Being Exceptional in Higher Dimensions

Study shows that the strong coupling of photons and spin waves in magnetic materials creates an “exceptional surface” for new phenomena, potentially advancing the field of information processing.

A navy waved fluted tunnel connected to a red waved fluted tunnel intersecting at the tips with a waved sheet of gray.
Image courtesy of Argonne National Laboratory
Image caption: The middle layer in this image is a 3D “exceptional surface” calculated from quasiparticles called magnon polaritons. The surfaces above and below the intersection with the exceptional surface display unique behaviors.

The Science

By connecting electromagnetic waves and magnetism to create a system made of magnon polaritons, scientists demonstrated the existence of an “exceptional surface” for the first time. Magnon polaritons are a type of “quasiparticles.” These are disturbances that act like a particle despite not being a particle, like a bubble in a glass of soda. Exceptional surfaces were originally a purely mathematical concept. But recent research shows they have potential physical, real-world applications.

The Impact

Exceptional surfaces could have applications in information processing and sensing, including in new quantum technology. For example, the behaviors displayed by these surfaces could help make quantum sensors more sensitive to slight changes. They could also improve the how well quantum circuits handle environmental noise. In addition, the use of magnon polaritons to create these surfaces provides a new way to reveal unusual behavior at the quantum level. This ability would help scientists explore quantum physics and design quantum information systems.


An exceptional point can occur under certain conditions when quanta of magnetic excitations (spin waves) and microwave energy coalesce into quasiparticles called magnon polaritons.

For the first time, scientists have observed an exceptional surface–a continuous saddle-shaped surface comprised of exceptional points. Past research has detected exceptional points and lines, but this is the first time that researchers have plotted surfaces with potential impact on physical processes. They demonstrated that there is an exceptional saddle point in the exceptional surface that originates from the unique couplings between magnons and microwave photons. The exceptional surface displays unique phenomena, such as complex anisotropic behavior, that can enable desirable behavior such as high sensitivity and unidirectional signal propagation in materials.

Scientists at the Center for Nanoscale Materials and the Imperial College London worked together to produce the surface. The scientists constructed a magnon polariton system using a small magnetic sphere of yttrium iron garnet and a specialized circuit board that confines microwaves. They characterized the system using a magneto-electro-optical spectrometer at the Center for Nanoscale Materials, a Department of Energy Office of Science user facility. They then plotted the results in three-dimensional graphs to produce the exceptional surface.

This phenomenon has applications in various signal processing technologies including microwave devices, high sensitivity sensors, and secure communications. These results also lay the groundwork for a new avenue for producing and studying new physical phenomena including the quantum physics realm.


Xufeng Zhang
Center for Nanoscale Materials, Argonne National Laboratory  

Kun Ding
Imperial College, London


This work was performed in part at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science user facility, one of the five DOE Nanoscale Science Research Centers supported by the U.S. Department of Energy Office of Science. Additional funding came from the Gordon and Betty Moore Foundation.


Zhang, X. et al., “Experimental observation of exceptional surface in synthetic dimensions with magnon polaritons”, Physical Review Letters 123, 237202 (2019). [DOI: 10.1103/PhysRevLett.123.237202]

Related Links

Advancing information processing with exceptional points and surfaces, Argonne National Laboratory

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