Rotational Locking of Charged Microparticles in Quadrupole Ion Traps

Electric quadrupole traps are a leading technology for suspending charged objects ranging in size from single protons to atomic and molecular ions, and even to nano- and micron-sized bodies. If the levitated objects' charge distribution contains multipoles, the time-dependent trapping fields can significantly impact its rotational motion. Here, we experimentally observe the transition from librational motion to a regime where a microparticle rotates in sync with the trap drive. We theoretically explain that the locked motion is caused by the torques acting on the electric quadrupole, which can thus be expected to be ubiquitous for nonspherical micro-objects. We demonstrate the versatility of this method by spinning diverse particles such as silicon microrods, magnetic particles, and microdiamonds. For diamonds, we show that the rotational motion can be precisely characterized by stroboscopic readout of the embedded nitrogen vacancy centers. Given its generality, we anticipate that this technique will become an important tool for future experiments in levitated quantum nanomechanics.