Electronic feedback control of single ion motion

The paper demonstrates an alternative method to cool the ion's motion below its Doppler-limit by using "cold damping" or homodyne feedback control. For cold damping, an additional friction force proportional to the instantaneous velocity of the ion is created, by supplying additional electric fields along the direction of the motion of the ion, which are -90/spl deg/ phase-shifted with respect to the measured amplitude of the motion.