  Metasurfaces are an emerging platform for manipulating light on a
two-dimensional plane. Existing metasurfaces comprise arrays of optical
resonators such as plasmonic antennas or high-index nanorods. In this letter,
we describe a new approach to realize metasurfaces based on electronic
transitions in two-level systems (TLSs). These metasurfaces can reproduce all
of the major results in conventional metasurfaces. In addition, since TLSs can
be lossless, tunable, and orders of magnitude smaller than optical resonators,
atomic metasurfaces can realize functions that are difficult to achieve with
optical resonators. Specifically, we develop quantum scattering theory to
describe the interaction between single photons and atomic metasurfaces. Based
on the theory, we use a sheet of three-level {\Lambda}-type atoms as an example
and theoretically demonstrate dynamically tunable single-photon steering. We
further show directional beaming of spontaneously emitted photons from atomic
metasurfaces.
