NbN Superconducting Resonator for Strong Magnon–Photon Coupling

Fabrication and measurement of NbN superconducting resonators demonstrating strong coupling with YIG spheres.

Superconducting resonators enable strong interactions between microwave photons and magnons, forming a key platform for hybrid quantum systems. This project demonstrates:

  • NbN microstrip resonators integrated with yttrium iron garnet (YIG) spheres to realize strong magnon–photon coupling at cryogenic temperatures.
  • The DC lines and Indium bumps were aimed for current-controlled magnon–photon coupling.
  • Details of the electroplating are described on CPW Fabrication and Electroplating Indium.

Design and Fabrication

  1. I designed meander-type microstrip NbN resonators (10 mm × 5 mm) optimized for magnetic-field-dependent coupling experiments.
  2. I have fabricated the devices on intrinsic Si substrates (525 µm thick) with 250 nm NbN traces deposited by DC reactive sputtering.
  3. Al contact pads were added for microwave wirebonding, and a NbN backside ground plane completed the structure.
Left: NbN Resonator layout. Middle: Fabricated resonators on DSP Si wafers. Right: YIG sphere positioned for coupling measurement.

Measurement and Characterization

  • The assembly was tested in a Quantum Design PPMS for cryogenic and magnetic-field measurements.
  • A VNA (Agilent N5227A) was used to perform S-parameter measurements (S₁₁, S₂₁) from 100 MHz – 30 GHz at 1.8 K.
  • An in-plane and out-of-plane magnetic field was applied to drive the YIG into ferromagnetic resonance (FMR) and observe magnon–photon hybridization.
Top: S-parameters of a resontor with and without the YIG sphere
Bottom: Corresponding Q-factors of a resontor with and without the YIG sphere.
Reprinted with permission from M. Mahdi et al., IEEE Trans. Appl. Supercond., vol. 35, no. 5, Aug 2025. © IEEE.

Key Results

  • At 1.8 K, the system exhibited clear avoided crossing in the S₂₁ spectra, confirming strong coupling between microwave photons and magnons.
  • These results establish a scalable pathway toward hybrid quantum magnonic devices based on superconducting circuits.

“Hybrid systems like NbN–YIG platforms bridge the gap between superconducting quantum circuits and magnonic information carriers.”

Acknowledgment

This research is supported by the Air Force Office of Scientific Research (AFOSR) under grant funding for hybrid quantum systems.

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