  Metasurfaces are a key photonic platform to manipulate classical light using
sub-wavelength structures with designer optical response. Static metasurfaces
have recently entered the realm of quantum photonics, showing their ability to
tailor nonclassical states of light. We introduce the concept of space-time
quantum metasurfaces for dynamical control of quantum light. We provide
illustrative examples of the impact of spatio-temporally modulated metasurfaces
in quantum photonics, including the creation of frequency-spin-path
hyperentanglement on a single photon and the realization of space-time
asymmetry at the deepest level of the quantum vacuum. Photonic platforms based
on the space-time quantum metasurface concept have the potential to enable
novel functionalities, such as on-demand entanglement generation for quantum
communications, nonreciprocal photon propagation for free-space quantum
isolation, and reconfigurable quantum imaging and sensing.
