Bound state in the continuum by spatially separated ensembles of atoms in a coupled-cavity array

We present an analytic solution of bound states in the continuum (BICs) for photons and atoms in a one-dimensional coupled cavity array. These bound states are formed by two ensembles of twolevel atoms confined in separated cavities of the array. We show that in the regime where the photon hopping rate between neighboring cavities is high compared with the collective Rabi frequency, the BIC corresponds to a subradiant collective atomic state in which the two ensembles of atoms are strongly entangled. On the other hand in the low photon hopping rate regime, the BIC behaves a quantum cavity in which photons can be trapped between the two ensemble of atoms.