  Closed-loop or feedback controlled ratchets are Brownian motors that operate
using information about the state of the system. For these ratchets, we compute
the power output and we investigate its relation with the information used in
the feedback control. We get analytical expressions for one-particle and
few-particle flashing ratchets, and we find that the maximum power output has
an upper bound proportional to the information. In addition, we show that the
increase of the power output that results from changing the optimal open-loop
ratchet to a closed-loop ratchet also has an upper bound that is linear in the
information.
