Ground-state probability
= Ground-state probability
{title2=$P_1=|\langle\phi_1,\psi\rangle|^2$}
The probability that an <energy measurement> selects a <ground state>. For a nondegenerate normalized mode $\phi_1$, it is $|\langle\phi_1,\psi\rangle|^2$; for a degenerate ground eigenspace, use its full orthogonal projection.