Inelastic transitions and counterflow tunneling in double-dot quantum ratchets

The ratchet regime of unbiased double quantum dots driven out of equilibrium by an independently biased nearby detector has been theoretically studied using the nonequilibrium Keldysh formalism and the random-phase approximation for the Coulomb effects. When the detector is suitably biased the energy exchange between the two systems removes the Coulomb blockade on the double dot via inelastic interdot tunneling. The energy detuning determines whether the current flows in the same direction as the driving current (positive flow) or in the opposite direction (electronic counterflow). In both cases the intradot transitions lead to negative-differential conductance. Besides the ratchet contribution to the current we also single out a Coulomb drag component.