Redefining the Kilogram and Ampere with Graphene

Groundbreaking research by the National Physical Laboratory's (NPL) Quantum Detection Group and an international team is underpinning the biggest change in the Système Internationale d'unités (SI Units) since the system began 50 years ago.

New research using graphene presents highly precise measurements of the quantum Hall effect, one of the key steps in the process to redefine two SI units.

The quantum Hall effect in graphene is such that it could be the material of choice for quantum resistance metrology.

"Many metrology laboratories around the world have been striving to do this experiment, and it is a real achievement that the NPL team and its co-workers were the first to get this key result. It turns out that the quantum Hall effect in graphene is very robust and easy to measure — not bad for a material that was only discovered six years ago," said J.T. Janssen, NPL Science Fellow and lead author of the research.

It has been the goal of scientists to relate all of the unit definitions to fundamental constants of nature, making them stable and universal, and giving them closer links to each other and the quantities they measure.

Graphene has the potential to surpass conventional materials in many applications, including quantum resistance metrology. (Image: NPL)

Key units to be redefined are the kilogram (mass) and the ampere (electric current). The kilogram is defined now by a physical lump of platinum-iridium, and the ampere is defined via the force produced between two wires.

The goal is to define the kilogram in terms of the Planck constant h and the ampere in terms of the electron charge e.

Making this change relies on the exactness of the relationships that link these constants to measurable quantities.

The quantum Hall effect defines a relationship between these two fundamental physical constants. Experiments are needed to test the quantum Hall effect in different materials to prove whether it is truly universal.

Until recently, the effect was observed exclusively in a few semiconductor materials. A few years ago, the quantum Hall effect was observed by the same team in graphene also, a completely different type of material with an electronic structure that was very dissimilar.

This research directly compared the quantum Hall effect in graphene with that observed in a traditional semiconductor material.

Graphene could surpass conventional materials in many important applications, partly due to its extraordinary electrical properties.

The results confirmed that the quantum Hall effect is truly universal with an uncertainty level of 86 parts per trillion, supporting the redefinition of the kilogram and ampere.

The discovery was highlighted in the journal Nature. The paper can be read in the New Journal of Physics.

The research was conducted in collaboration with the Bureau International des Poids et Mesures, Chalmers University of Technology, Lancaster University, and Linköping University.

For more information, vist: www.npl.co.uk