Double-dot quantum ratchet driven by an independent quantum point contact

Vadim S. Khrapay

Institute for Solid State Physics, Russian Academy of Sciences, Chernogolovka, Russia

V.S. Khrapay, S. Ludwig, J.P. Kotthaus, H.P. Tranitz, W. Wegscheider

Broken inversion symmetry can lead to directed motion in a system subjected to nonequilibrium fluctuations Such systems, called ratchets, appear in variety of examples from classically moving Brownian motors to tunnelling quantum ratchets. A double quantum dot (DQD) with an internal asymmetry, caused by the spatial distribution of quantized charge, represents a special-type quantum ratchet. We study a system of a weakly coupled DQD and a nearby quantum point contact (QPC), realized in a lateral nanostructure in AlGaAs/GaAs. The DQD and the QPC are embedded into separate electric circuits. We observe a novel dynamic DQD-QPC interaction effect, which manifests itself in current flowing through the Coulomb blockaded DQD in response to a strong source bias on the QPC. The current direction is determined by the position of the highest energy electron in the DQD, localized in one of the two quantum dots. The magnitude of the current depends upon the mismatch of the energy levels in individual dots. We prove that an inelastic interdot tunnelling, similar to photon-assisted tunnelling, is the actual electron transfer process in the DQD. The absorbtion of energy quanta is possible within a 250~GHz bandwidth. The observation is interpreted in terms of a quantum ratchet phenomenon in the DQD, driven by the strongly biased QPC.

In dependence on the QPC transmission, the driven DQD current shows a pronounced peak at the onset of the 1st quantized plateau of the QPC conductance, and a much smaller peak at the onset of the 2nd plateau. This implies the relevance of partition noise in the QPC for the ratchet driving mechanism. The QPC bias dependence shows that most probably a relaxation of the electrons in the QPC is responsible for the driving mechanism of the DQD ratchet, rather than shot noise voltage fluctuations. Possible microscopic origin of the QPC-mediated excitation of the DQD is discussed.