Informationally complete three-observable qubit tomography
= Informationally complete three-observable qubit tomography
Three Hermitian expectation values determine an arbitrary qubit state exactly when the traceless parts of the three observables span the three-dimensional space generated by the Pauli matrices. Mere linear independence as Hermitian matrices is insufficient: $I,\sigma_x,\sigma_z$ are independent but cannot detect the sign or magnitude of the Bloch $y$ component.