 Entropy increase under nonselective projective measurement (source code)

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= Entropy increase under nonselective projective measurement

If $\sigma=\sum_iP_i\rho P_i$ is a <nonselective projective measurement>, then $S(\sigma)\geq S(\rho)$. Equality holds exactly when $\rho$ is already block diagonal in the measurement decomposition, equivalently $[\rho,P_i]=0$ for every $i$.

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