  The evolution of a quantum system is supposed to be impeded by measurement of
an involved observable. This effect has been proven indistinguishable from the
effect of dephasing the system's wave function, except in an individual quantum
system. The coherent dynamics, on an optical E2 line, of a single trapped ion
driven by light of negligible phase drift has been alternated with
interrogations of the internal ion state. Retardation of the ion's nutation,
equivalent to the quantum Zeno effect, is demonstrated in the statistics of
sequences of probe-light scattering ''on'' and ''off'' detections, the latter
representing back-action-free measurement.
