Levitated particles are unique among optomechanical systems in that they benet from the absence of physicalcontact with the external environment. Recently, a new research direction known as levitated optomechanicshas attracted interest in numerous research groups, with a major focus on optically suspended particles. Incontrast to optical trapping experiments, we levitate charged silica nanospheres in high vacuum by means of aPaul trap. This method provides a deeper conning potential than that of optical traps and enables trappingof optically opaque objects. A detection system based on back-focal-plane interferometry allows us to observecenter-of-mass (CoM) motion of the particle. We introduce an additional laser beam that is focused on theparticle and provides optical forces with projections on all three principal axes of the Paul trap. This additionalbeam is intensity-modulated by an acousto-optic modulator controlled by feedback electronics. In this way, weare able to cool the secular motion of the CoM below 1 K, the eective temperature in all three directions beingcurrently limited only by the detection eciency. This is the rst time, to the best of our knowledge, that lasercooling of mechanical motion of a nanoparticle in a Paul trap potential has been demonstrated. Such cooling actslocally on a single particle, in contrast to feedback provided by auxiliary electric elds, and opens up possibilitiesfor sympathetic cooling of particles levitated in Paul traps when other methods are not suitable, for example, inthe case of highly absorptive particles.
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