Readout-induced leakage of the fluxonium qubit

TL;DR

This paper investigates how dispersive readout photons can induce non-QND effects in fluxonium qubits, causing state transitions that impact measurement fidelity and quantum error correction.

Contribution

It provides the first experimental mapping of non-QND effects in fluxonium and models the mechanisms behind photon-induced state transitions.

Findings

Resonator photons induce fluxonium state transitions both within and outside the qubit subspace.

Transitions to higher excited states and coupling to spurious modes explain non-QND effects.

Experimental mapping of fluxonium state evolution under readout photons.

Abstract

Dispersive readout is widely used to perform high-fidelity measurement of superconducting qubits. Much work has been focused on the qubit readout fidelity, which depends on the achievable signal-to-noise ratio and the qubit relaxation time. As groups have pushed to increase readout fidelity by increasing readout photon number, dispersive readout has been shown to strongly affect the post-measurement qubit state. Such effects hinder the effectiveness of quantum error correction, which requires measurements that both have high readout fidelity and are quantum non-demolition (QND). Here, we experimentally investigate non-QND effects in the fluxonium. We map out the state evolution of fluxonium qubits in the presence of resonator photons and observe that these photons induce transitions in the fluxonium both within and outside the qubit subspace. We numerically model our system and find that transitions to higher-excited states and coupling to an external spurious mode are necessary to explain observed non-QND effects.