In ‘flat optics’ approach, bulky optical elements are replaced by subwavelength layers or metasurfaces. ‘Flat’ elements are broadband, ultrafast, multifunctional, and can use practically any materials, including highly absorptive or nonlinear. They proved very efficient in nonlinear optics and are now conquering quantum optics. In particular, spontaneous parametric down-conversion (SPDC) in ultrathin layers and metasurfaces opens new paths for the generation and engineering of entangled photons.

Due to their tiny thickness, ‘flat’ SPDC sources enable momentum non-conservation between the pump photon and the emitted photon pair. As a result, photon pairs can have ultrabroad spectra and giant degrees of energy-time entanglement. There is also enormous freedom in the engineering of their polarization state. In metasurfaces, the rate of pair generation can be enhanced by orders of magnitude using Mie resonances or bound states in the continuum. Our recent experiments demonstrate SPDC with either one or both photons at resonance wavelengths.

We also observed generation of states with multiple entanglement links and versatile directivity of the photon emission. In the last part of my talk, I will discuss the possibility to enhance the efficiency of SPDC from nanoscale sources, leading to the generation of squeezed vacuum.