Graphene could hold the secret to redefining the basic unit of electric current, the ampere.
The work by the National Physical Laboratory (NPL) and the University of Cambridge could pave the way for redefining the ampere in terms of fundamental constants of physics, after the two institutions revealed the world's first graphene single-electron pump (SEP).
The present definition of the ampere is vulnerable to drift and instability, which means it is not sufficient to meet the accuracy needs of future electrical measurement, and the highest global measurement authority, the Conférence Générale des Poids et Mesures, has proposed that the ampere be re-defined in terms of the electron charge.
The SEP, described in a paper in Nature Nanotechnology, provides the speed of electron flow needed to create this new standard for electrical current based on electron charge.
Malcolm Connolly, a research associate based in the Semiconductor Physics group at Cambridge, said: "This paper describes how we have successfully produced the first graphene single-electron pump. We have work to do before we can use this research to redefine the ampere, but this is a major step towards that goal."
SEPs create a flow of individual electrons by shuttling them in to a quantum dot – a particle-holding pen – and emitting them one at a time and at a well-defined rate.
The newly published paper describes how a graphene SEP has been successfully produced and characterised for the first time, and confirms its properties are extremely well suited to this application.
A good SEP pumps precisely one electron at a time to ensure accuracy, and pumps them quickly to generate a sufficiently large current, but up to now the development of a practical electron pump has been a two-horse race.
Tuneable barrier pumps use traditional semiconductors and have the advantage of speed, while the hybrid turnstile uses superconductivity and has the advantage that many can be put in parallel.
Previous traditional metallic SEPs made of aluminium are very accurate, but pump electrons too slowly for making a practical current standard, but the researchers have given this form of pump a new lease of life by fabricating them out of graphene.
Graphene's unique semimetallic 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 – at near gigahertz frequency – to create a current standard.
The scientists at NPL and Cambridge still need to optimise the material and make more accurate measurements, but today's 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.
The realisation of the ampere is currently derived indirectly from resistance or voltage, which can be realised separately using the quantum Hall effect and the Josephson Effect.
A fundamental definition of the ampere would allow a direct realisation that National Measurement Institutes around the world could adopt. This would shorten the chain for calibrating current-measuring equipment, saving time and money for industry.
The paper will also have important implications beyond measurement as accurate SEPs operating at high frequency and accuracy can be used to make electrons collide and form entangled electron pairs – a fundamental resource for quantum computing, and for answering fundamental questions in quantum mechanics.