Lower limit on decoherence introduced by entangling two   spatially-separated qubits
  It is shown that a generalization of the fluctuation-dissipation theorem places an upper bound on the figure of merit for any quantum gate designed to entangle spatially-separated qubits. The bound depends solely on the spectral properties of the environment. The bound applies even to systems performing a quantum computation within a decoherence-free subspace, but might be optimized by the use of non-equilibrium squeezed states of the environment, or by using an environment system constructed to have response confined to certain frequencies. 
A.J. Fisher