  Successful feedback control of small systems allows for the rectification of
thermal fluctuations, converting them into useful energy; however, control
itself requires work. This paper emphasizes the fact that the controller is a
physical entity interacting with the feedback-controlled system. For a
specifically designed class of controllers, reciprocal interactions become
nonreciprocal due to large timescale separation, which considerably simplifies
the situation. We introduce a minimally dissipative controller model,
illustrating the findings using a simple example. We find that the work
required to run the controller must at least compensate for the decrease in
entropy due to the control operation.
