  Metamaterials have been a major research area for more than two decades now,
involving artificial structures with predesigned electromagnetic properties
constructed from deep subwavelength building blocks. They have been used to
demonstrate a wealth of fascinating phenomena ranging from negative refractive
index and epsilon-near-zero to cloaking, emulations of general relativity
effects, and super-resolution imaging, to name a few. In the past few years,
metamaterials have been suggested as a new platform for quantum optics, and
several pioneering experiments have already been carried out with single
photons. Here, we employ a dielectric metasurface to generate entanglement
between spin and orbital angular momentum of single photons. We demonstrate
experimentally the generation of the four Bell states by utilizing the
geometric phase arising from the photonic spin-orbit interaction. These are the
first experiments with entangled states with metasurfaces, and as such they are
paving the way to the new area of quantum metamaterials.
