Kinetic inductance coupling for circuit QED with spins

  1. S. Günzler,
  2. D. Rieger,
  3. M. Spiecker,
  4. T. Koch,
  5. G. A. Timco,
  6. R. E. P. Winpenny,
  7. I. M. Pop,
  8. and W. Wernsdorfer
In contrast to the commonly used qubit resonator transverse coupling via the σxy-degree of freedom, longitudinal coupling through σz presents a tantalizing alternative: it does not hybridize the modes, eliminating Purcell decay, and it enables quantum-non-demolishing 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 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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