Multi-channel waveguide QED with atomic arrays in free space

We study light scattering off a two-dimensional (2D) array of atoms driven to Rydberg levels. We show that the problem can be mapped to a generalized model of waveguide QED, consisting of multiple 1D photonic channels (transverse modes), each of which directionally coupled to a corresponding Rydberg surface mode of the array. In the Rydberg blockade regime, collective excitations of different surface modes block each other, leading to multi-channel correlated photonic states. Using an analytical approach, we characterize inter-channel quantum correlations, and elucidate the role of collective two-photon resonances of the array. Our results open new possibilities for multimode many-body physics and quantum information with photons in a free-space platform.