= Collective dephasing
{title2=$U_\phi=\exp[-i\phi\sum_jZ_j/2]$}
Correlated <dephasing> caused by the same fluctuating $z$ rotation on all <qubits>. Averaging $U_\phi\rho U_\phi^\dagger$ over the unknown angle suppresses <quantum coherence> between sectors with different total $Z$ <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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