  Quantum information, encoded within the states of quantum systems, represents
a novel and rich form of information which has inspired new types of computers
and communications systems. Many diverse electron spin systems have been
studied with a view to storing quantum information, including molecular
radicals, point defects and impurities in inorganic systems, and quantum dots
in semiconductor devices. In these systems, spin coherence times can exceed
seconds, single spins can be addressed through electrical and optical methods,
and new spin systems with advantageous properties continue to be identified.
Spin ensembles strongly coupled to microwave resonators can, in principle, be
used to store the coherent states of single microwave photons, enabling
so-called microwave quantum memories. We discuss key requirements in realising
such memories, including considerations for superconducting resonators whose
frequency can be tuned onto resonance with the spins. Finally, progress towards
microwave quantum memories and other developments in the field of
superconducting quantum devices are being used to push the limits of
sensitivity of inductively-detected electron spin resonance. The
state-of-the-art currently stands at around 65 spins per root-Hz, with
prospects to scale down to even fewer spins.
