High-fidelity QND readout and measurement back-action in a Tantalum-based high-coherence fluxonium qubit

TL;DR

This paper demonstrates high-fidelity, quantum nondemolition measurement of a tantalum-based fluxonium qubit, highlighting its potential for quantum computing due to high coherence and low measurement back-action.

Contribution

The study provides the first detailed characterization of measurement fidelity and back-action in a tantalum-based fluxonium qubit, showing significant improvements over previous designs.

Findings

Single-shot measurement fidelity of 97.8% with a Josephson Parametric Amplifier

QND fidelity of 99.6%, indicating low measurement back-action

Measurement fidelity limited by state-mixing errors, suggesting further optimization needed

Abstract

Implementing a precise measurement of the quantum state of a qubit is very critical for building a practical quantum processor as it plays an important role in state initialization and quantum error correction. While the transmon qubit has been the most commonly used design in small to medium-scale processors, the fluxonium qubit is emerging as a strong alternative with the potential for high-fidelity gate operation as a result of the high anharmonicity and high coherence achievable due to its unique design. Here, we explore the measurement characteristics of a tantalum-based high-coherence fluxonium qubit and demonstrate single-shot measurement fidelity (assignment fidelity) of 96.2% and 97.8% without and with the use of a Josephson Parametric Amplifier respectively. We study the back-action of the measurement photons on the qubit and measure a QND (repeatability) fidelity of 99.6%. We find that the measurement fidelity and QND nature are limited by state-mixing errors and our results suggest that a careful study of measurement-induced transitions in the fluxonium is needed to further optimize the readout performance.