One-dimensional metal may provide the perfect on/off switch for quantum devices


Purple bronze, a one-dimensional metal, contains molybdenum, pictured here. Image: Wikipedia.


By James Myers

A unique one-dimensional metal called purple bronze may hold the key to development of a crucial switching mechanism for quantum computers.

Led by researchers at the University of Bristol, a team of scientists discovered that the metal can hold two opposite electrical states. Composed of conducting chains of lithium, molybdenum and oxygen atoms, purple bronze has the potential to switch instantly between an insulating state and a superconducting state.

As an insulator, the metal would provide resistance and turn off the flow of electricity in the circuits of a quantum device like a computer. As a superconductor, purple bronze would provide unlimited conductivity to keep electricity on within the circuits. The metal’s switch between the two states, in either order, is triggered by a small amount of stimulus, for example from heat or light.

“This polarised versatility, known as ‘emergent symmetry’, has the potential to offer an ideal On/Off switch in future quantum technology developments,” the University of Bristol reported.  Emergent symmetry is present certain properties of a system can change while the system as a whole remains the same.

Professor Nigel Hussey, who co-authored the paper on the discovery published in Science, had been curious about purple bronze for thirteen years since two PhD students in his lab measured the effects of a magnetic field, a phenomenon called magnetoresistance, on the metal. They observed a perfect linear correlation between temperature and conductivity that was the same irrespective of the direction in which the electrical current or magnetic field were aligned.

Without practical applications at the time, the observation was not pursued for some years. However, “Such simplicity in the magnetic response invariably belies a complex origin and as it turns out, its possible resolution would only come about through a chance encounter,” said Professor Hussey. In 2017, he attended a seminar by Dr. Piotr Chudzinski, now a Research Fellow at Queen’s University Belfast, on the then-rare subject of purple bronze.

With the original observational data, the two scientists began collaborating and, with the help of PhD student at Radboud University Maarten Berben, conducted extensive tests on the ability of purple bronze to transition from a state of broken-symmetry – a familiar state to physicists – to a symmetric state.

As Dr. Chudzinski explained, “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. The figure in question is our material, purple bronze, while our magician is nature itself.”

At the boundary between insulating and superconducting states, the probability of the purple bronze system being in either state is essentially the same. As Professor Hussey stated, “Such physical symmetry is an unusual state of affairs and to develop such symmetry in a metal as the temperature is lowered, hence the term ‘emergent symmetry’, would constitute a world-first.”

What’s next for purple bronze and its potential for efficient electrical switching in quantum computers remains to be seen, as the viability of quantum computing continues to evolve rapidly.

However, as the authors of the paper published in Science concluded, the discovery could unlock similar properties elsewhere. “By establishing a direct link between quantum field theory and an experimentally measurable quantity, we uncover a path through which emergent symmetry might be identified in other candidate materials.”

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