  The root-mean-square (rms) nuclear charge radius of ^8He, the most
neutron-rich of all particle-stable nuclei, has been determined for the first
time to be 1.93(3) fm. In addition, the rms charge radius of ^6He was measured
to be 2.068(11) fm, in excellent agreement with a previous result. The
significant reduction in charge radius from ^6He to ^8He is an indication of
the change in the correlations of the excess neutrons and is consistent with
the ^8He neutron halo structure. The experiment was based on laser spectroscopy
of individual helium atoms cooled and confined in a magneto-optical trap.
Charge radii were extracted from the measured isotope shifts with the help of
precision atomic theory calculations.
