlinear processes, which are spontaneous parametric down-conversion (SPDC) and spontaneous four-wave mixing (SFWM). The main aims of the thesis are to understand how the photon pair generation is realized via SPDC and SFWM on subwavelength-scale platforms, and to make a bridge between photon pair generation via two nonlinear processes under the presence of optical resonances. The thesis covers two experimental parts related to SPDC and SFWM, and one future expectation of the SFWM in metamaterials. The first part of the thesis addresses one of the multi-functionalities that has recently been reported: the emission direction control of the photon pairs. A 150 nm-thick gallium phosphide (GaP) metasurface is used as a source. The metasurface supports a quasi-bound state in the continuum (quasi-BIC) resonance, enhancing the electric field inside the material. I generate the entangled photons through the SPDC process. The generated photons are in two different wavelengths under the given pump wavelength: signal photon at the resonance, and idler photon out of the resonance. The photon pair generation rate is enhanced by 66 times compared to the unpatterned GaP slab since the vacuum field in the signal mode is enhanced by the resonance. The generated photons obey the transmission spectrum of the metasurface. As a result, the signal photons at resonance are mostly reflected, and the idler photons are transmitted after generation. This result is the first work of photon demultiplexing by a metasurface. The second part of the thesis covers the photon pair generation through SFWM in thin nonlinear films. Silicon-rich silicon nitride (SiN) layers are selected as the sources. The layers have an intriguing characteristic that the third-order nonlinear susceptibility (chi-3) differs with the nitrogen doping rate. I generate the photon pairs through chi-3 variant SiN layers. The thicknesses of the samples are all sub-wavelength level, having a few hundred nanometers. Time-energy correlation measurements are conducted for all samples, and the second-order correlation functions (g(2)(0)) are calculated. The maximum g(2)(0) of the samples exceeds 2. Interestingly, I observed two-photon interference effects between photon pairs generated in the layers and the fused-silica substrate, which allows me to estimate the chi-3 values of the samples by knowing the reference: chi-3 value of the substrate. The last part of the thesis introduces designs of metasurfaces and possible experimental schemes for photon pair generation through SFWM. Yet, due to its weak light-matter interaction in third-order nonlinear processes, the photon pair generation through SFWM has not been observed in metasurfaces. However, it is fairly possible to enhance the vacuum fields at the output photon modes and generate photon pairs because preliminary work in unpatterned slabs has already been observed in this thesis. Finally, I introduce a fabricated metasurface. It needs improvements, but it is quite promising for the ongoing project.

•Jan 1, 2026•0 citations

Enhanced tunable photon-pair generation from a nonlinear metasurface with a guided-mode cavity

Tongmiao Fan, Jihua Zhang, Andrey A. Sukhorukov

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.

arXiv:2505.15026•Physical Review A•May 19, 2025•3 citations

Entangled photons from subwavelength nonlinear films

Tomás Santiago‐Cruz, Vitaliy Sultanov, Haizhong Zhang +2 more

Miniaturized entangled photon sources, in particular based on subwavelength metasurfaces, are highly demanded for the development of integrated quantum photonics. Here, as a first step towards the development of quantum optical metasurfaces (QOMs), we demonstrate generation of entangled photons via spontaneous parametric down-conversion (SPDC) from subwavelength films. We achieve photon pair generation with a high coincidence-to-accidental ratio in lithium niobate and gallium phosphide nanofilms. By implementing the fiber spectroscopy of SPDC in nanofilms, we measure a spectrum with a bandwidth of 500 nm, limited only by the overall detection efficiency. The spectrum reveals vacuum field enhancement due to a Fabry-Perot resonance inside the nonlinear films. It also suggests a strategy for observing SPDC from QOM. Our experiments lay the groundwork for future development of flat SPDC sources, including QOM.

Optics Letters•Dec 22, 2020•69 citations

Spontaneous Parametric Down-Conversion from Resonant Metasurfaces

Tomás Santiago‐Cruz, Anna N. Fedotova, Vitaliy Sultanov +7 more

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.

arXiv:2103.08524•Mar 15, 2021•2 citations

Quantum Pair Generation in Nonlinear Metasurfaces with Mixed and Pure Photon Polarizations

Jiho Noh, Tomás Santiago‐Cruz, Vitaliy Sultanov +4 more

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.

arXiv:2409.04569•Nano Letters•Nov 21, 2024•21 citations
Two-photon interference, a quantum phenomenon arising from the principle of indistinguishability, is a powerful tool for quantum state engineering and plays a fundamental role in various quantum technologies. These technologies demand robust and efficient sources of quantum light, as well as scalable, integrable, and multifunctional platforms. In this regard, quantum optical metasurfaces (QOMs) are emerging as promising platforms for the generation and engineering of quantum light, in particular pairs of entangled photons (biphotons) via spontaneous parametric down-conversion (SPDC). Due to the relaxation of the phase-matching condition, SPDC in QOMs allows different channels of biphoton generation, such as those supported by overlapping resonances, to occur simultaneously. In previously reported QOMs, however, SPDC was too weak to observe such effects. Here, we develop QOMs based on [110]-oriented GaAs that provide an order-of-magnitude enhancement in SPDC rate, after accounting for the spectral bandwidth, compared to any other QOMs studied to date. This boosted efficiency allows the QOMs to support the simultaneous generation of SPDC from several spectrally overlapping optical modes. Using a linear polarizer, we intentionally erase the distinguishability between the biphotons from a high-Q quasi-bound-state-in-the-continuum resonance and a low-Q Mie resonance, which results in the first-time observation of two-photon interference, shown in the form of a Fano contour, in the spectrum of biphotons. This quantum interference can enrich the generation of entangled photons in metasurfaces. Their advanced multifunctionality, improved nonlinear response, ease of fabrication, and compact footprint of [110]-GaAs QOMs position them as promising platforms to fulfill the requirements of photonic quantum technologies.
Two-photon interference, a quantum phenomenon arising from the principle of indistinguishability, is a powerful tool for quantum state engineering and plays a fundamental role in various quantum technologies. These technologies demand robust and efficient sources of quantum light, as well as scalable, integrable and multifunctional platforms. In this regard, quantum optical metasurfaces (QOMs) are emerging as promising platforms for quantum light generation, namely biphotons via spontaneous parametric down-conversion (SPDC), and its engineering. Due to the relaxation of phase matching, SPDC in QOMs allows different channels of biphoton generation, such as those supported by overlapping resonances, to occur simultaneously. In previously reported QOMs, however, SPDC was too weak to observe such effects. Here we develop QOMs based on [110]-oriented GaAs that provide more than an order of magnitude enhancement in SPDC rate, after accounting for the spectral bandwidth, compared to any other QOMs studied to date. This boosted efficiency allows the QOMs support the simultaneous generation of SPDC from several spectrally overlapping optical modes. Using polarization components in the interferometer analyzer, we intentionally erase the distinguishability between the biphotons from a high-QQQ quasi-bound-state-in-the-continuum resonance and a low-QQQ Mie resonance, which results in the first-time observation of two-photon interference in the spectral domain in these types of devices. This quantum interference can considerably enrich the generation of entangled photons in metasurfaces. Their advanced multifunctionality, improved nonlinear response, ease of fabrication and compact footprint of [110]-GaAs QOMs position them as promising platforms to fulfill the requirements for photonic quantum technologies.
Entangled photons have acted as a workhorse in many quantum technologies, including cryptography, imaging, sensing, and computation. With the growing demands of quantum technology, the entangled photon pair sources are required to be compact and integrable. For the last few years, the field of quantum optics in the nanoscale has demonstrated photon pair generation in ’flat’ optics such as thin layers, Van der Waals crystals, micro-, nano-antennas, and metasurfaces. The word ’flat’ is used because such optical platforms have sub-wavelength thicknesses. Due to the short travel length inside the sources, they can possess multiple functions that their bulky counterparts cannot have. This doctoral thesis addresses photon pair generation in ’flat’ optics via second- and third-order nonlinear processes, which are spontaneous parametric down-conversion (SPDC) and spontaneous four-wave mixing (SFWM). The main aims of the thesis are to understand how the photon pair generation is realized via SPDC and SFWM on subwavelength-scale platforms, and to make a bridge between photon pair generation via two nonlinear processes under the presence of optical resonances. The thesis covers two experimental parts related to SPDC and SFWM, and one future expectation of the SFWM in metamaterials. The first part of the thesis addresses one of the multi-functionalities that has recently been reported: the emission direction control of the photon pairs. A 150 nm-thick gallium phosphide (GaP) metasurface is used as a source. The metasurface supports a quasi-bound state in the continuum (quasi-BIC) resonance, enhancing the electric field inside the material. I generate the entangled photons through the SPDC process. The generated photons are in two different wavelengths under the given pump wavelength: signal photon at the resonance, and idler photon out of the resonance. The photon pair generation rate is enhanced by 66 times compared to the unpatterned GaP slab since the vacuum field in the signal mode is enhanced by the resonance. The generated photons obey the transmission spectrum of the metasurface. As a result, the signal photons at resonance are mostly reflected, and the idler photons are transmitted after generation. This result is the first work of photon demultiplexing by a metasurface. The second part of the thesis covers the photon pair generation through SFWM in thin nonlinear films. Silicon-rich silicon nitride (SiN) layers are selected as the sources. The layers have an intriguing characteristic that the third-order nonlinear susceptibility (chi-3) differs with the nitrogen doping rate. I generate the photon pairs through chi-3 variant SiN layers. The thicknesses of the samples are all sub-wavelength level, having a few hundred nanometers. Time-energy correlation measurements are conducted for all samples, and the second-order correlation functions (g(2)(0)) are calculated. The maximum g(2)(0) of the samples exceeds 2. Interestingly, I observed two-photon interference effects between photon pairs generated in the layers and the fused-silica substrate, which allows me to estimate the chi-3 values of the samples by knowing the reference: chi-3 value of the substrate. The last part of the thesis introduces designs of metasurfaces and possible experimental schemes for photon pair generation through SFWM. Yet, due to its weak light-matter interaction in third-order nonlinear processes, the photon pair generation through SFWM has not been observed in metasurfaces. However, it is fairly possible to enhance the vacuum fields at the output photon modes and generate photon pairs because preliminary work in unpatterned slabs has already been observed in this thesis. Finally, I introduce a fabricated metasurface. It needs improvements, but it is quite promising for the ongoing project.
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.
Miniaturized entangled photon sources, in particular based on subwavelength metasurfaces, are highly demanded for the development of integrated quantum photonics. Here, as a first step towards the development of quantum optical metasurfaces (QOMs), we demonstrate generation of entangled photons via spontaneous parametric down-conversion (SPDC) from subwavelength films. We achieve photon pair generation with a high coincidence-to-accidental ratio in lithium niobate and gallium phosphide nanofilms. By implementing the fiber spectroscopy of SPDC in nanofilms, we measure a spectrum with a bandwidth of 500 nm, limited only by the overall detection efficiency. The spectrum reveals vacuum field enhancement due to a Fabry-Perot resonance inside the nonlinear films. It also suggests a strategy for observing SPDC from QOM. Our experiments lay the groundwork for future development of flat SPDC sources, including QOM.
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.
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.