Metrology - the science of measurement - underpins almost everything we encounter on a daily basis. An everyday example of metrology is when very small weights of medicine are weighed out to give precise doses. In this instance, without a clear and common understanding of the unit of mass, there could easily be dire consequences for the patient. The unit of mass - the kilogram - was historically defined by a physical object made of platinum and housed in a vault in the outskirts of Paris, with several copies held around the world. These copies were unavoidably imperfect, in that one could never have the exact same number of platinum atoms in each copy, and hence small errors in the definition of the kilogram were inevitable.

Since 2019, however, the kilogram has been redefined in terms of the fundamental constants of nature and, somewhat counterintuitively, measured electronically. This therefore requires a common agreement in the units of electrical measurement - the volt (voltage), the Ohm (resistance), and the Ampere (current) - as is familiar from any light bulb packaging. Of these three electrical units, we have a very precise agreement on the magnitude of a volt and an Ohm, both of which are defined by the results of quantum mechanical experiments and are precise to a very high degree. The Ampere, however, still lacks a quantum mechanical definition of its own and is defined in terms of other units.

There are numerous proposals for systems that exploit quantum mechanics to provide an independent and precise definition of the Ampere. One such proposal uses superconductors - materials that lose all electrical resistance at very low temperatures and are large scale quantum mechanical objects in of themselves. Using superconductors to make a quantum current standard, however, has so far been difficult because it has been thought necessary to make very small structures - many hundreds of times narrower than a typical human hair - to induce the necessary behaviour to define the Ampere. The research proposed here will work towards a quantum mechanical definition of the Ampere that uses an alternative material - superconducting diamond - in place of more traditional superconducting materials.

In previous work, we have found that the internal structure of thin diamond films allows us to reproduce the prerequisite behaviours necessary for the definition of the Ampere, but at a comparatively large physical scale - only tens of times narrower than a human hair! Though this still seems small, making and measuring objects of this size is vastly more simple than previous approaches. We will make electrical circuits out of thin superconducting diamond films that are designed to help us quantum mechanically define the magnitude of the Ampere.

The unique internal structure of superconducting diamond results in a host of other promising applications in quantum technologies that will also be explored during the course of this research.