Results of the Investigations of the Nature of the Long -Term   Instability in Quantum Frequency Standards and Magnetometers
  The problem analysis results made the author to draw a conclusion that the nature of the resonance frequency long-term instability and drift at harmonic excitation is related to the phase dynamics of the "atom + field" system in the small vicinity of the resonance. The investigation is based on the strictly substantiated asymptotic Krylov-Bogolyubov perturbation theory. A time-dependent (drift) first-order correction of the perturbing field amplitude to the resonance frequency was disclosed. It was found that this correction is always present and is responsible for the frequency drift and long-term instability. The necessary and sufficient conditions of accurate resonance, as well as the conditions of realization of a stable (stationary, steady-state) drift-free oscillation regime in a quantum system, are obtained. 
A.G.Chirkov