  Electron spins in solids are promising candidates for quantum memories for
superconducting qubits because they can have long coherence times, large
collective couplings, and many quantum bits can be encoded into the spin-waves
of a single ensemble. We demonstrate the coupling of electron spin ensembles to
a superconducting transmission-line resonator at coupling strengths greatly
exceeding the cavity decay rate and comparable to spin linewidth. We also use
the enhanced coupling afforded by the small cross-section of the transmission
line to perform broadband spectroscopy of ruby at millikelvin temperatures at
low powers. In addition, we observe hyperfine structure in diamond P1 centers
and time domain saturation-relaxation of the spins.
