In Coulomb drag, a current flowing in one conductor can induce a voltage across an adjacent conductor via the Coulomb interaction. The mechanisms yielding drag effects are not always understood, even though drag effects are sufficiently general to be seen in many low-dimensional systems. In this Letter, we observe Coulomb drag in a Coulomb-coupled double quantum dot and, through both experimental and theoretical arguments, identify cotunneling as essential to obtaining a correct qualitative understanding of the drag behavior. DOI: 10.1103/PhysRevLett.117.066602 Coulomb-coupled quantum dots yield a model system for Coulomb drag [1], the phenomenon where a current flowing in a so-called drive conductor induces a voltage across a nearby drag conductor via the Coulomb interaction [2]. Though charge carriers being dragged along is an evocative image, as presented in early work on coupled 2D-3D [3] or 2D-2D [4] semiconductor systems, later measurements in graphene [5,6], quantum wires in semiconductor 2DEGs [7][8][9][10], and coupled double quantum dots [11] have indicated that the microscopic mechanisms leading to Coulomb drag can vary widely. For example, collective effects are important in 1D, but less so in other dimensions. All drag effects require interacting subsystems and vanish when both subsystems are in local equilibrium.A perfect Coulomb drag with equal drive and drag currents has been observed in a bilayer 2D electron system: effectively a transformer operable at zero frequency [12]. Coulombcoupled quantum dots can rectify voltage fluctuations to unidirectional current, with possible energy harvesting applications [13,14]. This rectification of nonequilibrium fluctuations is similar to a ratchet effect, as observed in charge- [15][16][17][18] and spin-based nanoelectronic devices [19], as well as in rather different contexts such as suspended colloidal particles in asymmetric periodic potentials [20]. Coulomb-coupled dots have also been proposed as a means for testing fluctuation relations out of equilibrium [1].An open question is how higher-order tunneling events in the quantum coherent limit contribute to Coulomb drag processes [21]. In this Letter, we present experimental measurements and theoretical arguments showing that simultaneous tunneling of electrons (cotunneling) is crucial to describe drag effects qualitatively in Coulomb-coupled double quantum dots (CC-DQDs). Previous theoretical work has obtained drag effects with sequential tunneling models [1] (for an exception, see Ref.[22]), and these models have been invoked in measurements of stacked graphene quantum dots [21]. We demonstrate here that for a DQD, cotunneling contributes to the drag current at the same order as sequential tunneling in a perturbation expansion. This has profound consequences in experiment, notably a measurable drag current even when the drag dot is far off resonance, and a gate voltage-dependent vanishing of the Coulomb gap above which the drag current can be measured. Our experiment shows that the drag mechanisms c...

We consider an interacting quantum dot working as a coherent source of single electrons. The dot is tunnel coupled to a reservoir and capacitively coupled to a gate terminal with an applied ac potential. At low frequencies, this is the quantum analog of the RC circuit with a purely dynamical response. We investigate the quantized dynamics as a consequence of ac pulses with large amplitude. Within a Keldysh-Green function formalism we derive the time-dependent current in the Coulomb blockade regime. Our theory thus extends previous models that considered either noninteracting electrons in nonlinear response or interacting electrons in the linear regime. We prove that the electron emission and absorption resonances undergo a splitting when the charging energy is larger than the tunnel broadening. For very large charging energies, the additional peaks collapse and the original resonances are recovered, though with a reduced amplitude. Quantization of the charge emitted by the capacitor is reduced due to Coulomb repulsion and additional plateaus arise. Additionally, we discuss the differential capacitance and resistance as a function of time. We find that to leading order in driving frequency the current can be expressed as a weighted sum of noninteracting currents shifted by the charging energy.Comment: 13 pages, 9 figures. Minor changes. Published versio

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