Quantum uncertainty
= Quantum uncertainty
{title2=$\Delta Q=\|(Q-\langle Q\rangle)\psi\|$}
For a normalized state with finite second moment of a <quantum observable> $Q$, its uncertainty is the standard deviation of the measurement outcome: $(\Delta Q)^2=\langle Q^2\rangle-\langle Q\rangle^2$. This equals $\|(Q-\langle Q\rangle)\psi\|^2$. The <Robertson uncertainty principle> bounds products of uncertainties using the expectation of the <commutator>.