Kinetic inductance coupling for circuit QED with spins

TL;DR

This paper demonstrates a novel longitudinal coupling method between molecular spin qubits and superconducting resonators, enabling frequency-independent readout and long spin relaxation times, with potential applications in quantum computing.

Contribution

It introduces a new longitudinal coupling technique using kinetic inductance for spin-resonator interactions, distinct from traditional transverse coupling methods.

Findings

Achieved frequency-independent coupling allowing full magnetization measurement.

Measured a 0.38 s spin relaxation time limited by phonon decay.

Proposed coupling of single spins to fluxonium qubits.

Abstract

In contrast to the commonly used qubit resonator transverse coupling via the $\sigma_{xy}$-degree of freedom, longitudinal coupling through $\sigma_z$ 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 {Cr$_7$Ni} 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 {Cr$_7$Ni} magnetization curve spanning 0-600 mT, corresponding to a spin frequency range of $f_\text{spin}=$0-15 GHz. For 2 GHz detuning from the readout resonator, we measure a $1/e$ spin relaxation time $\tau=$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.