The NPL single-electron group, in collaboration with the University of New South Wales (UNSW), Australia, has recently measured the current from a silicon single electron pump with an uncertainty of just 0.27 parts per million (ppm) [1]. Previous measurements of a silicon pump measured by NPL and NTT, Japan were limited to an accuracy of 0.91 ppm.

These results are of comparable accuracy to that seen in high-precision measurements of GaAs-based electron pumps. This suggests that a tunable-barrier electron pump can be a universal quantum standard of current at this uncertainty level, whether or not the pump is fabricated from silicon or gallium arsenide. This is somewhat like the the quantum hall effect, which can give a precise resistance standard irrespective of whether the device is fabricated from silicon, gallium arsenide or graphene.

The electron pump used in this research was fabricated at UNSW using a state-of-the-art multi-layer fabrication process. UNSW guest worker Ruichen Zhao then characterised and measured the pump using NPL’s precision current measuring system, which compares the single electron current to a reference current generated from primary standards of voltage and resistance.

To reduce the uncertainty in the reference current, NPL re-evaluated the uncertainty in the resistor calibration, and decreased the interval between resistor calibrations. The largest component in the uncertainty budget now comes from the drift in the reference resistor on time-scales of a few days – in other words, we are reaching the limits of conventional current sources based on monolithic resistance elements (current dividing resistance networks, such as the one in the ULCA, can have slightly better performance by cancellation of some systematic effects).

At least this current level (~160 pA), one could argue that the electron pump is demonstrating some advantages over the resistance based standards, but more research is needed. In particular, experiments in which the currents from two electron pumps are directly compared are planned for the near future.

[1] R. Zhao, A. Rossi, S. P. Giblin, J. D. Fletcher, F. E. Hudson, M. Möttonen, M. Kataoka and A. S. Dzurak,
“Thermal-error regime in high-accuracy gigahertz single-electron pumping”.
Subscription: Physical Review Applied 8, 044021 (2017).
Preprint (open access): https://arxiv.org/abs/1703.04795

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