Theory of the Energy Variance in a Quantum Bit

We define a new quantum Hermitian operator (namely, the energy variance operator) which is simply duplicated from the statistical definition of energy variance in classical physics. Its expectation value yields the standard deviation of the energy about the mean value of this latter. We show by use of an exact Hamiltonian description that this standard deviation is due to the high-frequeny energy oscillations which are usually discarded in the rotating wave aproximation. We check the present theory by recovering the duration of an abrupt quantum jump that has been described in a recent experiment.