  The quantum Brownian motion of a charged particle in the electromagnetic
vacuum fluctuations is investigated near a perfectly reflecting flat boundary,
taking into account the smooth switching process in the measurement.
Constructing a smooth switching function by gluing together a plateau and the
Lorentzian switching tails, it is shown that the switching tails have a great
influence on the measurement of the Brownian motion in the quantum vacuum.
Indeed, it turns out that the result with a smooth switching function and the
one with a sudden switching function are qualitatively quite different. It is
also shown that anti-correlations between the switching tails and the main
measuring part plays an essential role in this switching effect. The switching
function can also be interpreted as a prototype of an non-equilibrium process
in a realistic measurement, so that the switching effect found here is expected
to be significant in actual applications in vacuum physics.
