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arXiv:2410.07817 · Physical Review A

Microwave-activated two-qubit gates for fixed-coupling and fixed-frequency transmon qubits

Ling Jiang, Peng Xu, Shengjun Wu, Jianan Sun, Fu-Quan Dou

Published Mar 11, 2025• 5 citations
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Abstract

All-microwave control of fixed-frequency superconducting quantum systems offers the potential to reduce control circuit complexity and increase system coherence. Nevertheless, due to the limited control flexibility in qubit parameters, one has to address several issues, such as quantum crosstalk and frequency crowding, for scaling up qubit architecture with nontunable elements. This paper proposes a microwave-activated two-qubit gate scheme for two fixed-frequency transmon qubits coupled via a fixed-frequency transmon coupler. The protocol relies on applying a microwave pulse exclusively to the coupler, enabling the implementation of a controlled-$Z$ gate. We show that the gate fidelity exceeding 0.999 can be achieved within 150 ns, excluding decoherence effects. Moreover, we also show that leakage from the computational subspace to noncomputational states can also be effectively suppressed.

DOI
10.1103/physreva.111.032609

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All-microwave control of fixed-frequency superconducting quantum systems offers the potential to reduce control circuit complexity and increase system coherence. Nevertheless, due to the limited control flexibility in qubit parameters, one has to address several issues, such as quantum crosstalk and frequency crowding, for scaling up qubit architecture with nontunable elements. This paper proposes a microwave-activated two-qubit gate scheme for two fixed-frequency transmon qubits coupled via a fixed-frequency transmon coupler. The protocol relies on applying a microwave pulse exclusively to the coupler, enabling the implementation of a controlled-ZZZ gate. We show that the gate fidelity exceeding 0.999 can be achieved within 150 ns, excluding decoherence effects. Moreover, we also show that leakage from the computational subspace to noncomputational states can also be effectively suppressed.
Achieving high-fidelity two-qubit gates is a key requirement for the realization of large-scale quantum computation and simulation. In this paper, we propose a microwave-control protocol for the implementation of a two-qubit gate employing two transmon qubits coupled via a fixed-frequency transmon coupler. This protocol entails applying two microwave pulses exclusively to the coupler, thereby inducing interaction between fixed-frequency transmon qubits. This interaction facilitates the realization of iSWAP\sqrt{i\mathrm{SWAP}}iSWAP​ gates. Additionally, we explore the implementation of the gate scheme in two distinct qubit architectures (ABA and ABC). Without considering decoherence, the ABA architecture iSWAP\sqrt{i\mathrm{SWAP}}iSWAP​ gate has a fidelity of 94.60% in 346 ns, while the ABC architecture achieves 99.63% in 378 ns. When considering the implementation of the iSWAP\sqrt{i\mathrm{SWAP}}iSWAP​-like gate, we show that gate fidelity can exceed 99.9% in both architectures.
Scalable superconducting quantum processors require balancing critical constraints in coherence, control complexity, and spectral crowding. Fixed-frequency architectures suppress flux noise and simplify control via all-microwave operations but remain limited by residual ZZ crosstalk. Here we propose a microwave-activated three-qubit gate protocol for fixed-frequency transmon qubits in the large-detuning regime (∣Δ∣≫g|Δ| \gg g∣Δ∣≫g), leveraging the third-order nonlinear interaction to coherently exchange ∣001⟩↔∣110⟩\ket{001} \leftrightarrow \ket{110}∣001⟩↔∣110⟩ states. By incorporating a phase-compensated optimization protocol, numerical simulations demonstrate a high average gate fidelity exceeding 99.9%99.9\%99.9%. Systematic error analysis identifies static long-range ZZ coupling as the dominant error source in multi-qubit systems, which can be suppressed via operations in the large-detuning regime (∼1\sim 1∼1 GHz). The protocol maintains process fidelities exceeding 98%98\%98% under decoherence, while demonstrating intrinsic robustness to fabrication-induced parameter variations and compatibility with existing all-microwave two-qubit gate architectures. This hardware-efficient strategy advances scalable quantum computing systems by improving coherence properties, reducing spectral congestion, and expanding the experimental toolkit for error-resilient quantum operations in the noisy intermediate-scale quantum era.
High-fidelity iSWAP\sqrt{i\text{SWAP}}iSWAP​ gates using a fixed coupler driven by two microwave pulses
Attaining high-fidelity two-qubit gates represents a pivotal quantum operation for the realization of large-scale quantum computation and simulation. In this study, we propose a microwave-control protocol for the implementation of a two-qubit gate employing two transmon qubits coupled via a fixed-frequency transmon coupler. This protocol entails applying two microwave pulses exclusively to the coupler, thereby inducing interaction between the fixed-frequency transmon qubits. This interaction facilitates the realization of iSWAP\sqrt{i\text{SWAP}}iSWAP​ gates. Additionally, we explore the implementation of the gate scheme in two distinct qubit architectures. Demonstrating with experimentally accessible parameters, we show that high-fidelity iSWAP\sqrt{i\text{SWAP}}iSWAP​ gates can be achieved
Abstract High-quality entangling gates are crucial for scalable quantum information processing. Implementing all-microwave two-qubit gates on fixed-frequency transmons offers advantages in reducing wiring complexity, but the gate performance is often limited due to the residual ZZ interaction and the frequency crowding problem. Here, we introduce a novel scheme that enables a microwave drive-activated CZ gate compatible with the coupler structure to suppress the residual ZZ interaction. The microwave drive is applied to the coupler and the microwave drive frequency remains far detuned from the system’s transition frequency to alleviate the frequency crowding problem. We model the gate process analytically and demonstrate a theoretical gate fidelity up to 99.9% numerically. Our scheme is compatible with current coupler-structure-based circuits, and insensitive to microwave crosstalk, showing a possible path for all-microwave quantum operations at scale.
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 (ZZ) 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-Z (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 99.922% Æ 0.009%, 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 ZZ interactions.
Generating high-fidelity, tunable entanglement between qubits is crucial for realizing gate-based quantum computation. In superconducting circuits, tunable interactions are often implemented using flux-tunable qubits or coupling elements, adding control complexity and noise sources. Here, we realize a tunable ZZ interaction between two transmon qubits with fixed frequencies and fixed coupling, induced by driving both transmons off resonantly. We show tunable coupling over 1 order of magnitude larger than the static coupling, and change the sign of the interaction, enabling cancellation of the idle coupling. Further, this interaction is amenable to large quantum processors: the drive frequency can be flexibly chosen to avoid spurious transitions, and because both transmons are driven, it is resilient to microwave cross talk. We apply this interaction to implement a controlled phase (CZ) gate with a gate fidelity of 99.43(1)% as measured by cycle benchmarking, and we find the fidelity is limited by incoherent errors.