  We report a precise measurement of the isotope shifts in the $4^2$S$_{1/2}
\rightarrow 3^2$D$_{5/2}$ electric quadrupole transition at 729~nm in the $^{40
- 42,44,48}$Ca$^+$. 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 9,990,382,525.0(4.9) Hz between
$^{42,44,48}$Ca$^+$and $^{40}$Ca$^+$, 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 systems. The resulting measurement
precision provides a benchmark for tests of theoretical isotope shift
calculations, and also offers a step towards probing New Physics via isotope
shift spectroscopy.
