High-precision isotope-shift measurement of23S-23Ptransition in helium

The isotope shift for the $2^3$S-$2^3$P transition between ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ has been measured with a laser-microwave spectroscopy technique to a precision of 1 ppm, or 30 kHz. Our result is 100 times more precise than previous measurements and is by far the most precise isotope-shift measurement ever made for a multielectron system. It provides, for the first time, not only a check for the specific mass shift, but also for the calculations of the nuclear volume shift, the relativistic corrections involving the coupling between electrons, and the QED contributions.