'Forbidden' light in superconductors heralds new quantum frontier

May 21, 2020 //By Rich Pell
'Forbidden' light in superconductors heralds new quantum frontier
Scientists at Iowa State University say they are using light waves to accelerate supercurrents - electricity that moves through materials without resistance, usually at super cold temperatures - giving researchers access to a new class of quantum phenomena.

In a new paper, the scientists report that they have seen unexpected things in supercurrents that break symmetry and are supposed to be forbidden by the conventional laws of physics. This includes, say the researchers, "forbidden" light emissions that one day could be applied to high-speed quantum computers, communications, and other technologies.

The researchers, who have been using light pulses at terahertz frequencies to accelerate electron pairs - known as Cooper pairs - within supercurrents, in this case tracked light emitted by the accelerated electrons pairs. What they found were "second harmonic light emissions," or light at twice the frequency of the incoming light used to accelerate electrons.

That, say the researchers, is analogous to color shifting from the red spectrum to the deep blue.

"These second harmonic terahertz emissions are supposed to be forbidden in superconductors," says Jigang Wang, a professor of physics and astronomy at Iowa State University, a senior scientist at the U.S. Department of Energy’s Ames Laboratory, and the leader of the project. "This is against the conventional wisdom."

Ilias Perakis, professor and chair of physics at the University of Alabama at Birmingham, and a collaborator on the project adds, "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."

The researchers' work was made possible by the use of a measurement tool using quantum terahertz spectroscopy , which can visualize and steer electrons. It uses terahertz laser flashes as a control knob to accelerate supercurrents and access new and potentially useful quantum states of matter. The National Science Foundation has supported development of the instrument as well as the current study of forbidden light.

Access to this and other quantum phenomena, say the researchers, could help drive major innovations.

"Just like today's gigahertz transistors and 5G wireless routers replaced megahertz vacuum tubes or thermionic


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