  Quantum optics based on highly excited atoms, also known as Rydberg atoms,
has cemented itself as a powerful platform for the manipulation of light at the
few-photon level. The Rydberg blockade, resulting from the strong interaction
between individual Rydberg atoms, can turn a large ensemble of atoms into a
system which collectively resembles a single two-level emitter, a so-called
Rydberg superatom. The coupling of this artificial emitter to a driving
photonic mode is collectively enhanced by Rydberg interactions, enabling strong
coherent coupling at the few-photon level in free-space. The exquisite level of
control achievable through this has already demonstrated its utility in
applications of quantum computing and information processing. Here, we review
the derivation of the collective coupling between a Rydberg superatom and a
single light mode and discuss the similarity of this free-space setup to
waveguide quantum electrodynamics systems of quantum emitters coupled to
photonic waveguides. We also briefly review applications of Rydberg superatoms
to quantum optics such as single-photon generation and single-photon
subtraction.

Quantum Physics