Quantum processing by adiabatic transfer through a manifold of dark states

We consider a network whose nodes are electromagnetic cavities, each coupled to a single three-level atom. The nodes are connected by optical fibers. Each atom is addressed by a control laser, which along with the cavity field drives atomic transitions. The network can be in the form of chain or two- and three-dimensional arrays of N cavities connected by N _B fibers. Following the work on a two-cavity system by Pellizzari, we find that under certain conditions the system possesses two kinds of dark states. The first kind are N states corresponding to atomic excitations at each node and these are our logical states for quantum processing. The second kind are N _B degenerate dark states on pairs of sites connected by a fiber. By manipulating intensities and phases of control lasers on the cavities, one can pass adiabatically among these dark states due to their degeneracy. This network operates as an N-level quantum system in which one can generate computationally useful states by protocols of external controls. We obtain numerical results for small chains and lattices to demonstrate some quantum operations such as the transport of states across the array, generation of W states, and Fourier-like states. We also discuss effects of dissipation and limitations of the model.