Fano interference of photon pairs from a 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 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 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, shown in the form of a Fano contour, in the spectral domain in these types of devices. 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 for photonic quantum technologies.