PERTURBATIVE EXPANSIONS FOR THE FIDELITIES AND SPATIALLY CORRELATED DISSIPATION OF QUANTUM BITS

We construct generally applicable short-time perturbative expansions f or some fidelities, such as the input-output fidelity, the entanglemen t fidelity, and the average fidelity. Successive tens of these expansi ons yield characteristic times for the damping of the fidelities invol ving successive powers of the Hamiltonian. The: second-order results, which represent the damping rates of the fidelities, are extensively d iscussed. As an interesting application of these expansions, we use th em to study spatially correlated dissipation of quantum bits. Spatial correlations in the dissipation are described by a correlation functio n. Explicit conditions are derived for independent decoherence and for collective decoherence. [S1050-2947(97)02712-1].