  We demonstrate super-resolution optical sensing of the size of the wave
packet of a single trapped ion. Our method extends the well known ground state
depletion (GSD) technique to the coherent regime. Here, we use a hollow beam to
strongly saturate a coherently driven dipole-forbidden transition around a
sub-diffraction limited area at its center and observe state dependent
fluorescence. By spatially scanning this laser beam over a single trapped
$^{40}\mathrm{Ca}^+$ ion, we are able to measure the wave packet sizes of
cooled ions. Using a depletion beam waist of $4.2(1)\,\mu$m we reach a spatial
resolution which allows us to determine a wave packet size of $39(9)\,$nm for a
near ground state cooled ion. This value matches an independently deduced value
of $32(2)\,$nm, calculated from resolved sideband spectroscopy measurements.
Finally, we discuss the ultimate resolution limits of our adapted GSD imaging
technique in view of applications to direct quantum wave packet imaging.
