Correlated dephasing caused by the same fluctuating rotation on all qubits. Averaging over the unknown angle suppresses quantum coherence between sectors with different total eigenvalues. Within one eigenspace, however, every state acquires the same global phase, so its density matrix is preserved. Such an eigenspace is a decoherence-free subspace for this noise model. The assumption concerns the shared rotation; independent local phase noise generally destroys this protection.
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