Fast, Lifetime-Preserving Readout for High-Coherence Quantum Annealers

TL;DR

This paper introduces a quantum flux parametron (QFP) that enables fast, high-fidelity readout of high-coherence quantum annealers by acting as an isolator and amplifier without degrading qubit lifetime.

Contribution

The work demonstrates a novel use of QFP as both isolator and amplifier in qubit readout, preserving qubit coherence while achieving rapid, high-fidelity measurements.

Findings

QFP acts as isolator and amplifier in readout circuit.

QFP preserves qubit lifetime despite low-Q resonator.

Achieves >98.6% fidelity in 80 ns, 99.6% in 1 μs.

Abstract

We demonstrate, for the first time, that a quantum flux parametron (QFP) is capable of acting as both isolator and amplifier in the readout circuit of a capacitively shunted flux qubit (CSFQ). By treating the QFP like a tunable coupler and biasing it such that the coupling is off, we show that $T_1$ of the CSFQ is not impacted by Purcell loss from its low-Q readout resonator ($Q_e = 760$) despite being detuned by only $40$ MHz. When annealed, the QFP amplifies the qubit's persistent current signal such that it generates a flux qubit-state-dependent frequency shift of $85$ MHz in the readout resonator, which is over $9$ times its linewidth. The device is shown to read out a flux qubit in the persistent current basis with fidelities surpassing $98.6\%$ with only $80$ ns integration, and reaches fidelities of $99.6\%$ when integrated for $1$ $\mu$s. This combination of speed and isolation is critical to the readout of high-coherence quantum annealers.