A new joint innovation by the University of Cambridge and the National Physical Laboratory (NPL), the UK’s National Measurement Institute, could lead to redefining the ampere in terms of fundamental constants of physics.
The international system of units (SI) comprises seven base units (the metre, kilogram, second, Kelvin, ampere, mole and candela) which should be stable over time and universally reproducible. This requires definitions based on fundamental constants of nature which are the same wherever you measure them. As of now, however, the definition of the Ampere is prone to instability. 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 world’s first graphene single-electron pump (SEP) provides the speed of electron flow needed to create a new standard for electrical current based on electron charge, ans so it is a leading option in this race to redefine the ampere. SEPs create a flow of individual electrons by shuttling them into a quantum dot and emitting them one at a time and at a well-defined rate. A graphene SEP has been successfully produced and characterized for the first time, with properties that are extremely well suited to this application.
A reliable SEP pumps precisely one electron at a time to ensure accuracy, and pumps them quickly to generate a sufficiently large current. Up to now the development of a practical electron pump has suffered a lack of diversity. Tuneable barrier pumps use traditional semiconductors and have the advantage of speed, while hybrid turnstile utilizes superconductivity and has the advantage that many can be put in parallel.
Traditional metallic pumps, thought to be not worth pursuing, have been given a new lease of life by fabricating them out of graphene. Previous metallic SEPs made of aluminium are very accurate, but pump electrons too slowly for making a practical current standard. Graphene’s unique semimetallic 2D structure has 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. While Cambridge and NPL scientists still need to optimize the material and make more accurate measurements, this graphene SEP marks a major step forward in the road towards using graphene to redefine the ampere.
Beyond redefining the ampere, accurate SEPs operating at high frequency and accuracy can be used to make electrons collide and form entangled electron pairs, believed to be a fundamental resource for quantum computing, and for answering fundamental questions in quantum mechanics.