Tailored particle current in an optical lattice by a weak time-symmetric harmonic potential

Quantum ratchets exhibit asymptotic currents when driven by a time-periodic potential of zero mean if the proper spatiotemporal symmetries are broken. Recently, there has been debate on whether directed currents may arise for potentials which do not break these symmetries. We show here that in the presence of degeneracies in the quasienergy spectrum, long-lasting directed currents can be induced, even if the time-reversal symmetry is not broken. Our model can be realized with ultracold atoms in optical lattices in the tight-binding regime, and we show that the time scale of the average current can be controlled by extremely weak fields.