Nonresonant corrections to the 1s-2s two-photon resonance for the   hydrogen atom
  The nonresonant (NR) corrections are estimated for the most accurately measured two-photon transition 1s-2s in the hydrogen atom. These corrections depend on the measurement process and set a limit for the accuracy of atomic frequency measurements. With the measurement process adopted in the modern experiments the NR contribution for 1s-2s transition energy can reach 10^{-3} Hz while the experimental inaccuracy is quoted to be +/- 46 Hz. 
L.N. Labzowsky, D.A. Solovyev, G. Plunien, and G. Soff