NPL is developing nano-scale Single Electron Transport (SET) devices for moving electrons one at a time around an electrical circuit. In these devices, known as electron pumps, a periodic clock signal at a frequency f causes a precise number n (usually one) of electrons to pass through during each clock period. The current is then given by I = nef, where e is the charge on each electron. This behaviour is in contrast to everyday electronics – even the most miniaturised transistors in modern computers move thousands of electrons per clock cycle.

The main motivation for developing pumps is to enable the metrological triangle experiment to be carried out. This experiment will verify the consistency of the existing electrical standards – the quantum Hall effect resistance standard and the Josephson effect voltage standard. To perform the metrological triangle with a reasonable signal-to-noise ratio demands a current of at least 1 nA from the electron pump, corresponding to f ≥ 5 GHz.

A tuneable-barrier electron pump fabricated
from a surface-gated Gallium Arsenide wire

Electron pumps are made from one or more nano-scale conducting islands, isolated from the rest of the circuit by tunnel barriers. At sufficiently low temperatures, the number of electrons on each island is stable, and can be controlled using voltages applied to nearby electrodes. There are two approaches to pumping electrons; fixed- and tuneable-barrier pumping. In the first, the number of electrons on the islands of a multi-island device is changed in sequence. This results in the controlled transfer of one electron through the device. In the second, the tunnel barriers of a single-island device are raised and lowered to allow an electron to enter the island from one side, and leave from the other.

The NPL group, in collaboration with the Semiconductor Physics group at Cambridge University, recently achieved an important milestone in tuneable-barrier pumping^[1]. Using a pump fabricated in a 2-dimensional electron gas (2-DEG), we showed that the current was given by I = nef to an accuracy of 100 parts per million (ppm) up to frequencies of a few GHz. We are currently improving the coupling of the clock signals to the sample. This will allow us to make a detailed investigation of how the relation I = nef breaks down at high frequencies, and improve our understanding of errors in this type of pump.

In the fixed-barrier pumping scheme, the maximum current is limited by the time constant of the tunnel barriers. To pump a current of 1 nA with 0.01 ppm accuracy will require tunnel barriers with capacitance of 1 aF (10^-18 F) or less. This is not possible with traditional two-angle evaporation of metals. We have recently started collaborating with the Microelectronics Research Centre at the University of Cambridge to investigate novel methods of fabricating low-capacitance tunnel barriers in silicon.

As an alternative to electron pumping, we have also started an investigation of quantum phase-slip devices. These devices could generate quantised currents by exploiting phase-slips in ultra-narrow superconducting wires.

Reference

1. M. D. Blumenthal et al, Nature Physics 3, 343 (2007).

Current NPL collaborators

• University of Cambridge Semiconductor Physics Group
• University of Cambridge Microelectronics Research Centre
• PTB, Germany

Other labs with an interest in metrological applications of SET

• Helsinki University, Finland
• LNE, France
• METAS, Switzerland
• NIST, USA