The future of Quantum Electrical Metrology: Coherent Quantum Phase Slip

Current quantization in superconducting nanowires or in small Josephson junctions is a phenomena dual to quantization of voltage, the ac Josephson effect. The effect can one day be used to realise a metrology standard for the ampere. When a nanowire or small Josephson junction is irradiated with microwaves of a well-defined frequency f, so-called inverse Shapiro steps results in plateaus of constant current given by I=2ef, where e is the elementary charge. Such a quantum standard for current would be the exact dual to the Josephson voltage standard. Importantly, a quantum current standard will close the “quantum metrology triangle” which is the three pillars of quantum electronic metrology standards: resistance, voltage, and current. At present the current standard is the block under development by different approaches, whilst the quantum Hall resistance standard and the Josephson voltage standard are well-established. The current quantization by the dual Josephson effect was firstly predicted shortly after the development of Josephson junctions & devices back in the 80’s, but has been clearly experimentally demonstrated only recently [1].

This opens up a new field of research and in order to advance the present proof-of-concept into practical metrology devices many aspects of this now accessible effect must be characterised and improved in detail.

The main objective of this PhD project is to improve the quality of present devices based on superconducting nanowires and Josephson Junctions to enable them for the practical use in electrical metrology. It targets three main areas: (1) understanding the operation and current limitations of the devices due to the external electromagnetic environment, (2) thermal management in these micron scale circuits to reduce dissipation and improve coherence, (3) investigating the device performance in current quantisation. The first will be done by making and studying a range of devices with controllably varied parameters of the environment, the second will involve detailed modelling to understand and adapt design and materials to improve coherence, and the third will be done using an SI traceable measurement system available at NPL.

In addition to being essential element for electrical metrology, the devices will also form novel components in the toolbox for quantum circuits, with promising applications in high frequency electronics and superconducting quantum technology. Furthermore, advances in device and materials engineering pursued in the development of a current standard goes hand in hand with understanding sources of decoherence limiting scaling of quantum circuits, thus broadly underpinning the NQTP.

NPL and RHUL have in place all needed technologies and expertise for successful research: nanofabrication facilities for processing of superconducting nanowires and electromagnetic environment, facilities for low temperature experiments, and expertise in precise low current measurements. Superconducting thin films will be accessed by close collaboration with IPHT (Germany) and a theoretical support is envisaged from a close collaboration with Aalto University (Finland), giving the student ample opportunities to engage in collaboration and start building an international network.