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