  We analyze the magnetic dipole coupling of an ensemble of spins to a
superconducting microwave stripline structure, incorporating a Josephson
junction based transmon qubit. We show that this system is described by an
embedded Jaynes-Cummings model: in the strong coupling regime, collective
spin-wave excitations of the ensemble of electrons pick up the nonlinearity of
the cavity mode, such that the two lowest eigenstates of the coupled spin-wave
+ microwave-cavity + Josephson-junction system define a hybrid two-level
system. The proposal described here enables the use of spin ensembles as qubits
which can be coherently manipulated and coupled using the same
nonlinear-cavity. Possibility of strong-coupling cavity-QED with
magnetic-dipole transitions opens up the possibility of extending previously
proposed quantum information processing protocols to spins in silicon or
graphene, without the need for single-electron confinement.
