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Photonics for Quantum (PfQ)

Biphoton interference from spectrally overlapping resonances in nonlinear metasurfaces

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

Published Jun 13, 2025• 0 citations
Publisher

Abstract

Two-photon interference between two photon pairs, or biphotons, requires the interfering biphotons to be indistinguishable. Previously, researchers have demonstrated this by coherently pumping two identical but separate nonlinear materials or using multipass interferometric setups. In contrast, we demonstrate two-photon interference of biphotons simultaneously emitted by two overlapping resonances in the same nonlinear source—a nonlinear metasurface.

DOI
10.1117/12.3062911

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Two-photon interference between two photon pairs, or biphotons, requires the interfering biphotons to be indistinguishable. Previously, researchers have demonstrated this by coherently pumping two identical but separate nonlinear materials or using multipass interferometric setups. In contrast, we demonstrate two-photon interference of biphotons simultaneously emitted by two overlapping resonances in the same nonlinear source—a nonlinear metasurface.
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.
Nonlinear metasurfaces have been demonstrated as ultrathin photon-pair sources, bringing resonant nanophotonics into a field traditionally dominated by bulk crystals and waveguides. Early experiments established spontaneous parametric down-conversion from nanoantennas and metasurfaces and showed resonant enhancement. Recent experiments and theoretical proposals address a more specific objective: a metasurface that emits the biphoton state required by a particular quantum-optical experiment. Resonant dispersion, mode symmetry, quasi-bound states in the continuum, guided-mode resonances, flatband design, multi-resonance interference, polarisation engineering and tunable emission geometry can shape both the pair-generation probability and the spatial, spectral and polarisation structure of the emitted state. This Perspective examines how these physical parameters determine the joint amplitude and which measurements establish the resulting two-photon state.
We have experimentally demonstrated two-photon interference between biphoton amplitudes arising from two well-separated pump pulses. It is important to note the following. 1) The pump pulse intensity was low enough so that single counting rates of the detectors were kept much smaller than pulse repetition rates: the probability of having one spontaneous parametric down conversion (SPDC) photon pair per pulse in our experiment is negligible. Hence the interference cannot be explained as two SPDC photons from two pump pulses (one SPDC photon pair from each pulse) interfering at the detectors. The two pump pulses simply provide two biphoton amplitudes which could result in coincidence counts, and interference occurs between these two biphoton amplitudes only when they are indistinguishable. 2) The BBO crystal in our experiment was only 100 /spl mu/m thick, so the two-pulse did not exist in BBO at the same time at any moment. 3) Due to the delays in the interferometer (/spl tau/ /spl tau//sub 1/), the signal-idler never met at the beamsplitter. Therefore, the existence of two-photon interference cannot be viewed as the interference between the signal and idler photons. These three points again emphasize the fact that it is the indistinguishability of the two-photon Feynman alternatives for the biphoton amplitudes which is responsible for the two-photon quantum interference effects.
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.
Nonlinear metasurfaces have recently emerged as subwavelength-thin sources of photon pairs with complex quantum states [1]. Although metasurfaces are specially known for their capabilities in manipulating the properties of light, these capabilities have not yet been explored in depth for controlling photon-pair generation. In this work, we study photon-pair generation by a nonlinear metasurface, focusing on effects that arise from a periodic or disordered arrangement of the meta-atoms. We also find a special detection configuration, where the two-photon directionality pattern is independent of the meta-atom arrangement and even robust to disorder. The symmetry in the physics of two-photon interference is identified as the cause of such an effect.