  It has been shown that the combination of a broken spatial symmetry in the
potential (or ratchet potential) and time correlations in the driving are
crucial, and enough to allow transformation of the fluctuations into work. The
required broken spatial symmetry implies a specific molecular arrangement of
the proteins involved. Here we show that a broken spatial symmetry is not
required, and that temporally asymmetric fluctuations (with mean zero) can be
used to do work, even when the ratchet potential is completely symmetric.
Temporal asymmetry, defined as a lack of invariance of the statistical
properties under the operation to temporal inversion, is a generic property of
nonequilibrium fluctuation, and should therefore be expected to be quite common
in biological systems.
