  The proton magnetic moment in nuclear magnetons is measured to be
$\mu_p/\mu_N \equiv g/2 = 2.792\,846 \pm 0.000\,007$, a 2.5 ppm (parts per
million) uncertainty. The direct determination, using a single proton in a
Penning trap, demonstrates the first method that should work as well with an
antiproton as with a proton. This opens the way to measuring the antiproton
magnetic moment (whose uncertainty has essentially not been reduced for 20
years) at least $10^3$ times more precisely.
