Previous metallic SEPs made of aluminium are very accurate, but pump electrons too slowly for making a practical current standard. 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 Achillies heel of metallic pumps, slow pumping speed, has thus been overcome by exploiting the unique properties of graphene.
The scientist 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.
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 industries billing for electricity and using ionising radiation for cancer treatment.
Current, voltage and resistance are directly correlated. Because we measure resistance and voltage based on fundamental constants electron charge and Planck's constant - being able to measure current would also allow us to confirm the universality of these constants on which many precise measurements rely.
Graphene is not the last word in creating an ampere standard. NPL and others are investigating various methods of defining current based on electron charge. But today's paper suggests graphene SEPs could hold the answer. Also, any redefinition wil
|Contact: David Lewis|
National Physical Laboratory