Precision spectroscopy of Helium

Accurate Quantum‐Electrodynamics (QED) tests of the simplest bound three body atomic system are performed by precise laser spectroscopic measurements in atomic Helium. In this paper, we present a review of measurements between triplet states at 1083 nm (23S‐23P) and at 389 nm (23S‐33P). In 4He, such data have been used to measure the fine structure of the triplet P levels and, then, to determine the fine structure constant when compared with equally accurate theoretical calculations. Moreover, the absolute frequencies of the optical transitions have been used for Lamb‐shift determinations of the levels involved with unprecedented accuracy. Finally, determination of the He isotopes nuclear structure and, in particular, a measurement of the nuclear charge radius, are performed by using hyperfine structure and isotope‐shift measurements.