Superconducting NbSi Quantum Phase-Slip Nanowire Devices for Electronics

We will develop superconducting devices to enable applications in high-technology electronics, based on a quantum-mechanical phenomenon known as quantum phase-slip (QPS). The proposal as a whole is concerned with developing this so-far-little-developed manifestation of quantum mechanics within superconducting nanowires. The developments will be both in our fundamental understanding of the physics, through experimental tests of recently proposed theories, and in moving towards concrete applications by development of circuit elements which will be useful in future nanoscale applications. One specific area of potential for future application is as elements for qubit technologies and thus the research fits into the broader research effort of enabling technologies for quantum computing.

The development of coherent QPS devices would open up avenues for developing a gamut of new devices of potential technological importance, as well as for fundamental research. Coherent QPS devices are likely to find applications in both fundamental science and metrology, and the impact could be as marked as the superconducting quantum interference device (or SQUID), which is dual to one of the coherent QPS devices we will develop. The devices are also likely to be of relevance to quantum computing applications, both as qubits in the own right and also as useful circuit elements in future realisations using previously developed solid-state qubits. There is also potential for a QPS-based quantum current standard to be developed and this would have substantial impact on the metrology community.

A quantum phase-slip occurs when quantum fluctuations in the superconducting order parameter are sufficiently strong that the phase of the order parameter slips. The key element of each device will be a superconducting nanowire. If such a nanowire has a cross-section sufficiently small, quantum fluctuations may have significant effects and QPS events may occur. In that case, the fundamental quantum-mechanical Heisenberg uncertainty relation between the charge and superconducting phase may noticeably change the current flow through the device and even, counterintuitively, completely block charge transport along the wire, even though it is superconducting. This is a result of the quantum-mechanical nature of the physics.

In this research we will develop devices based on the quantum phase-slip, using the state-of-the-art nanofabrication facilities in the London Centre for Nanotechnology. We will experimentally realise a range of coherent QPS devices - including the flux-biased and current-biased QPS transistors and the QPS box - for the first time, with a view to future applications in devices, and we will build on and refine previous experimental work with the aim of realising a prototype QPS-based current standard.

Much of the proposed research constitutes early underpinning work which will in the medium term enable future applications with economic impact. Apart from the academic beneficiaries (described in the Academic Beneficiaries section), the principal beneficiaries in the short term will be the metrology community, primarily based outside academia in national standards laboratories worldwide.

The International Committee for Weights and Measures (CIPM) has stated it is seeking a redefinition of the ampere and is currently assessing options for a new definition. The development of a prototype current standard based on QPS would be an excellent fit with other efforts within the metrology community to develop a practical realisation of the ampere based on the fundamental constant e. The discovery in the 1960s of the AC Josephson effect had a substantial effect on the metrology community. This led to a change in 1990 in the SI definition of the volt, relating it to quantum physics via the Josephson effect. The potential exists for QPS to have a similarly marked influence on electrical metrology, providing a realisation of the ampere based upon the fundamental physical constant e. Primary beneficiaries of this development will be national standards laboratories internationally. The benefit will be a more precisely determined ampere, linked to a fundamental constant and so fitting the international drive away from standards defined with respect to physical artefacts or requiring reference to other standards. The UK National Physical Laboratory and Germany's PTB will benefit most directly due to direct contacts which I already have with them.

Users of the calibration services of the national standards laboratories, including high-technology industries and research organisations, will subsequently benefit through having more accurately determined current standards. Important developments in devices for low-temperature electrical transport as a result of the research will lead to a knock-on economic benefit to manufacturers of instrumentation and cryogenic apparatus in order to implement the new technology.

A distinct additional benefit of a more precisely determined current standard is the "closing of the metrological triangle", allowing a consistency test of the physics underlying the present realisations of the voltage standard and the resistance standard and will either confirm that our understanding of the physics is correct, or will identify a problem with this understanding. The metrology community and more widely those engaged in fundamental science research related to the present realisations of the voltage standard and the resistance standard will benefit.

Outcomes of the research which are applicable in qubit technology can be expected to contribute to the huge economic impact quantum computing technology will have if it matures and fulfils its potential as a future computing technology with power far outstripping the present state-of-the-art.

On a personal level, the fellowship will be a significant step towards my own career goals. To date I have developed my research skills through postdoctoral research positions, and this fellowship will provide me with the opportunity to demonstrate independence and start to establish my own research group. I aim to recruit a PhD student during the course of the fellowship - I will investigate the possibilities of funding for a studentship from my department and from the UCL Impact scheme in collaboration with NPL. Following completion of the fellowship I would expect to be in a strong position for obtaining a permanent position in a UK university and with an increasingly established international influence. The PDRA will gain valuable skills and experience through the research and the professional development of the PDRA will be supported through UCL's programme of staff training and development provided by its Organisational and Staff Development department.