Quantum computing in the solid state: the challenge of decoherence

The current status of solid-state implementations of quantum computing is briefly described. There are numerous candidate proposals, but only comparatively recently have some of them begun to progress to the point of demonstrating coherent motion of the individual quantum bits (qubits), and the controlled coupling of more than one qubit remains a significant challenge. We also present a generalization of the fluctuation-dissipation theorem that provides a relationship between the coherent evolution entangling two spatially separated qubits, and the associated irreducible decoherence. This relationship may be used to bound the maximum attainable figures of merit in proposed two-qubit gates.