Superconducting fluxonium qubits provide a promising alternative to transmons on the path toward large-scale superconductor-based quantum computing due to their better coherence and larger anharmonicity. A major challenge for multi-qubit fluxonium devices is the experimental demonstration of a scalable crosstalk-free multi-qubit architecture with high-fidelity single-qubit and two-qubit gates, single-shot readout, and state initialization. Researchers present a two-qubit fluxonium-based quantum processor with a tunable coupler element. They experimentally demonstrate fSim-type and controlled-Z-gates with 99.55 and 99.23% fidelities, respectively. The residual ZZ interaction is suppressed down to the few kHz levels. Using a galvanically coupled flux control line. Researchers implement high-fidelity single-qubit gates and ground state initialization with a single arbitrary waveform generator channel per qubit.
They implement two-qubit gates using a parametric flux modulation to bring the qubits into resonance with each other. The qubits have low transition frequencies. They also proposed and implemented an unconditional reset mechanism for qubits initialization. The tunable coupling scheme helped us to obtain high-fidelity two-qubit operations and suppress residual ZZ-coupling rate (here less than 1 kHz), allowing for parallel high-fidelity single-qubit operations.
Taken together, this work reveals an interesting and promising approach towards fault-tolerant quantum computing with low-frequency qubits that can be a good alternative and competitive to the transmon system. They believe that the low frequency of data qubits opens the possibility of using sub-gigahertz wiring and electronics for gate operations and individual qubit control, which in turn allows to reduce the complexity of the control system via using a single flux bias line for each qubit.
Superconducting qubits have become one of the most successful platforms for quantum computing during the past decade. One of the pillars of this success was the development of the transmon qubit. The typical transmon-based toolkit consists of a coplanar waveguide (CPW) resonator for dispersive readout and capacitive coupling that facilitates two-qubit gates. One of the key limitations of transmon-based quantum computing is dielectric loss, which limits the qubit coherence time. Incremental progress in material science and fabrication technology over the years has enabled an increase in coherence times from few microseconds4 to hundreds of microseconds. Despite this remarkable progress, dielectric loss is still a major issue on the route to large-scale quantum computing with superconducting qubits. Another fundamental issue with transmon qubits is their low relative anharmonicity, which leads to longer gate times and, ultimately, lower gate fidelities. Nevertheless, transmon qubits have been hugely successful in the development of noisy intermediate-scale quantum information processing device. Recent implementations of two-qubit gates on transmons demonstrate two-qubit gate fidelities around 99.5%. Another major issue for large-scale devices is crosstalk suppression. Among transmon qubits, one of the most critical types of crosstalk is static ZZ interaction. Recently, tunable couplers have been widely used as a tool to mitigate ZZ crosstalk in a scalable and effective wa
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