Nanoscale virtual potentials using optical tweezers

We combine optical tweezers with feedback to impose arbitrary time-dependent potentials on a colloidal particle. The feedback trap detects a particle's position, calculates a force based on an imposed “virtual potential,” and shifts the trap center to generate the desired force. This kind of feedback trap can both extend the capabilities of optical tweezers and test fundamental ideas in stochastic thermodynamics. For the former, we have controlled trap stiffness to make tweezer response truly isotropic in three dimensions, allowing unbiased measurement of dynamics. For the latter, we have created virtual double-well potentials with well separations of 11 nm and dwell times of a few ms, scales that approach those of protein-folding dynamics. Such a speed-up of dynamics will allow measurement of stochastic trajectory-based quantities for systems far from equilibrium.