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Conference on Precision Electromagnetic Measurements

Robustness of single-electron pumps at sub-ppm level

Friederike Stein, Ralf Behr, Martin Götz, Eckart Pesel, Thomas Weimann, K. Pierz, Hans Werner Schumacher, F. Hohls

Published Jul 1, 2016• 4 citations
Publisher

Abstract

Single-electron pumps (SEP's) based on tunable barrier quantum dots show the most promise as high-precision nanoampere quantum sources. They are treated as a good candidate in the context of a quantum representation of the SI base unit ampere. A key point for such a realization is the robustness of operation. Here we show the invariance of current at a sub-ppm level against variation of magnetic flux density, bias voltage on source and drain and the entrance gate voltage.

DOI
10.1109/cpem.2016.7540622

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Single-electron pumps (SEP's) based on tunable barrier quantum dots show the most promise as high-precision nanoampere quantum sources. They are treated as a good candidate in the context of a quantum representation of the SI base unit ampere. A key point for such a realization is the robustness of operation. Here we show the invariance of current at a sub-ppm level against variation of magnetic flux density, bias voltage on source and drain and the entrance gate voltage.
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.
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.
Semiconductor-based quantum dot single-electron pumps are currently the most promising candidates for the direct realization of the emerging quantum standard of the ampere in the International System of Units. Here, we discuss a silicon quantum dot single-electron pump with radio frequency control over the transparencies of entrance and exit barriers as well as the dot potential. We show that our driving protocol leads to robust bidirectional pumping: one can conveniently reverse the direction of the quantized current by changing only the phase shift of one driving waveform with respect to the others. We anticipate that this pumping technique may be used in the future to perform error counting experiments by pumping the electrons into and out of a reservoir island monitored by a charge sensor.
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.
Single-electron pumps based on quantum dots (QDs) with tunable barriers are promising candidates for the emerging quantum standard of electrical current, but accuracy at elevated reservoir temperatures can be challenging. In this study, a planar silicon QD achieves accurate, high-speed single-electron pumping in the thermal regime, where reservoir temperature could rise to a few kelvin. This device operates without the demanding conditions of previous experiments, indicating that it could deliver a cost-effective and easily distributed standard of current---and, furthermore, could be useful for long-range transfer of quantum information.
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.