Ja n 20 02 Nonresonant corrections to the 1 s - 2 s two - photon resonance for the hydrogen atom

Nonresonant corrections to the 1s-2s two-photon resonance for the hydrogen atom. Abstract 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. Nonresonant (NR) corrections have been first introduced in Ref. [1] where the modern QED theory of the natural line profile in atomic physics has been formulated. The NR corrections indicate the limit up to which the concept of the energy of an excited atomic state has a physical meaning-that is the resonance approximation. In the resonance approximation the line profile is described by the two parameters: energy E and width Γ. Beyond this approximation the specific role of E and Γ should be replaced by the complete evaluation of the line profile for the particular process. If the distortion of the Lorentz profile is small one can still consider the NR correction as an additional energy shift. Unlike all other energy corrections, this correction depends on the particular process which has been employed for the measurement of the energy difference. Quite independent of the accuracy 1