Physicists measure quantum geometry for the first time

Physicists and colleagues at MIT have for the first time measured the geometry, or shape, of electrons in solids at the quantum level. Scientists have long known how to measure the energy and speed of electrons in crystalline materials, but until now, the quantum geometry of the systems could only be determined theoretically, or sometimes not at all.

The work, reported on November 25 issue on Nature Physics“opens new avenues for understanding and manipulating the quantum properties of materials,” said Riccardo Comin, MIT’s Class of 1947 Career Development Associate Professor of Physics and leader of the work.

“We created a blueprint to get a new information that was not available before,” said Comin, who is also with MIT’s Materials Research Laboratory and the Research Laboratory of Electronics.

The work could be applied to “any kind of quantum material, not just the one we’re working on,” said Mingu Kang, first author of the paper in Nature Physics and a Kavli Postdoctoral Fellow at Cornell’s Laboratory of Atomic and Solid State Physics. Kang, MIT Ph.D. 2023, doing work as a graduate student at MIT.

Kang was also invited to write a companion Research Briefing of work, including its implications, for the November 25 issue of Nature Physics.

A wonderful world

In the strange world of quantum physics, an electron can be described as a point in space and a wave form. At the heart of the present work is a basic object known as a wave function which describes the latter. “You can think of it as a surface in a three-dimensional space,” Comin said.

There are different types of wave functions, from simple to complex. Imagine a ball. That is similar to a simple, or trivial wave function. Now picture a Mobius strip, the type of structure explored by MC Escher in his art. That’s in a complex, or non-trivial wave function. And the quantum world is filled with materials composed of the latter.

But until now, the quantum geometry of wave functions could only be determined theoretically, or sometimes not at all. And the property is becoming more and more important as physicists discover more quantum materials with potential applications in everything from quantum computers to advanced electronic and magnetic devices.

The MIT team solved the problem using a technique called angle-resolved photoemission spectroscopyor ARPES. Comin, Kang, and some of the same colleagues have used the technique in other research. For example, in 2022 they report discover the “secret sauce” behind the exotic properties of a new quantum material known as kagome metal. That work, too, shows the Nature Physics.

In the current work, the team adapted ARPES to measure the quantum geometry of a kagome metal.

Close collaboration

Kang emphasized that the new ability to measure the quantum geometry of materials “comes from close cooperation between theorists and experimenters.”

The COVID pandemic, too, has an impact. Kang, who is from South Korea, was based in that country during the pandemic. “That facilitated a collaboration with theorists in South Korea,” said Kang, an experimenter.

The pandemic has also brought an unusual opportunity for Comin. He traveled to Italy to help run the ARPES experiments at the Italian Light Source Elettra, a national laboratory. The lab was closed during the pandemic, but began to reopen with Comin’s arrival.

She finds herself alone, however, when Kang tests positive for COVID and she can’t join him. So he inadvertently ran the experiments himself with the support of local scientists.

“As a professor, I lead the projects but the students and postdocs actually carry out the work.