Period of two-state quantum interference
= Period of two-state quantum interference
{title2=$T=2\pi\hbar/|E_2-E_1|$}
A superposition of two distinct <energy eigenstates> gives the <expectation value> of a time-independent <observable> an interference term proportional to $e^{-i(E_2-E_1)t/\hbar}$. The displayed beat period applies when that term has nonzero amplitude. A vanishing off-diagonal <matrix element>, zero state coefficient or equal <energies> instead gives a constant <expectation value>.