‘Forbidden’ light in superconductors heralds new quantum frontier

Technology News | May 21, 2020

By Rich Pell

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 valves over half a century ago,” says Perakis, “scientists are searching for a leap forward in design principles and novel devices in order to achieve quantum computing and communication capabilities. 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. The National Science Foundation has included quantum studies in its ‘10 Big Ideas‘ for future research and development critical to our nation.”

Wang adds, “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. Second harmonic generation is a fundamental symmetry probe. This will be useful in the development of future quantum computing strategies and electronics with high speeds and low energy consumption.”

To achieve this, say the researchers, they need to do more exploring of the quantum world. And, says Wang, this forbidden second harmonic light emission in superconductors represents “a fundamental discovery of quantum matter.”

For more, see “Terahertz Second-Harmonic Generation from Lightwave Acceleration of Symmetry-Breaking Nonlinear Supercurrents.”