  Light-matter interactions lie at the heart of condensed matter physics,
providing physical insight into material behaviour while enabling the design of
new devices. Perhaps this is most evident in the push to develop quantum
information and spintronic technologies. On the side of quantum information,
engineered light-matter interactions offer a powerful means to access and
control quantum states, while at the same time new insights into spin-photon
manipulation will benefit the development of spintronic technologies. In this
context the recent discovery of hybridization between ferromagnets and cavity
photons has ushered in a new era of light-matter exploration at the crossroads
of quantum information and spintronics. The key player in this rapidly
developing field of cavity spintronics is a quasiparticle, the
cavity-magnon-polariton. In this early review of recent work, the fundamental
behaviour of the cavity-magnon-polariton is summarized and related to the
development of new spintronic applications. In the last few years a
comprehensive theoretical framework of spin-photon hybridization has been
developed. On an intuitive level many features can be described by a model of
coupled oscillators, however the origin of hybridization is only revealed by
considering a comprehensive electrodynamic framework. Here both approaches are
summarized and a quantum description using the input-output formalism is
outlined. Based on this foundation, in depth experimental investigations of the
coupled spin-photon system have been performed. For example, the influence of
hybridization on spin current generation has been revealed and several in-situ
coupling control mechanisms have been developed. The many recent developments
within this field represent only the first steps in what appears to be a bright
future for cavity spintronics.
