Tuneable-barrier electron pump

Electron pumps are made from one or more nanoscale 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 National Physical Laboratory (NPL) in Britain, in collaboration with the Semiconductor Physics group at Cambridge University, has achieved a milestone in tuneable-barrier pumping.

Using a pump fabricated in a two-dimensional electron gas (2-DEG), it showed that the current was given by I = nef to an accuracy of 100 ppm up to frequencies of a few GHz.

Researchers are improving the coupling of the clock signals to the sample. This will allow a detailed investigation of how the relation I = nef breaks down at high frequencies and improve 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.

The laboratory has 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, it has also started an investigation of quantum phase-slip devices. These could generate quantised currents by exploiting phase-slips in ultra-narrow superconducting wires.