Part-per-billion measurement of the 4 2S1 /2→3 2D5 /2 electric-quadrupole-transition isotope shifts between +42,44,48Ca and +40Ca
We report a precise measurement of the isotope shifts in the 4 2S1 /2→3 2D5 /2 electric quadrupole transition at 729 nm in +40 -42,44,48Ca. The measurement has been made via high-resolution laser spectroscopy of co-trapped ions, finding measured shifts of 2 771 872 467.6(7.6), 5 340 887 394.6(7.8), and 9 990 381 870.0(6.3) Hz between +42,44,48Ca and +40Ca, respectively. By exciting the two isotopes simultaneously, using frequency sidebands derived from a single laser, systematic uncertainties resulting from laser frequency drifts are eliminated. This permits far greater precision than similar previously published measurements in other alkaline-earth-metal systems. The resulting measurement accuracy provides a benchmark for tests of theoretical isotope shift calculations and also offers a step towards probing new physics via isotope shift spectroscopy.