Tiny quantum electron vortices can circulate in superconductors in ways never seen before

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A new study from KTH Royal Institute of Technology and Stanford University revises our understanding of quantum eddies in superconductors. Pictured is an artist’s impression of quantum vortices. Credit: Greg Stewart, SLAC National Accelerator Laboratory

Small tornadoes of electrons, known as quantum eddies, can occur within superconductors, which have important implications in superconducting applications such as quantum sensors. Now a new type of superconducting vortex has been found, reports an international team of researchers.

Egor Babaev, a professor at KTH Royal Institute of Technology in Stockholm, says the study revises the prevailing understanding of how electron flux can occur in superconductors, based on work on quantum vortices that was recognized in 2003 with the Nobel Prize. The KTH researchers, together with researchers from Stanford University, the TD Lee Institute in Shanghai and AIST in Tsukuba, have discovered that the magnetic flux produced by vortices in a superconductor can be split over a wider range of values ​​than previously thought. thought.

This represents a new insight into the fundamentals of superconductivity and can potentially also be applied to superconducting electronics.

A magnetic flux vortex occurs when an external magnetic field is applied to a superconductor. The magnetic field penetrates the superconductor in the form of tubes of quantized magnetic flux which form vortices. Babaev says research originally claimed that quantum vortices passed through superconductors each carrying a quantum of magnetic flux. But arbitrary fractions of the quantum flux were not a possibility in previous theories of superconductivity.

Using the Superconducting Quantum Interference Device (SQUID) at Stanford University, Babaev’s co-authors, researcher Yusuke Iguchi and professor Kathryn A. Moler, showed at the microscopic level that quantum vortices can exist in a single electronic band. The team was able to create and move around these fractional quantum vortices, says Moler.

“Professor Babaev has been telling me for years that we might see something like this, but I didn’t believe it until Dr. Iguchi saw it and conducted a series of detailed checks,” he says.

The Stanford researchers found the initial observation of this phenomenon “so incredibly unusual,” says Iguchi, that they repeated the experiment 75 times at various locations and temperatures.

The work confirms a prediction that Babaev published 20 years ago, according to which in certain types of crystals, a part of a population of electrons of a superconducting material can form a vortex circulating in a clockwise direction, while other electrons can simultaneously form a vortex in a clockwise direction. Counterclockwise. “These combined quantum tornadoes can carry an arbitrary fraction of the quantum flux,” he says.

“This revises our understanding of quantum eddies in superconductors,” he says.

Moler affirmed this conclusion. “I’ve been observing eddies in new superconductors for over 25 years and I’ve never seen this before,” he says.

Babaev says the robustness of quantum vortices and the ability to control them suggests that quantum vortices could potentially be used as information carriers in superconducting computers.

“The knowledge we gain, the spectacular methods pioneered by our colleagues Dr. Iguchi and Professor Moler at Stanford, could potentially be useful for some platforms for quantum computing in the long run,” Babaev says.

The study is published in the journal Science.

More information:
Yusuke Iguchi, Superconducting vortices carrying a temperature-dependent fraction of the flux quantum, Science (2023). DOI: 10.1126/science.abp9979. www.science.org/doi/10.1126/science.abp9979

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