Cavity-photon induced state transitions in a coupled Fluxonium qubit system

Superconducting qubits are a subject of intense research as a platform for scalable quantum computing. While transmon qubits have received a lot of attention, the less ubiquitous Fluxonium qubit has been shown to have long life-times and gates unlimited by level anharmonicity [1]. Despite this, little research has been put into studying multi-Fluxonium devices. Here, one of the difficulties is understanding how their rich level structure can make them prone to measurement photons inducing transitions out of the qubit subspace [2]. We present a systematic study of a system comprising two capacitively coupled Fluxonium qubits sharing the same read-out cavity. By tracking the state dependent transmission of the read-out pulse, we determine the transition rates from state i to state j of the coupled system using a forward-backward analysis [3] to characterize these cavity induced transitions. By varying the flux bias of the system and the population of the cavity, we characterize the fidelity of this read-out.

[1] L.B. Nguyen et al., Phys. Rev. X 9 041041, 2019
[2] D. Sank et al., Phys. Rev. Lett. 117 190503, 2016
[3] T. Rybarczyk et al., Physical Review A 91 062116, 2015