Quantum Tech

Future quantum switches found in rare metal

23rd November 2023
Harry Fowle

Quantum scientists have discovered a phenomenon in purple bronze that could pave the way for 'perfect quantum switches' in quantum devices, capable of shifting between insulator and superconductor states.

This research, led by the University of Bristol and published in Science, found that purple bronze, a unique one-dimensional metal, harbours both these opposing electronic states.

In this material, minor changes, possibly induced by small stimuli like heat or light, can instantly switch the material from an insulating state with zero conductivity to a superconducting state with unlimited conductivity, and vice versa. This ability, known as 'emergent symmetry', holds the potential for creating ideal On/Off switches in quantum technology.

Nigel Hussey, Professor of Physics at the University of Bristol, expressed his enthusiasm about the discovery: “It’s a really exciting discovery which could provide a perfect switch for quantum devices of tomorrow." He recalled the project's inception 13 years ago when two PhD students, Xiaofeng Xu and Nick Wakeham, first measured the magnetoresistance of purple bronze in his lab.

Despite the complexity in the material's resistance at different temperatures, its magnetoresistance exhibited an unexpected simplicity, consistent regardless of the direction of the current or field and maintaining a perfect linear temperature dependence down to the superconducting transition.

Hussey explained the initial lack of a coherent explanation for this behaviour and how the data remained unpublished for seven years. The resolution to this puzzle came through a chance meeting with Dr Piotr Chudzinski, who proposed that the resistive upturn might be caused by interference between conduction electrons and 'dark excitons'. Their collaboration confirmed this theory.

Dr Chudzinski, now a Research Fellow at Queen’s University Belfast, compared the phenomenon to a magical transformation: “Imagine a magic trick where a dull, distorted figure transforms into a beautiful, perfectly symmetric sphere. This is, in a nutshell, the essence of emergent symmetry."

Further tests on additional crystals by PhD student Maarten Berben at Radboud University completed the story, revealing why different crystals exhibited varied ground states. Hussey noted, “Looking ahead, it might be possible to exploit this ‘edginess’ to create switches in quantum circuits whereby tiny stimuli induce profound, orders-of-magnitude changes in the switch resistance.”

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