Charge radii of neutron-deficient ^{55, 56}Ni

The ⁵⁶Ni nucleus is doubly magic with the neutron number N = 28, which occurs due to a shell gap driven by the spin-orbit force, but is considered to be a soft core. The mean-square charge radii <r²> were determined for ^{55, 56}Ni using collinear laser spectroscopy technique at BECOLA at NSCL/MSU to investigate how the softness manifests itself in <r²> at and below N = 28, and to benchmark nuclear theories. Details of the experiment and preliminary results will be presented.