Abstract
We propose and demonstrate an architecture for fluxonium-fluxonium two-qubit gates mediated by transmon couplers (FTF, for fluxonium-transmon-fluxonium). Relative to architectures that exclusively rely on a direct coupling between fluxonium qubits, FTF enables stronger couplings for gates using noncomputational states while simultaneously suppressing the static controlled-phase entangling rate () down to kilohertz levels, all without requiring strict parameter matching. Here, we implement FTF with a flux-tunable transmon coupler and demonstrate a microwave-activated controlled- (CZ) gate whose operation frequency can be tuned over a 2-GHz range, adding frequency allocation freedom for FTFs in larger systems. Across this range, state-of-the-art CZ gate fidelities are observed over many bias points and reproduced across the two devices characterized in this work. After optimizing both the operation frequency and the gate duration, we achieve peak CZ fidelities in the 99.85%–99.9% range. Finally, we implement model-free reinforcement learning of the pulse parameters to boost the mean gate fidelity up to , averaged over roughly an hour between scheduled training runs. Beyond the microwave-activated CZ gate we present here, FTF can be applied to a variety of other fluxonium gate schemes to improve gate fidelities and passively reduce unwanted interactions.
- Received 3 May 2023
- Revised 23 July 2023
- Accepted 3 August 2023
DOI:https://doi.org/10.1103/PhysRevX.13.031035
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
The realization of large-scale quantum computation offers the promise of solving certain classes of problems exponentially faster than classical computers. A major bottleneck for the fulfillment of such a processor is the error rate of single- and two-qubit operations. Here, we use a relatively underdeveloped type of superconducting qubit, the fluxonium qubit, to improve the state-of-the-art fidelities for both single- and two-qubit gate operations.
In comparison to the more common transmon superconducting qubit, fluxonium offers higher qubit lifetimes and a closer approximation to a two-level system. But two-qubit gates are more challenging with fluxonium due to the additional design constraints imposed by the circuit. We introduce a novel method of coupling fluxonium qubits, using a tunable transmon coupler. By mixing qubit types, we show that this scheme offers advantages in robustness, extensibility, and gate fidelities. By flux-tuning the frequency of the transmon qubit in situ, we demonstrate a flexible gate operation frequency over a 2-GHz bandwidth, with fully optimized gate fidelities of up to 99.92%.
Our results mark a technological advancement in both engineering fluxonium systems and superconducting qubit gate fidelities. This work also demonstrates that fluxonium is a serious candidate qubit for large-scale quantum processors.
