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Journal of the Korean Physical Society

Parallelized Single-Electron Pumps Based on Gate-Tunable Quantum Dots

Young-Seok Ghee, Ye-Hwan Ahn, Sungguen Ryu, H.-S. Sim, Changki Hong, Bum-Kyu Kim, Myung‐Ho Bae, Nam Kim

Published Aug 1, 2019• 6 citations
Publisher

Abstract

Parallelization of pump devices is a direct way to increase the output level of the single-electron pump, which is required for metrological purposes. We fabricated a pair of single-electron pumps in parallel on a chip level and investigated their synchronized electron pumping phenomena. In the investigation, the pumping error was estimated to see whether the error was increased after the parallelization. We found that a proper choice of rf gates must be made in accordance with the direction of the applied magnetic field. In relation with the chirality of the edge state, the rf modulating gates should be chosen not to produce rf-induced heating effects.

DOI
10.3938/jkps.75.331

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Parallelization of pump devices is a direct way to increase the output level of the single-electron pump, which is required for metrological purposes. We fabricated a pair of single-electron pumps in parallel on a chip level and investigated their synchronized electron pumping phenomena. In the investigation, the pumping error was estimated to see whether the error was increased after the parallelization. We found that a proper choice of rf gates must be made in accordance with the direction of the applied magnetic field. In relation with the chirality of the edge state, the rf modulating gates should be chosen not to produce rf-induced heating effects.
The parallelization of single-electron pumps is a promising method to increase the quantized current level produced from a semiconductor-based single-electron system. In the parallelization of multiple pumps with common gate electrodes, the key process is to fabricate them with high reproducibility, resulting in an overlap of the most accurate regions in current plateaus at the same gate voltages. However, because of the lack of gating reproducibility, we here adopted a separate gate-tuning scheme to realize the overlap of the current plateaus instead of using a common gate scheme. To minimize the number of gates, we used entrance gates in common but an exit gate in separate with an additional in-common gate located outside the quantum dot but near the exit gates. The combination of the additional gate and separate exit gates led to an optimal current plateau overlap with a pair of pumps among six pumps in parallel. Under the optimal plateau-tuned condition, we achieved a relative type-A uncertainty of 1.4 × 10−6 at a 100 pA level with f = 160 MHz in the parallelized mode with the second current plateaus for both pumps.
Tunable barrier single-electron pumps (SEPs) have recently shown their great potential as high precision current source with direct link to the electron charge. The quantization of SEPs realized in gallium-arsenide semiconductors and driven by a single alternating control voltage routinely shows sub-ppm accuracy when applying high magnetic field. However, their accuracy without field is presently much lower. We now have found that modulation of two gates can increase accuracy in zero magnetic field and have confirmed the current quantization I=ef for a frequency of 84.5 MHz within the measurement uncertainty of less than 1 ppm.
We report on our recent progress in applying semiconductor quantum dots for spin-based quantum computation, as proposed by Loss and DiVincenzo (1998 Phys. Rev. A 57 120). For the purpose of single-electron spin resonance, we study different types of single quantum dot devices that are designed for the generation of a local ac magnetic field in the vicinity of the dot. We observe photon-assisted tunnelling as well as pumping due to the ac voltage induced by the ac current driven through a wire in the vicinity of the dot, but no evidence for ESR so far. Analogue concepts for a double quantum dot and the hydrogen molecule are discussed in detail. Our experimental results in laterally coupled vertical double quantum dot device show that the Heitler–London model forms a good approximation of the two-electron wavefunction. The exchange coupling constant J is estimated. The relevance of this system for two-qubit gates, in particular the SWAP operation, is discussed. Density functional calculations reveal the importance of the gate electrode geometry in lateral quantum dots for the tunability of J in realistic two-qubit gates.
Single Electron Pump (SEP) devices that can deliver single electrons at high frequencies are fabricated using two different gate geometries. The pump quantum dot (QD) can be defined either using two finger gates where the pump operates in what is known as the conventional pumping regime (CPR), or alternatively, the QD can be designed with the use of a single finger gate and split gate, where the pump operates in the long pumping regime (LPR). Here we investigate pump-maps produced in both the CPR and LPR under the effects of a low magnetic field (1 T to -1 T) and observe both the evolution of the pump-map and pump accuracy. Measurements were carried out in a cryogen free dilution refrigeration system with the use of a superconducting magnetic. Clear discrepancies between the behaviour of Med current in the different regimes can be seen indicating a fundamental difference in the operation of the pumps in the two different pumping regimes.
Abstract We studied the dynamics of a pair of single-electron double quantum dots (DQDs) under longitudinal and transverse static magnetic fields and time-dependent harmonic modulation of their interaction couplings. We propose to modulate the tunnel coupling between the QDs to produce one-qubit gates and the exchange coupling between DQDs to generate entangling gates, the set of operations required for quantum computing. We developed analytical approximations to set the conditions to control the qubits and applied them to numerical calculations to test the accuracy and robustness of the analytical model. The results shows that the unitary evolution of the two-electron state performs the designed operations even under conditions shifted from the ideal ones.