Single-parameter quantized charge pumping in high magnetic fields

Abstract

We study single-parameter quantized charge pumping via a semiconductor quantum dot in high magnetic fields. The quantum dot is defined between two top gates in an AlGaAs/GaAs heterostructure. Application of an oscillating voltage to one of the gates leads to pumped current plateaus in the gate characteristic, corresponding to controlled transfer of integer multiples of electrons per cycle. In a perpendicular-to-plane magnetic field the plateaus become more pronounced indicating an improved current quantization. Current quantization is sustained up to magnetic fields where full spin polarization of the device can be expected. PACS numbers:Generating well defined currents by manipulating single charges has attracted considerable interest in the last two decades from both fundamental and applied points of view [1]. A particular potential of application lies in the field of metrology to provide a link between time and current units [2]. Different approaches have been studied, such as arrays of tunnel-connected metallic islands controlled by a number of phase shifted ac signals [3,4,5,6] or semiconducting channels along which the potential can be modulated continuously [7,8,9,10,11]. The pumping mechanism demonstrated in Ref.[10] allows gigahertz pumping comparable to surface-acoustic-wave pumps [8] while promising a higher degree of control. It employs three electrodes of which two are modulated at a fixed phase shift and with different amplitudes. In Ref. [11] it was shown that a single modulated voltage signal is sufficient to operate the pump and a numerical investigation indicated the importance of the tunnel barrier shape for improving the accuracy. A possible way to manipulate these tunnel couplings might be the application of a magnetic field owing to its influence on the wave function and the corresponding rearrangement of electrons between quantum states (see for instance [12]). Therefore the operation of such a single-parameter charge pump has been realized in the presented work when a perpendicular-to-plane magnetic field was applied.Two devices have been investigated which were both realized in an AlGaAs/GaAs heterostructure with a carrier concentration of 2.1 × 10 15 m −2 and a mobility of 97 m 2 /Vs in the dark. A 700 nm wide wire connected to two-dimensional electron gases was created by wetetching the doped AlGaAs layer. This channel is crossed by two 100 nm wide Ti-Au finger gates of 250 nm separation. A schematic is shown in the inset of Fig. 1(a). A quantum dot (QD) with discrete quasibound states between the gates can be created by applying sufficiently large negative voltages V 1 and V 2 to gate 1 and gate 2, * Electronic address: Bernd.Kaestner@ptb.de respectively. An additional radiofrequency (rf) signal is coupled to gate 1. The resulting voltages are thereforeat gate 1 and gate 2, respectively. If the oscillation amplitude is high enough, then the bound state drops below the Fermi level during the first half-cycle of the periodic signal and can be loaded with electrons from source. During th...

An Accurate Single-Electron Pump Based on a Highly Tunable Silicon Quantum Dot

“…To date, the latter scheme provides the lowest uncertainty of 1.2 parts per million (ppm) yielding current in excess of 150 pA 27 . This remarkable result has been attained by exploiting the beneficial effect of an external magnetic field on the quantization accuracy 23,24,27 . An effect that has been ascribed to the suppression of non-adiabatic excitations and the reduction of initialization errors, as a consequence of the increased magnetic confinement.…”

“…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][12][13] , silicon-based quantum dot pumps [14][15][16][17][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 of 150 pA 27 .…”

Quantum Fluctuations and Coherence in High-Precision Single-Electron Capture

“…This ambitious goal is complemented by a long-standing challenge in quantum metrology [11] to untie the definition of ampere from the mechanical units of SI [12] and implement a current standard based on direct counting of discrete charge carriers. So far the overlap between these research directions [13,14] has been rather limited arguably because metrological applications strive to maximize the particle nature of on-demand excitations.Optimizing the trade-off between speed and accuracy of single-electron isolation [2] does require consideration of quantum error mechanisms such as non-adiabatic excitation [15][16][17] or backtunneling [18][19][20][21]. However, these effects have been hard to differentiate experimentally owing to complexity of non-equilibrium many-particle quantum dynamics [22] and experimental challenges in exercising high-speed control of the electrostatic landscape.…”

“…(3). We define a crossover moment t c as the earliest time from which the quantum phase of the level can be preserved, +∞ tc Γ(t)dt =h, and explore exponential timedependence [16] of Γ(t) around t c ,accompanied by a linear shift of energy [19,20,22,36],The shape of ε(t) and Γ(t) for t t c as well as the initial conditions for Eq. (1) are irrelevant for n f if the ansatz (5) holds from a few τ before t c .…”

Counting Statistics for Electron Capture in a Dynamic Quantum Dot

“…The goal is to attain a predictable low dispersion distribution thus making dynamic QDs reliable and reproducible sources of electrons on demand. Deviations from this ideal case may be caused, for instance, by backtunneling [9][10][11] or non-adiabatic excitations [12][13][14]. …”