  We study a hybrid quantum system consisting of spin ensembles and
superconducting flux qubits, where each spin ensemble is realized using the
nitrogen-vacancy centers in a diamond crystal and the nearest-neighbor spin
ensembles are effectively coupled via a flux qubit.We show that the coupling
strengths between flux qubits and spin ensembles can reach the strong and even
ultrastrong coupling regimes by either engineering the hybrid structure in
advance or tuning the excitation frequencies of spin ensembles via external
magnetic fields. When extending the hybrid structure to an array with equal
coupling strengths, we find that in the strong-coupling regime, the hybrid
array is reduced to a tight-binding model of a one-dimensional bosonic lattice.
In the ultrastrong-coupling regime, it exhibits quasiparticle excitations
separated from the ground state by an energy gap. Moreover, these quasiparticle
excitations and the ground state are stable under a certain condition that is
tunable via the external magnetic field. This may provide an experimentally
accessible method to probe the instability of the system.
