Realization of a feedback controlled flashing ratchet
  A flashing ratchet transports diffusive particles using a time-dependent, asymmetric potential. Particle speed is predicted to increase when a feedback algorithm based on particle positions is used. We have experimentally realized such a feedback ratchet using an optical line trap, and observed that use of feedback increases velocity by up to an order of magnitude. We compare two different feedback algorithms for small particle numbers, and find good agreement with simulations. We also find that existing algorithms can be improved to be more tolerant to feedback delay times. 
Benjamin J. Lopez, Nathan J. Kuwada, Erin M. Craig, Brian R. Long, and   Heiner Linke