  Information-driven engines that rectify thermal fluctuations are a modern
realization of the Maxwell-demon thought experiment. We introduce a simple
design based on a heavy colloidal particle, held by an optical trap and
immersed in water. Using a carefully designed feedback loop, our experimental
realization of an "information ratchet" takes advantage of favorable "up"
fluctuations to lift a weight against gravity, storing potential energy without
doing external work. By optimizing the ratchet design for performance via a
simple theory, we find that the rate of work storage and velocity of directed
motion is limited only by the physical parameters of the engine: the size of
the particle, stiffness of the ratchet spring, friction produced by the motion,
and temperature of the surrounding medium. Notably, because performance
saturates with increasing frequency of observations, the measurement process is
not a limiting factor. The extracted power and velocity are at least an order
of magnitude higher than in previously reported engines.
