### Overview & objectives

The ability to perform more and more accurate and precise measurements is driving the development of new and technologies and the quality of life. At the core of our measurement capabilities lies the SI, *systeme international*, which sets out the standards and definitions for the fundamental physical units that underpins all measurement. In 2019 something remarkable took place, then parts of the SI was redefined in terms of fundamental constants of nature, eliminating the need for the last remaining artifacts to realise the fundamental SI units. This means that in practice, most units no longer need to be realised by a specific method or device, as long as the underlying fundamental physical relation based on constants of natures is realised. In particular, this opens up new possibilities for realising more accurate measurements of electrical currents through novel physics, one of the base SI units that today is most challenging to disseminate with high accuracy.

**The Ampere – improving measurements of electrical currents**

In the new SI the unit of electrical current, the ampere, can be disseminated by any device that realised the simple relation I [A] = e*f, where f is a frequency defined through the caesium atomic hyperfine transition that forms the basis for the worlds time standards, e is the fixed charge of the electron.

The easiest way to think of this definition is that the current is defined in terms of the number of electrons that pass through a device per unit of time, and if we somehow could count the electrons, we would know the current.

The challenge is that the fundamental electron charge is a very small number, e=1.602176634×10^{-19}, which means that in order to produce any appreciable currents required for disseminating the ampere (>nA) we would need to be able to count individual electrons at incredibly high rates, several billion counts per second, without a single error. Semiconductor charge pumps that precisely control the flow of electrons one by one have been developed in the last decades to record-breaking precision (link) by pushing the devices and operating conditions to the limit, however, achieving rates exceeding a few GHz remains extremely challenging.

We are therefore exploring alternative solutions based on different physical concepts to go beyond the current state of the art in accurate current standards.

**The volt** – **robust quantum standard**

Complementary to the unit of the ampere, and of significant importance for all electrical measurements, is the volt. This is one of the first metrological standards that were realised through a quantum mechanical effect – the so-called Josephson effect – and based entirely on fundamental constants of nature rather than relying on an artefact. When a Josephson junction (a very thin piece of insulator sandwiched between two superconducting electrodes) is driven by an ac voltage (microwave radiation) the charge tunneling across this junction becomes phase locked to the external drive, producing constant voltages at integer multiples of (h/2e) = 2.067834 mV/GHz, V=(h/2e)f. This effect is extremely robust, and its universality has been shown down to a record-breaking accuracy of 10^{-19}. By integrating a large number of Josephson junctions in series the volt can be disseminated straightforwardly up to 10 V with very high accuracy.

**Coherent quantum phase slips**

Fundamental theory of quantum mechanics stipulates a duality in superconducting circuits between phase and charge. What this means is that there is a quantum mechanical duality which implies that, loosely speaking, what can be realised with Josephson junctions and the volt, should have a dual counterpart where similar effects can be unlocked for disseminating the ampere. This fundamental process is called Coherent Quantum Phase Slips (CQPS) which causes a phase slip of the superconducting order parameter (state of a continuous superconducting nanowire) and is analogous to the tunneling of a magnetic flux across the superconducting wire, as opposed to the tunneling of a charge (Cooper pair) across the insulating barrier of a Josephson junction.

Realising devices based on coherent quantum phase slip physics has been an open challenge in the last few decades, as the requirements puts stringent demands on materials and nano-fabrication of superconducting devices. However, we are now at a stage where these technologies have advanced to enable a range of new possibilities, several of which will be explored in this project.

Moreover, this duality allows the potential CQPS-based current standard to, in principle, be realised with equipment which is very much compatible with existing infrastructure for the dissemination of the volt. This vision opens a range of new possibilities in developing compact, versatile metrology systems capable of delivering all the electrical standards within the same measurement system which will greatly reduce metrological traceability chains and enhance measurement capabilities at end users.

This outlines the main goal of this project: To demonstrate a CQPS-based proof-of-principle for a current standard and to show that it is possible to integrate with existing voltage standards. This will be realised by bringing together a broad range of materials and fabrication technologies from various fields of physics, as outlined in more detail below. The project is divided into four technical work packages, see the main menu for details about each work package.