Frequency Metrology of Helium around 1083 nm and Determination of the Nuclear Charge Radius

We measure the absolute frequency of seven out of the nine allowed transitions between the 2 ³S and 2 ³P hyperfine manifolds in a metastable ³He beam by using an optical frequency comb synthesizer-assisted spectrometer. The relative uncertainty of our measurements ranges from 1×10^-11 to 5×10^-12, which is, to our knowledge, the most precise result for any optical ³He transition to date. The resulting 2 ³P--2 ³S centroid frequency is 276 702 827 204.8(2.4) kHz. Comparing this value with the known result for the ⁴He centroid and performing ab initio QED calculations of the ⁴He-³He isotope shift, we extract the difference of the squared nuclear charge radii δr² of ³He and ⁴He. Our result for δr²=1.074(3) fm² disagrees by about 4σ with the recent determination [R. van Rooij et al., Science 333, 196 (2011)].