  We present a new class of thermal ratchets operating under the action of a
symmetry breaking non-Hermitian perturbation which rectifies thermal
fluctuations, and driven by a unbiased periodic force. The peculiar
non-Hermitian dynamics which follows causes energy transduction from the force
to the system in such a way that an average `uphill' particle current is
induced. We discuss physical realizations in assemblies of orientable
particles, in itinerant oscillator models, and in problems of diffusion in
disordered media.
