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Accelerated Supercurrents Give Scientists Access To “Forbidden” Light


An illustration of the acceleration of supercurrents by light pulses. Jigang Wang/Iowa State University

In what is described as “a fundamental discovery of quantum matter,” a team of American researchers have accessed forbidden light emissions that could one day help to advance quantum technologies.

To make their discovery, the scientists directed trillions of light pulses every second towards pairs of electrons (called Cooper pairs) that were flowing through a superconductor at an extremely cold temperature. This caused the electron pairs in the “supercurrents” (an electrical current that moves without energy loss) to speed up. When examined, the light emitted by the accelerated electron pairs had twice the frequency of the incoming pulsed light, termed “second harmonic light emissions.”


“These second harmonic (terahertz) emissions are supposed to be forbidden in superconductors,” Jigang Wang, a professor of physics and astronomy at Iowa State University, explained in a statement. “This is against the conventional wisdom.”

Described in a paper published in Physical Review Letters, the pioneering technique also unlocked another phenomenon.

“The forbidden light gives us access to an exotic class of quantum phenomena – that's the energy and particles at the small scale of atoms – called forbidden Anderson pseudo-spin precessions,” Ilias Perakis, a co-author and professor at the University of Alabama at Birmingham, explained.

Named after the late Nobel Laureate Philip W. Anderson, Anderson pseudo-spins were proposed to describe the superconducting state. In this latest experiment, the precession of these twisted pseudo-spins broke the symmetry of the system of electrons in the Niobium-tin superconductor, thus allowing the “forbidden” light to be emitted.

The procession (far right) of Anderson pseudo-spins broke the symmetry of the system and led to the emission of "forbidden light". Jigang Wang/Iowa State University

“The determination and understanding of symmetry breaking in superconducting states is a new frontier in both fundamental quantum matter discovery and practical quantum information science,” Wang said. “This will be useful in the development of future quantum computing strategies and electronics with high speeds and low energy consumption."

“Finding ways to control, access and manipulate the special characteristics of the quantum world and connect them to real-world problems is a major scientific push these days,” Perakis added.

Wang and Perakis will be amongst many researchers in the coming years digging further into the exotic and magical world of quantum.

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