Reproducible Single-Electron Pumps for a Quantum Current Standard

In the Mise en pratique in the 2019 revised SI system, which is now based entirely on physical constants, the single electron transport clocked at the frequency f is proposed as a “unique and elegant, … but challenging” way of a direct realization of the base unit ampere. The generated current I = n e f depends only on the elementary charge e - defined as a constant- and the number of electrons n transported per cycle, whereby the frequency f is represented by the SI base unit second. The most promising technical implementation of this single-electron transport nowadays is the use of a dynamic quantum dot with tunable tunnel barriers, using the Coulomb blockade mechanism. From a technical point of view, precise single-electron pumps (SEPs) can emit quasi-free, ballistic single-electron wave packets, which predestine them for a variety of applications in research and technology in addition to metrology. One relevant example gaining high interest is the application of semiconductor-based SEPs in quantum computing as so-called spin qubit shuttles, a connecting path over several µm in length between different qubits, enabling the scaling of complex multiple qubit systems.

In the last years of development at different NMIs, science and industry, realizations of SEPs based on Ga/As or Si semiconductor material systems were demonstrated, typically generating 100 pA up to 300 pA of current with uncertainties on a sub-ppm level. Nevertheless, all of these devices were carefully hand-selected and individually tuned devices, raising the question of robustness and reproducibility for a broad application, especially in terms of the proposed scaling of current by parallelization of several devices. Reproducibility of SEPs as a key parameter itself inhibits open and yet to define multiple questions on e.g. comparable metrics, measurement setup & protocols, device fabrication & yield and overall reporting, paving the way to a possible future guideline and documentary standard.

Here we demonstrate our recent results on a reliable small-scale series production of GaAs/AlGaAs heterostructure based SEPs, showing remarkable results in terms of yield and reproducibility. Among other parameters, the relevant Coulomb blockade and its main energy scale, the charging energy for adding electrons, was reproduced in a narrow range (< 10%) on a high two-digit number of samples and different production batches. The ability to fabricate QDs with a high yield and reproducible properties allows us to systematically investigate potential candidates for a quantum current standard. A sample batch (total 208 SEPs) with an optimized layout was fabricated and 5 randomly selected samples were tested by means of a precision current measurement. All samples showed no significant deviation from the theoretical value of 1ef within the resolution of the setup in the order of 10^-6 A/A, making them ideal candidates for a future quantum current standard.

In conclusion, the presented results of the small-scale series production and the development of a verification and validation scheme for comparable measurements pave the way for a broader use of these devices in metrology, general science and a wide area of applications in quantum technologies.