Nanoscale single-electron pumps can be used to generate accurate currents, and can potentially serve to realize a new standard of electrical current based on elementary charge. Here, we use a silicon-based quantum dot with tunable tunnel barriers as an accurate source of quantized current. The charge transfer accuracy of our pump can be dramatically enhanced by controlling the electrostatic confinement of the dot using purposely engineered gate electrodes. Improvements in the operational robustness, as well as suppression of non-adiabatic transitions that reduce pumping accuracy, are achieved via small adjustments of the gate voltages. We can produce an output current in excess of 80 pA with experimentally determined relative uncertainty below 50 parts per million.As early as one and a half centuries ago, J. C. Maxwell envisaged the need for a system of standards based on phenomena at the atomic scale and directly related to invariant constants of nature. 1 However, Maxwell could not anticipate that, in order to harness the behaviour of the world at the nanometer scale, a completely new physical interpretation was needed, namely, quantum mechanics. At first, the laws of quantum mechanics seemed to reveal fundamental limits to the accuracy of physical measurements. Concepts like the Heisenberg uncertainty principle, which imposes intrinsic fluctuations on the values of non-commuting observables, and the wavefunction collapse, responsible for the randomization of a system configuration after performing a measurement, appeared to be at odds with the requirement of deterministic consistency that is paramount for metrological purposes. Nevertheless, quantum-based systems are today acknowledged as the most stable and reliable metrological tools, as they can be strongly intertwined with fundamental constants. Exquisitely quantum-mechanical phenomena such as the ac Josephson effect 2 and the quantum Hall effect 3 have paved the way towards new and more reliable reference standards for the units of voltage and resistance, respectively.Major efforts are currently ongoing to re-define the unit of electrical current, the ampere (A), in terms of the elementary charge, e, by means of quantum technologies 4,5 . A practical implementation of this standard may be the electron pump, a device in which a quantum phenomenon, namely tunnelling, and classical Coulomb repulsion, are combined to control the transfer of an integer number of elementary charges. This device ideally generates a quantized output current, I P = nef , where n is an integer and f is the frequency of an external periodic drive. Several enabling technologies have already been developed including metal/oxide tunnel barrier devices 6,7 , normal-metal/superconductor turnstiles 8,9 , graphene double quantum dots 10 , donor-based pumps 11-13 , silicon-based quantum dot pumps 14-18 and GaAs-based quantum dot pumps [19][20][21][22][23][24][25][26][27] . To date, the latter scheme provides the lowest uncertainty of 1.2 parts per million (ppm) yielding current in excess o...

Semiconductor devices have been scaled to the point that transport can be dominated by only a single dopant atom. As a result, in a Si Fin Field Effect Transistor Kondo physics can govern transport when one electron is bound to the single dopant. Orbital (valley) degrees of freedom, apart from the standard spin, strongly modify the Kondo effect in such systems. Owing to the small size and the s-like orbital symmetry of the ground state of the dopant, these orbital degrees of freedom do not couple to external magnetic fields which allows to tune the symmetry of the Kondo effect. Here we study this tunable Kondo effect and demonstrate experimentally a symmetry crossover from a SU(4) ground state to a pure orbital SU(2) ground state as a function of magnetic field. Our claim is supported by theoretical calculations that unambiguously show that the SU(2) symmetric case corresponds to a pure valley Kondo effect of fully polarized electrons.PACS numbers: 71.27.+a, 71.30.+h, 73.23.Hk,72.15.Qm The resistance of metals with magnetic impurities anomalously increases as one decreases the temperature. This Kondo effect [1] can be explained as the screening of the localized spin of the magnetic impurity by the spins of the de-localized electrons in the metal. As a consequence of this screening, the localized spin and the itinerant ones form a many-body singlet with binding energy T K , which defines the low temperature scale at which Kondo physics appears. A few years ago, it was shown that quantum dots (QDs) [2] behave as Kondo impurities. The transport properties of QDs in the Kondo regime are quite remarkable: starting from an insulating QD in the Coulomb blockade regime at high temperatures, the linear conductance reaches the maximum unitary value of a perfect quantum conductor,2 /h as the temperature is reduced well below. At finite bias voltages V b , Kondo physics manifests as a zero-bias anomaly in the dI/dV b curves whose width is roughly given by T K . The Kondo effect in QDs originates from quantum fluctuations of the charge residing in the QD: electrons can transit through virtual states on a time-scale which is shorter than allowed by the Heisenberg uncertainty principle [1]. This mechanism generates effective spin flips which in turn lead to Kondo physics. Importantly, the role of the electron spin can be replaced by any other quantum degree of freedom such as e.g. orbital momentum [3][4][5][6][7][8][9], giving rise to exotic Kondo effects. Furthermore, the simultaneous presence of both a spin-and an orbital-degeneracy leads to an SU(4)-Kondo effect, where SU(4) refers to the symmetry of the corresponding Kondo ground state [3][4][5][6][7][8][9].In the past, SU(4) Kondo symmetry has been predicted to arise in parallel double quantum dot systems [3], but so far it has only been clearly observed in carbon nanotubes [5] and in single dopant devices in Si [9]. Si is a good candidate for observing SU(4) Kondo physics due to its six-fold valley (orbital) degeneracy of the conduction band and orbital effects are th...