Verifying the analogy between transversely coupled spin-1/2 systems and inductively-coupled fluxoniums

Abstract We report a detailed characterization of two inductively coupled superconducting fluxonium qubits for implementing high-fidelity cross-resonance gates. Our circuit stands out because it behaves very closely to the case of two transversely coupled spin- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:math> systems. In particular, the generally unwanted static ZZ -term due to the non-computational transitions is nearly absent despite a strong qubit–qubit hybridization. Spectroscopy of the non-computational transitions reveals a spurious LC -mode arising from the combination of the coupling inductance and the capacitive links between the terminals of the two-qubit circuit. Such a mode has a minor effect on the present device, but it must be carefully considered for optimizing future multi-qubit designs.