We study the effects of the electromagnetic subvacuum fluctuations on the
dynamics of a nonrelativistic charged particle in a wavepacket. The influence
from the quantum field is expected to give an additional effect to the velocity
uncertainty of the particle. In the case of a static wavepacket, the observed
velocity dispersion is smaller in the electromagnetic squeezed vacuum
background than in the normal vacuum background. This leads to the subvacuum
effect. The extent of reduction in velocity dispersion associated with this
subvacuum effect is further studied by introducing a switching function. It is
shown that the slow switching process may make this subvacuum effect
insignificant. We also point out that when the center of the wavepacket
undergoes non-inertial motion, reduction in the velocity dispersion becomes
less effective with its evolution, no matter how we manipulate the
nonstationary quantum noise via the choice of the squeeze parameters. The role
of the underlying fluctuation-dissipation relation is discussed.Comment: 30 pages, 2 figure