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Kinetic inductance coupling for circuit QED with spins

S. Günzler1,2,*,†, D. Rieger1,*, M. Spiecker1,2, T. Koch1, G. A. Timco3, R. E. P. Winpenny3, I. M. Pop1,2,4, and W. Wernsdorfer1,2,‡

  • *These authors contributed equally to this work.
  • †Contact author: simon.guenzler@kit.edu
  • ‡Contact author: wolfgang.wernsdorfer@kit.edu
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Phys. Rev. B 112, 115424 – Published 17 September, 2025

DOI: https://doi.org/10.1103/jz24-qpqv

Abstract

While transverse coupling (σx) is commonly used in qubit-resonator systems, longitudinal coupling via σz presents a tantalizing alternative: it does not hybridize the modes, eliminating Purcell decay, and it enables quantum-nondemolition qubit readout independent of the qubit-resonator frequency detuning. Here, we demonstrate longitudinal coupling between a {Cr7Ni} molecular spin qubit ensemble and the kinetic inductance of a granular aluminum superconducting microwave resonator. The inherent frequency independence of this coupling allows for the utilization of a 7.8-GHz readout resonator to measure the full {Cr7Ni} magnetization curve spanning 0–600 mT, corresponding to a spin frequency range of fspin=0–15 GHz. For 2-GHz detuning from the readout resonator, we measure a 1/e spin relaxation time of τ=0.38 s, limited by phonon decay to the substrate. Based on these results, we propose a path towards longitudinal coupling of single spins to a superconducting fluxonium qubit.

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Phys. Rev. B 112, 115424– Published 17 September, 2025
  • Received 17 February 2025
  • Revised 20 June 2025
  • Accepted 14 August 2025
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