= Informationally complete POVM
A <POVM> is informationally complete if its outcome probabilities determine every input <density operator> uniquely. Its effects must span the real space of Hermitian operators. For a qubit, normalization leaves at most $m-1$ independent probabilities from $m$ outcomes; an arbitrary <Bloch vector> has three real parameters, so one fixed informationally complete measurement requires at least four outcomes. Using several differently oriented binary measurements is a different experimental setting and can also perform <quantum state tomography>.
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