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arXiv:0906.5384 · Physical Review B

Parallel quantized charge pumping

S. J. Wright, M. D. Blumenthal, M. Pepper, David V. Anderson, G. A. C. Jones, Christine A. Nicoll, D. A. Ritchie

Published Sep 16, 2009• 44 citations
arXivHTMLPDF

Abstract

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 $<20\text{ }\text{ppm}$ 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.

DOI
10.1103/physrevb.80.113303

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•Jun 1, 2010•1 citation
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.
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In quantum metrology, semiconductor single-electron pumps are used to generate accurate electric currents with the ultimate goal of implementing the emerging quantum standard of the ampere. Pumps based on electrostatically defined tunable quantum dots (QDs) have thus far shown the most promising performance in combining fast and accurate charge transfer. However, at frequencies exceeding approximately 1 GHz the accuracy typically decreases. Recently, hybrid pumps based on QDs coupled to trap states have led to increased transfer rates due to tighter electrostatic confinement. Here, we operate a hybrid electron pump in silicon obtained by coupling a QD to multiple parasitic states and achieve robust current quantization up to a few gigahertz. We show that the fidelity of the electron capture depends on the sequence in which the parasitic states become available for loading, resulting in distinctive frequency-dependent features in the pumped current.
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.
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.
One major change of the future revision of the International System of Units is a new definition of the ampere based on the elementary charge e. Replacing the former definition based on Ampère's force law will allow one to fully benefit from quantum physics to realize the ampere.However, a quantum realization of the ampere from e, accurate to within 10 -8 in relative value and fulfilling traceability needs, is still missing despite the many efforts made for the development of single-electron tunneling devices.Starting again with Ohm's law, applied here in a quantum circuit combining the quantum Hall resistance and Josephson voltage standards with a superconducting cryogenic amplifier, we report on a practical and universal programmable quantum current generator.We demonstrate that currents generated in the milliampere range are accurately quantized in terms of ef J (f J is the Josephson frequency) with measurement uncertainty of 10 -8 .This new quantum current source, which is able to deliver such accurate currents down to the microampere range, can greatly improve the current measurement traceability, as demonstrated with the calibrations of digital ammeters.In addition, it opens the way to further developments in metrology and in fundamental physics, such as a quantum multimeter or new accurate comparisons to single-electron 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.