Resonant metasurfaces for generating complex quantum states

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. Metasurfaces are specially designed arrays of dielectric components that transform the function of bulk optical components into thin films. Exploiting the physics of bound states in the continuum for the highly efficient trapping of light, Santiago-Cruz et al. design and fabricate metasurfaces that can operate as quantum sources of light. Patterned in GaAs, the quantum source provides entangled pairs of photons across a broad range of wavelength, allowing the formation of complex quantum states. The approach will be useful for the development of integrated optical and quantum optical devices. —ISO Resonant metasurfaces can generate photon pairs at multiple selected wavelengths, enabling the formation of complex quantum states.