Cal Poly Physicists Conjure New Forms of Matter by Flicking a Magnetic Field On and Off

Two physicists at California Polytechnic State University have shown that by switching a magnetic field on and off in a precisely timed pattern, they can coax everyday materials into quantum states that simply do not exist under any static condition — a result that could remove one of the biggest obstacles to building practical quantum computers.

The research, published this month in Physical Review B under the title "Flux-Switching Floquet Engineering," was led by Ian Powell, a physics lecturer at Cal Poly's San Luis Obispo campus, and his former undergraduate student Louis Buchalter, who earned his bachelor's degree last year. Their central insight is that the symmetry of a quantum material is not fixed by its chemistry alone — it can be reshaped on the fly by how the material is driven in time. "Useful quantum properties can depend not just on what a material is, but on how it is driven in time," Powell said in an interview. "Once you accept that, the design space opens up enormously."

The technique sits within a fast-growing subfield called Floquet engineering, named after the 19th-century French mathematician Gaston Floquet, who developed the mathematics of differential equations with periodically varying coefficients. In Powell and Buchalter's setup, a topologically trivial two-dimensional material — one that on its own would behave like an ordinary conductor — is subjected to a magnetic flux that switches between two values at a carefully chosen frequency. The periodic kick rearranges the material's electronic structure, producing exotic, topologically protected quantum phases that have no static counterpart. Their numerical simulations show emergent edge states, hidden Berry curvature, and the appearance of so-called higher-order topological invariants normally seen only in much more complicated systems.

The practical payoff, the researchers argue, is robustness. The biggest barrier to building useful quantum computers is decoherence — the tendency of fragile quantum states to scramble themselves into noise within microseconds. Topologically protected states, by contrast, are theoretically immune to small perturbations because their defining properties are encoded in the global geometry of the system rather than in any local detail. "These driven phases inherit some of that protection," Buchalter said, "which means qubits built from them should be much harder to dephase than the ones we have today."

The Cal Poly result is theoretical, but the experimental community has been moving quickly. Groups at MIT, Harvard, Delft and the Max Planck Institute have all reported preliminary observations of Floquet-engineered states in graphene, in cold-atom lattices, and in arrays of superconducting qubits. "What Powell and Buchalter have done is provide a much cleaner mathematical recipe — flux-switching rather than continuous driving — that experimentalists can actually implement with standard radio-frequency hardware," said Mohammad Hafezi, a physicist at the University of Maryland who specializes in topological photonics and was not involved in the work. "It is the kind of paper that lab groups are going to download and try to run on the bench tomorrow."

The paper, identified by DOI 10.1103/c28t-x1dh and appearing in volume 113, issue 19 of Physical Review B, also hints at broader applications. The driven systems Powell describes share a mathematical structure with higher-dimensional quantum lattices that are otherwise impossible to build in three-dimensional space. That means flux-switching could give researchers a tabletop window into physics — say, the behavior of four- and five-dimensional topological insulators — that has so far existed only on paper. Funding for the research came from a combination of Cal Poly internal grants and the Henry Luce Foundation's undergraduate research program; Buchalter, now a graduate student in physics at the University of California, Santa Barbara, said the work began as a senior thesis project. "It's a reminder," Powell said, "that some of the most interesting physics doesn't require a $100 million accelerator. Sometimes you just need a clever way to flip a switch."