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  • Open Access

Overcoming Intrinsic Material Limitations through Cavity Feedback

M. Ebrahimi1, Y. Huang1, V. A. S. V. Bittencourt2, A. Rashedi1, A. Metelmann2,3, and J. P. Davis1,*

  • *Contact author: jdavis@ualberta.ca
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Phys. Rev. X 16, 031068 – Published 14 September, 2026

DOI: https://doi.org/10.1103/pbbp-bz28

Abstract

Magnons, the quanta of spin waves, have significant potential for use in modern technologies, especially when they are strongly coupled to another mode for readout and control. Although magnons strongly interact with microwave photons via the magnetic-dipole interaction to form hybrid cavity-magnon polariton modes, the magnon-phonon interaction in micrometer-sized ferromagnetic spheres is typically weak, which is further aggravated by the large polariton linewidths dominated by the magnon dissipation. The material-limited magnon dissipation rate in particular has been regarded as an unavoidable limitation in these systems. Here, we surpass this long-standing limitation by implementing an active microwave feedback loop to suppress the linewidth of cavity-magnon polaritons and reduce their effective decay rate below the magnon-limited linewidth, thereby enhancing the polariton-phonon cooperativity from C≃1 to C≃300. As a key milestone, we achieved normal-mode splitting between a cavity-magnon polariton and a mechanical mode, providing direct evidence of three-mode hybridization among photons, magnons, and phonons. Our results establish feedback as a general route to accessing strong-coupling regimes in systems previously thought to be limited by material properties and hence open new opportunities for coherent control in hybrid quantum systems.

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Physics Subject Headings (PhySH)

Popular Summary

Strong interactions between magnons and phonons are difficult to achieve because magnetic losses quickly destroy coherence. We overcome this limitation using an active feedback loop that compensates these losses and allows the different excitations in the system to interact more strongly.

With this approach, we demonstrate simultaneous strong coupling among microwave photons, magnons, and phonons. The feedback preserves coherent interactions that would otherwise be suppressed by magnetic damping and provides a practical way to control dissipation in hybrid systems.

This work shows that active control of loss can be used as a resource for engineering complex interactions, opening new possibilities for hybrid quantum technologies and sensitive measurements.

Article Text

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Phys. Rev. X 16, 031068– Published 14 September, 2026
  • Received 11 March 2026
  • Accepted 13 August 2026
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