  We present the first laser spectroscopic measurement of the neutron-rich
nucleus $^{68}$Ni at the \mbox{$N=40$} subshell closure and extract its nuclear
charge radius. Since this is the only short-lived isotope for which the dipole
polarizability $\alpha_{\rm D}$ has been measured, the combination of these
observables provides a benchmark for nuclear structure theory. We compare them
to novel coupled-cluster calculations based on different chiral two- and
three-nucleon interactions, for which a strong correlation between the charge
radius and dipole polarizability is observed, similar to the stable nucleus
$^{48}$Ca. Three-particle--three-hole correlations in coupled-cluster theory
substantially improve the description of the experimental data, which allows to
constrain the neutron radius and neutron skin of $^{68}$Ni.

Nuclear Experiment;Nuclear Theory