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

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.

14 pages, 11 figures; version accepted for publication in PRX Quantum

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• Charge insensitive qubit design derived from the Cooper pair box
• QuTiP 2: A Python framework for the dynamics of open quantum systems
• Low-decoherence flux qubit
• Tuning the Gap of a Superconducting Flux Qubit
• Protocols for optimal readout of qubits using a continuous quantum nondemolition measurement
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• Decoherence in adiabatic quantum computation
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Cited by in corpus (6)

• Customized quantum annealing schedules
• Superconducting Pulse Conserving Logic and Josephson-SRAM
• Demonstration of long-range correlations via susceptibility measurements in a one-dimensional superconducting Josephson spin chain
• Optimizing for periodicity: a model-independent approach to flux crosstalk calibration for superconducting circuits
• Decoherence of a tunable capacitively shunted flux qubit
• Exploring the Fidelity of Flux Qubit Measurement in Different Bases via the Quantum Flux Parametron