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Conference on Precision Electromagnetic Measurements

Increasing Quantized Single-Electron Current Accuracy in a Tunable Barrier Pump by Two-Gate Modulation

Friederike Stein, Martin Götz, Thomas Weimann, K. Pierz, Hans Werner Schumacher, F. Hohls

Published Jul 1, 2018• 0 citations
Publisher

Abstract

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.

DOI
10.1109/cpem.2018.8500988

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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.
Two quantized charge pumps are operated in parallel. The total current generated is shown to be far more accurate than the current produced with just one pump operating at a higher frequency. With the application of a perpendicular magnetic field the accuracy of quantization is shown to be <20 ppm<20\text{ }\text{ppm}<20 ppm for a current of 108.9 pA. The scheme for parallel pumping presented in this work has applications in quantum information processing, the generation of single photons in pairs and bunches, neural networking, and the development of a quantum standard for electrical current. All these applications will benefit greatly from the increase in output current without the characteristic decrease in accuracy as a result of high-frequency operation.
A gigahertz single-electron (SE) pump with a semiconductor charge island is promising for a future quantum current standard. However, high-accuracy current in the nanoampere regime is still difficult to achieve because the performance of SE pumps tends to degrade significantly at frequencies exceeding 1 GHz. Here, we demonstrate robust SE pumping via a single-trap level in silicon up to 7.4 GHz, at which the pumping current exceeds 1 nA. An accuracy test with an uncertainty of about one part per million (ppm) reveals that the pumping current deviates from the ideal value by only about 20 ppm at the flattest part of the current plateau. This value is two orders of magnitude better than the best one reported in the nanoampere regime. In addition, the pumping accuracy is almost unchanged up to 7.4 GHz, probably due to strong electron confinement in the trap. These results indicate that trap-mediated SE pumping is promising for achieving the practical operation of the quantum current standard.
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.
First I discuss various candidates of single-electron current pumps for quantum metrology. Then I focus on the hybrid normal-metal-superconductor turnstile in the form of a one-island single-electron transistor with one gate. The device demonstrates robust current plateaus at multiple levels of ef at frequency f. I discuss the various error mechanisms, based on our experiments and theoretical considerations. Ultimately the quantization accuracy is expected to be limited by either two-electron tunneling or by Cooper-pair-electron co-tunneling. We predict that it should be possible to achieve the metrological accuracy of 10-8, while maintaining the quantized current on the level of more than 10 pA, just by one turnstile with realistic parameters using aluminium as a superconductor. Recently we have managed to run ten turnstiles in parallel increasing the current level to above 100 pA. Work on suppressing the harmful sub-gap leakage current is in progress with encouraging experimental results.
Using an error-counting scheme, we evaluated the accuracy of single-electron (SE) transfer in Si tunable-barrier turnstiles in a dilution refrigerator. The error counting was performed by shuttling SEs between a lead and a charge-accumulating node and detecting in real time the number of electrons in the node with an SE-resolution charge sensor. The best experimentally obtained error rate is about 100 parts per million (ppm), where the SE capture in the SE island occurs in thermal equilibrium due to the heating effect on SEs caused by pulse voltages applied to modulate the potential barrier. When we reduce the heating effect by suppressing the pulse voltages, there is a change in the SE transfer mechanism to nonequilibrium SE capture. We theoretically discuss the crossover point of the change. Moreover, at the minimum pulse voltage, the theoretical lower bound of the error rate estimated by fitting is on the order of 0.01 ppm. This suggests that Si SE-transfer devices are promising candidates for use as quantum current standards in metrology.