Precision Separated-Oscillatory-Field Measurement of then=10+F3−+G4Interval in Helium: A Precision Test of Long-Range Relativistic, Radiative, and Retardation Effects

We have measured the ${}_{}{}^{+}{}_{}{}^{}F_3^{}{}_{}{}^{}$- ${}_{}{}^{+}{}_{}{}^{}G_4^{}{}_{}{}^{}$ interval in $n=10\mathrm{}$ of helium to be 2017.3258(4) MHz (an accuracy of 200 ppb). The measurement uses a fast neutral beam and Ramsey separated-oscillatory-field microwave-induced transitions. When compared to recent high-precision variational calculations, it provides a high-precision test of physics (including relativistic, radiative, and retardation effects) on the large distance scale of these Rydberg atoms. The comparison indicates either that the expected long-range Casimir effect is not present, or the presence of new long-range physics.