Efforts have long been underway to make the Système Internationale d'unités (SI Units) more accurate. If you know your science history, SI units were devised during the French Revolution as an alternative to the British System.   And so the French Academy of Sciences was tasked with the new system and promptly got the whole thing wrong.

But SI units caught on and so efforts have been ongoing to make them more accurate.  Scientists like the idea 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. 

Key units that still need to be redefined are the kilogram (for mass) and the ampere (for electric current) - the kilogram is defined 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 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 in order to prove whether or not it is truly universal. Until recently the effect was exclusively observed in a few semiconductor materials. A few years ago the quantum Hall effect was also observed by the same team in graphene, a completely different type of material with a very different electronic structure.

This research directly compared the quantum Hall effect in graphene with that observed in a traditional semiconductor material. Graphene is set to 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 quantum Hall effect in graphene is so good that it should be the material of choice for quantum resistance metrology.

JT Janssen, NPL Science Fellow and the lead author of the research, said, "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."