Quantum nanophotonics is an active research field with emerging applications that range from quantum computing to imaging and telecommunications. This has motivated scientists and engineers to develop sources for entangled photons that can be integrated into nano-scale photonic circuits. Practical application of nanoscale devices requires a high photon-pair generation rate, room-temperature operation, and entangled photons emitted at telecommunications wavelengths in a directional manner.

The most common way to create entangled photons is by a process known as Spontaneous Parametric Down Conversion (SPDC) which involves a being split into two entangled photons of lower frequencies, known as the signal and idler. Conventional approaches for SPDC rely on bulky devices that are up to several centimeters in length and are not optimal for photonic circuit integration. Conversely, at the nanoscale, the efficiency of the SPDC process is hindered by the small volume of the resonators, and the directionality of the emitted photons is challenging to control.

Dielectric metasurfaces offer a promising route to enhance and tailor SPDC emission. To date, however, metasurfaces have used relatively low quality-factor Mie resonances and have an accordingly broad emission spectrum, which restricts the spectral brightness of photons. New research reveals that extended Bound States in the Continuum (BIC) resonances make it possible to harness modes in the metasurface that have very high quality factors. This in turn means that the photon-pair generation inside the resonators is enhanced by many orders of magnitude and the wavelength of the photons will have a very narrow bandwidth. This results in a very high spectral brightness, which is beneficial for quantum network applications.

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