Pinching map
= Pinching map
{title2=$\mathcal P(\rho)=\sum_iP_i\rho P_i$}
= Projective pinching
{synonym}
For mutually orthogonal projectors summing to the identity, a pinching map discards coherences between their ranges while preserving each diagonal block. It is a <quantum channel> with <Kraus operators> $P_i$, and a <unital quantum channel>. A state is fixed by pinching exactly when it commutes with every projector.