Redefining the ampere

The National Physical Laboratory and the University of Cambridge have joined forces in redefining the ampere in terms of fundamental constants of physics.

Published in Nature Nanotechnology, the researchers describe the world’s first graphene single-electron pump (SEP)which provides the speed of electron flow needed to create a new standard for electrical current based on electron charge.

“This paper describes how we have successfully produced the first graphene single-electron pump. We have work to do before we can define the amphere, but this is a major step towards that goal,” said Malcolm Connolly, a research associate based in the Semiconductor Physics group at Cambridge.

The international systems of units (SI) comprises seven based units (the metre, kilogram, second, Kelvin, ampere, mole and candela). These should be stable over time and universally reproducible which requires definitions based on fundamental constants of nature which are the same wherever you measure them.

However, the present definition of the ampere is vulnerable to drift and instability which is not sufficient to meet the accuracy needs of present and future electrical measurement. SEPs create flow of individual electrons by shuttling them into a quantum dot and emitting them one at a time at a well-defined rate, pumping them quickly to generate a sufficiently large current.

Previous SEPs were made from aluminium and were very accurate, but pumped electrons too slowly for making a practical current standard. Graphene’ s unique semi metallic two-dimensional structure has just the right properties to let electrons on and off the quantum dot very quickly, creating a fast enough electron flow to create the current standard.

The researchers still need to optimise the material and make more accurate measurements, but the paper marks a major step forward in the road towards using graphene to redefine the ampere. “We have shown that graphene outperforms other materials used to make this style of SEP. It is robust, easier to produce and operates at higher frequency. Graphene is constantly revealing exciting new applications and as our understanding of the material advances rapidly we seem able to do more and more with it,” said Connolly.

Reference: Gigahertz quantized charge pumping in graphene quantum dots