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.
Highlights
Quantum annealing is a heuristic algorithm typically employed for solving optimization problems formulated in terms of finding ground states of classical Ising spin Hamiltonians [1], closely related to adiabatic quantum computing [2]
Once the readout signal is encoded in the quantum flux parametron (QFP) persistent current, it is protected from any errors due to flux qubit tunneling or thermal excitations, and can be integrated for as long as is necessary to facilitate single-shot readout
These results provide an outline of the parameter space occupied by high-performance QFP readout of the flux qubit, which can be accessed by moderate increases to qubitQFP coupling or moderate reductions in the QFP width
Summary
Quantum annealing is a heuristic algorithm typically employed for solving optimization problems formulated in terms of finding ground states of classical Ising spin Hamiltonians [1], closely related to adiabatic quantum computing [2]. Architectures for quantum annealers commonly employ superconducting circuits [7,8,9,10], and a promising route uses tunable flux qubits [11,12,13,14,15] that have small persistent currents [16] in order to take advantage of quantum coherence It has been shown, both in flux qubits [16,17,18] and fluxonium [19,20], that decreasing the magnitude of the qubit persistent current, Ip , is a key factor in achieving long coherence times. The QFP provides the isolation and amplification necessary for fast, high-fidelity readout without impacting qubit lifetime
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