Xie Chen, professor of theoretical physics at Caltech, has been named a 2021 Simons Investigator, an honor that comes with $100,000 in research support per year for five years. The intent of the award, according to the Simons Foundation, is to support "outstanding theoretical scientists in their most productive years, when they are establishing creative new research directions, providing leadership to the field and effectively mentoring junior scientists."
Chen is a condensed matter theorist who seeks to understand the exotic properties of what physicists call quantum many-body systems. These are materials in which hundreds, millions, and even more electrons (the "many bodies") can become entangled, meaning that the electrons remain connected without actually being in contact with each other, somewhat like a flock of birds. Her research may have applications in quantum technologies of the future including quantum computing, though her goal is not to build these systems herself but rather to understand the fundamental power and limitations of the materials from which they would be made.
"By asking questions about what can happen in these materials, we can figure out how to, for example, store information and correct errors in quantum computers," says Chen. "But first, we need to know what is possible and what is not. There's a long way to go—from figuring out whether something is possible in principle to actually making a device that does the job—but we are taking the first steps."
Chen has been inspired, for example, by one particular question: How does one store quantum information on a hard drive? With classical computers, a user might back up their data on a hard drive and then plop that device in a drawer. But this would be more difficult with quantum computers, machines that are inherently fragile. The encoded quantum information, like the pieces in a game of Jenga, can easily "topple over" and lose stability, which leads to errors. Quantum computers of the future will thus require error-correction protocols to be running at all times.
"Error correction is one of the biggest challenges in actually realizing a quantum computer, but, theoretically, there are some very well-designed protocols for that," says Chen. "But we don't want to keep a hard drive plugged into a power source and to run error-correction apps on it all the time. We need to somehow rely on the intrinsic entanglement pattern of the quantum material to protect the stored information. That puts a tougher requirement on the material than quantum computing does, and it is not even clear whether such materials exist."
To start thinking about a possible quantum hard drive, physicists turn to so-called toy models of materials; the word "toy" denotes the fact that the models are conceptually simple but impractical in reality. One such toy model came from Jeongwan Haah (PhD '13), a former Caltech student working with John Preskill, the Richard P. Feynman Professor of Theoretical Physics. With the help of a computer-based search, Haah found a theoretical material, now known as a fracton model, that may provide a better starting point for building a quantum hard drive than any other material known. (Other researchers independently discovered fractons.)
Like other materials discovered in recent decades, fractons exhibit global patterns of entanglement that twist and change shape but remain "topologically invariant," or stable. Topology refers to the geometric properties of a material; a coffee mug and a donut have the same topology because they each have one hole (the mug's hole is in the handle).
But fractons are particularly unique because, according to Chen, they go beyond topology. The patterns in some of these materials exhibit fractal-like shapes, hence their name. Fractals are geometric structures that appear the same at different scales. "Fractons came as a surprise because they defy the conventional wisdom on materials. We usually think of excitations in materials as coming from particles—electrons, phonons—that move around carrying energy and momentum. That breaks down in fracton models. The excitations in fracton models do not move! Moreover, even though they are made of discrete atoms, we can usually use a continuous description for the materials, such as density fields for fluids, magnetization fields for magnets, etc. But fracton models have features that are fundamentally discrete, so we need to be more creative when dealing with them."
For Chen, the study of exotic materials will lead to a new understanding of fundamental laws of nature. While fractons may have started out as outliers, scientists are now starting to study more examples and to find patterns. "We are painting a picture around these materials to see how they all fit together into something bigger."
More about the Simons Investigators can be found at: www.simonsfoundation.org/grant/simons-investigators/.