Period of two-state quantum interference ($T=2\pi \hslash /|E_2-E_1|$)

A superposition of two distinct energy eigenstates gives the expectation value of a time-independent observable an interference term proportional to $e^{-i(E_2-E_1)t/\hslash }$. 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.