Graphene joins the race to redefine the ampere
Electron pumps made from graphene work ten times faster than similar pumps made from conventional three-dimensional materials and can be used to generate larger currents. (Image credit: Malcolm Connolly, NPL/Cambridge)
A new joint innovation 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. 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. A good 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 been a two-horse race. Tuneable barrier pumps use traditional semiconductors and have the advantage of speed, while the hybrid turnstile utilises 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 the world's most famous super-material - graphene. Previous metallic SEPs made of aluminium are very accurate, but pump 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 - at near gigahertz frequency - to create a current standard. The Achilles' heel of metallic pumps, slow pumping speed, has thus been overcome by exploiting the unique properties of graphene.