Fluxon Time-Delay Readout of a Superconducting Qubit Protected by a Spectral Gap in a Josephson Transmission Line

TL;DR

This paper proposes a fluxon-based time-delay readout method for superconducting qubits using a Josephson transmission line, which suppresses decay and simplifies integration for scalable quantum computing.

Contribution

It introduces a novel fluxon time-delay readout scheme that leverages spectral gaps in JTLs to enhance qubit protection and reduce hardware complexity.

Findings

Time delay depends on qubit state, enabling readout.

Spectral gap suppresses qubit decay via the transmission line.

Absence of fluxon pinning improves readout fidelity.

Abstract

We theoretically investigate a readout scheme of the quantum state of a superconducting qubit based on time delay of a single flux quantum (SFQ), also known as a fluxon, propagating in a Josephson transmission line (JTL). We concretely study the time-delay readout based on capacitive coupling between a transmon qubit and a JTL, and we evaluate the time delay depending on the qubit state. We also reveal a feature of the absence of fluxon pinning and exponential suppression of nonadiabatic transitions caused by the propagating fluxon, which is advantageous for the time-delay readout. We extend the analysis to a multi-level transmon as well. Owing to the spectral gap in the JTL, the radiative decay of the qubit mediated by the JTL is exponentially suppressed, and thus the transmission line itself also serves as a filter protecting the qubit. The readout scheme requires neither complicated wiring to low-temperature stages nor bulky microwave components, which are bottlenecks for integration of a large-scale superconducting quantum computer.