Fig. 2.

a Setup for measuring the phase shift imprinted by a single ${}^{174}{\text{Yb}}^{+}$-ion. With an additional laser beam at a wavelength of 935.2 nm, we can pump the ion back from the metastable $D_{\text{3/2}}$ state into the $S_{\text{1/2}}$ ground state. This laser is sent onto the ion from a hole at the backside of the parabolic mirror. The same is done for a second laser beam at a wavelength of 369.5 nm that is used for ionization and for cooling the ion in certain steps of the experimental procedure (see text). SHWP segmented half-wave-plate, (P)BS (polarizing) beam splitter (other abbreviations in the text). b Intensity signal $I_{\text{TDC}}(t)$ obtained from the statistics of photon detection times on the TDC for the ion being in the bright state (red) and in the $D_{\text{3/2}}$ dark state (black)