Rare-earth doped crystals have attracted a significant amount of attention for
use in quantum systems. Available, long-lived, optical and microwave transitions
has lead to proposals for quantum transduction and quantum memories, both of
which are important in building large scale quantum networks. Ensembles of
rare-earth spins can be coupled to superconducting resonators, and high coupling
strengths (with cooperativity > 1) readily achievable. While such systems have
been constructed, a useful quantum memory which exploits highly coherent transitions has not yet been developed in the microwave domain. In this thesis we couple
high-Q superconducting resonators to Yb doped YSO. The spin system of Yb:YSO
is explored and the main causes of decoherence are outlined, these are found to
be instantaneous diffusion and spectral diffusion. In the process of this, new techniques are developed to determine decoherence sources, where nuclear spins within
the YSO crystal are found to limit coherence.
Two different regimes are explored to increase the coherence time. Using optimal field orientations and high magnetic field magnitudes, the coherence time is
extended to (6±2) ms. While the zero field clock transition is used, along with isotopic purification, to reach the same time ((6±1) ms). Using these techniques to increase coherence, the foundations for a microwave quantum memory with Yb:YSO
are laid. Cooperativities > 1 are measured for three different Yb spin systems, this
allows for these spin systems to be used in memory protocols and reach unit efficiency. New pulse sequences using adiabatic fast passage are developed to provide
control over the spin ensemble and for memory protocols.
Finally, we use the knowledge from all of these studies to propose a system
which would form the basis of an efficient, long-lived microwave quantum memory
using FIB-milled Yb:YSO.