Metrology is one of most important fields of physics, since it seeks to define chemical and physical units, which is extremely important not only for science but also, for example, for manufacturing and testing precision products of all kinds. Looking for the highest accuracy is often important in our daily life, for instance one of the highest accuracy commonly used devices is the satellite navigation system, which makes accurate geo-location possible.
It used to be the case that base SI quantities such as the meter were rather crudely defined using metal sticks and secondary quantities such as the Volt used comparison standards based on chemical batteries. These days the meter is defined in terms of the speed of light and the Volt is compared across the world using the Josephson Voltage standard, which relates the Volt to some fundamental constants of physics. Nowadays, it is understood that the best approach to defining metrological quantities is to use the fundamental constants of nature and fundamental physical, especially quantum, effects. Nevertheless, we still do not have a better definition of the Ampere than that provided by the force between two current carrying wires.
The "Quantum Hall Effect", an effect observed in two-dimensional electron gases, presently provides a comparison standard for the unit of resistance, the Ohm. "Shapiro steps" in Josephson junctions, as noted above, link to the unit of voltage. Through Ohm’s Law they both link to the unit of current, but we need a current comparison standard in order to check.
The structure of the superconducting phase transition, called a "gauge symmetry breaking", introduces a periodic relation between the superconducting phase and the electrical current. Recently, researchers proposed to build a superconducting “quantum phase slip” device and developed theories which linked the unit of electrical current to precise many-body states in superconductors. These states can be seen as an analogue or dual of the Josephson junction but instead of linking the Volt to fundamental constants would instead link the Ampere, the unit of current to fundamental constants - namely the charge of two electrons times a frequency.
It is remarkable that such solid state systems which are a priori "dirty" (that is, they contain imperfections and impurities) can lead to such precise relations, which have been checked up to a relative accuracy of 10^-16, in the case of Shapiro steps. There are profound and imperfectly understood reasons for this precise behaviour, it has to do with the topology of the quantum state created in these devices, which makes it robust against disorder ("dirtyness"), nevertheless, they are precise.
Such a device, if it can be constructed, would make a far superior current reference and would allow the so-called "metrological triangle" to be completed. In this advance the Josephson Voltage standard, the Quantum Hall Effect resistance standard and the new electrical current standard could all be checked against each other for internal consistency. Discrepancies could reveal new fundamental physics, making this is a high priority for metrologists and physicists!
Our goal for this project is to build and test a quantum phase slip current standard. We are working together with scientists at the UK National Physical Laboratory.