The foundation of discrete-variable quantum photonic technologies lies in the creation of multiphoton quantum states through spontaneous parametric down-conversion and spontaneous four-wave mixing. However, the conventional approach using nonlinear crystals and waveguides faces limitations due to the strict requirements of phase-matching conditions, energy conservation, and momentum conservation for the involved photons. These limitations restrict the minimum system size and the versatility of the produced states. To overcome these challenges, nonlinear metasurfaces composed of nanoresonators offer new possibilities by exploiting innovative physical principles. By engineering the metasurface eigenmodes, phase-matching conditions can be met within a subwavelength-scale structure at any desired optical wavelength. This allows for the simultaneous generation of multiple spectrally shifted entangled photon pairs. Additionally, the concept of bound states in the continuum will be utilized to combine phase matching with a significant enhancement of the local optical field inside a nonlinear crystal. As a result, metasurfaces will significantly boost the emission of spatially entangled photons across narrow resonance bands and a wide spectral range. The utilization of ultrathin metasurfaces as efficient sources of entangled photons holds immense potential for the miniaturization of various quantum devices. These devices play a critical role in space-related applications such as quantum communication and quantum metrology. By leveraging the unique capabilities of metasurfaces, advancements in these technologies can be achieved, leading to improved performance and expanded possibilities in space-based quantum applications.
