= Computational-basis intercept-resend attack
{title2=$\rho\mapsto\sum_{b=0}^1(I\otimes|b\rangle\langle b|)\rho(I\otimes|b\rangle\langle b|)$}
Measuring the transmitted <qubit> in the <computational basis> and forwarding its measured state produces a <dephasing channel> for users who do not know the outcome. For a <Bell state> $|\Psi^+\rangle$, the reduced state becomes $(|01\rangle\langle01|+|10\rangle\langle10|)/2$. It retains perfect $Z$ anticorrelation while destroying $X$ correlation. Thus a single perfectly correlated key basis need not reveal an <intercept-resend attack>; incompatible-basis correlation tests can.
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