Dephasing channel
= Dephasing channel
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= Dephasing
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A <dephasing channel> suppresses off-diagonal entries of a <density matrix> in a preferred basis while preserving <quantum state populations>. Complete <dephasing> is identical to a <projective measurement> in that basis with its outcome discarded.
For the <Bell correlations versus a classical mixture> example, completely <dephasing> Alice's <qubit> in the <computational basis> maps $\rho_{\mathrm{Bell}}$ to $\rho_{\mathrm{mix}}$. It preserves their perfect $Z$ agreement and each <reduced density matrix>, yet destroys the $X$ correlation. This is why testing only populations can miss damage to a quantum memory: the stored <quantum coherence> may have disappeared while every preferred-basis measurement still looks correct. The joint <Von Neumann entropy> rises from zero to one <bit>, although each local entropy stays fixed.