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We present a model of an N-qubit channel where consecutive qubits experiencecorrelated random rotations. Our model is an extension to the standarddecoherence-free subsystems approach (DFS) which assumes that all the qubitsexperience the same disturbance. The variation of rotations acting onconsecutive qubits is modeled as diffusion on the SU(2) group. The model may beapplied to spins traveling in a varying magnetic field, or to photons passingthrough a fiber whose birefringence fluctuates over the time separation betweenphotons. We derive an explicit formula describing the action of the channel onan arbitrary N-qubit state. For N=3 we investigate the effects of diffusion onboth classical and quantum capacity of the channel. We observe thatnonorthogonal states are necessary to achieve the optimal classical capacity.Furthermore we find the threshold for the diffusion parameter above whichcoherent information of the channel vanishes.Comment: 11 pages, 6 figures, improved clarity, more discussion, many new  references and the title change

Effects of imperfect noise correlations on decoherence-free subsystems: SU(2) diffusion model