Charge quantum interference device and applications

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The project aims to develop the technology of a novel type of quantum device, the Charge Quantum Interference Device (CQUID), and demonstrate its operation. This is a new quantum sensor, which detects ultralow charges with reduced backaction, with potentially a broad range of applications. The focus application of this project will be to realize the prototype of a new quantum standard for electrical current. In our previous work we demonstrated the new and fundamental phenomenon of superconductivity - Coherent Quantum Phase Slip (CQPS) - which is the basis of the new CQUID device. The effect was discovered in nanowires patterned in a new class of materials - highly disordered thin film superconductors. The CQUID acts in the opposite way to the well known superconducting quantum interference device (SQUID). Both devices rely on the quantum interference effect. A SQUID is based on the quantum interference of electric currents (supercurrents) in a superconducting loop with two tunnel junctions sensitive to the applied magnetic flux. By contrast, a CQUID is based on the quantum interference of a pair of tunnelling magnetic flux flows via two nanowires and sensitive to the induced electric charge. Thus the core of the CQUID is the tunnel junction for magnetic flux quanta. This is dual to a Josephson junction, which is the tunnel junction for a Cooper pair. The development of CQUIDs will particularly pave the way for metrological applications. The coherent flux tunnelling will be the core of a new quantum standard for electrical current, the prototype of which is the ultimate objective of this project. This quantum current standard will allow precise measurement of the flow of electricity at the single electron level. As a new class of quantum sensor, it will also play a wider role in nanoscale electronics beyond this project. Building the prototype of the quantum current standard is a challenging goal: we will theoretically analyse and simulate the device; develop a robust nano-technological process for the controllable phase-slip junction fabrication; investigate mechanisms of dissipation and dephasing hindering operation of the current standard; understand so-called quasiparticle poisoning and minimise its effect; simulate, design and investigate an optimal environment for the phase-slip current standards, such as compact hybrid inductances (from highly disordered films and Josephson junctions). Crucially, we will demonstrate the so-called inverse Shapiro steps, which are current plateaus in the current-voltage characteristic of the device, observed when the phase-slip junction (a superconducting nano-wire) is irradiated by microwaves of a particular frequency. The technology will be developed exploiting the newly established nano-fabrication facilities at Royal Holloway (SuperFab), which has been built with joint EPSRC and institutional capital investment support, and which will start full operation in early 2019. SuperFab is equipped with unique fabrication facilities dedicated specifically for technologies of the superconducting quantum systems. This project, with a focus on superconducting quantum technology, will form part of the UK Quantum Technology Programme. An important project partner is the National Physical Laboratory, where expertise in metrology will play an important role in the development and optimisation of the new quantum current standard prototype.