  Optically levitated nanoparticles in vacuum are a promising model system to
test physics beyond our current understanding of quantum mechanics. Such
experimental tests require extreme control over the dephasing of the levitated
particle's motion. If the nanoparticle carries a finite net charge, it
experiences a random Coulomb force due to fluctuating electric fields. This
dephasing mechanism can be fully excluded by discharging the levitated
particle. Here, we present a simple and reliable technique to control the
charge on an optically levitated nanoparticle in vacuum. Our method is based on
the generation of charges in an electric discharge and does not require
additional optics or mechanics close to the optical trap.
