Exploring magnons with superconducting qubits

Devices taking advantage of the collective quantum behavior of spin excitations in magnetic materials—known as magnons—have the potential to improve quantum computing devices. However, using magnons in quantum devices requires an in-depth understanding of their nature and limitations. A new experimental technique uses superconducting qubits to sensitively characterize magnon behavior in previously unexplored regimes.

Researchers in the Grainger College of Engineering at the University of Illinois Urbana-Champaign have reported in the journal Physical Review Applied that highly excited magnon behavior in ferromagnetic materials can be accurately characterized by coupling the material to a superconducting qubit via a microwave cavity. This setup allowed the researchers to characterize both the number of magnons and their lifetimes when thousands of excitations are present, a regime that has not been studied well.

“To be useful in quantum computing applications, limitations on magnon systems need to be understood properly,” said Sonia Rani, the study’s lead author. “The problem is that there isn’t a good theory for when certain effects become important, and if we should expect them to lead to detrimental effects.

“Our experiment shows that we can use superconducting qubits as flexible detectors to study magnons in these magnetic systems over a large range, which is exciting both for the quantum computing connection and for fundamental science.”

Magnon devices can enhance quantum computers with functionalities such as nonreciprocity, in which the device only allows information to flow in one direction, and transduction, or conversion between different operating frequencies for different systems.

These functionalities assume linear magnon behavior, or that the magnon excitations in the material do not interact with each other, and that their damping is well-described by simple, empirical models. It is unclear at which points these assumptions break down, and whether that matters for quantum device applications.

To explore systems with large numbers of magnons present, Rani and her coworkers in the research group of Illinois physics professor Wolfgang Pfaff used a device called a superconducting qubit—a quantum computing component whose sensitivity to electric fields also makes it useful for precision measurement. They connected it to the magnon device, which is sensitive to magnetic fields, with a microwave cavity, in which electromagnetic waves facilitate conversion between electric and magnetic fields.

The experiment allowed the researchers to explore the magnon dynamics with two techniques. The first, dispersive frequency shift, takes advantage of a relationship between the number of magnons present and the superconducting qubit’s frequency of operation. This allowed the researchers to pinpoint the number of magnons present with an error of 0.5%.

The second, parametric pumping, can be used to temporarily create an interaction between the qubit and the magnons and control its strength. This provided another measurement of the number of magnons present, and it allowed the decay of the magnon excitations to be tracked.

“Parametric pumping is particularly appealing because it allows us to accurately explore the magnon dynamics as the system evolves in time, and it does it in a way that does not degrade the superconducting qubit sensor,” Rani said. “It’s a flexible, integrated approach ideal for monitoring magnon dynamics.”

The material explored by the researchers in this study, yttrium-iron-garnet, exhibited linear behavior with well-understood damping characteristics of up to 2,000 magnon excitations. Future studies could explore more magnetic materials with stronger excitations to explore the onset and impact of nonlinear effects, where the magnons interact with each other.

“This work sets the stage for new kinds of quantum devices in two ways,” Pfaff said. “First, it shows in a very practical way how we can integrate superconducting qubits and magnetic systems—two platforms that are normally very much at odds with each other. And second, it allows us to ‘screen’ magnetic systems for nonidealities that would compromise the performance of a quantum device.”