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arXiv:2302.06930 · Physical Review Letters

All-Microwave Manipulation of Superconducting Qubits with a Fixed-Frequency Transmon Coupler

Shotaro Shirai, Yuta Okubo, Kohei Matsuura, Alto Osada, Yasunobu Nakamura, Atsushi Noguchi

Published Jun 29, 2023• 20 citations
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Abstract

All-microwave control of fixed-frequency superconducting quantum computing circuits is advantageous for minimizing the noise channels and wiring costs. Here we introduce a swap interaction between two data transmons assisted by the third-order nonlinearity of a coupler transmon under a microwave drive. We model the interaction analytically and numerically and use it to implement an all-microwave controlled-Z gate. The gate based on the coupler-assisted swap transition maintains high drive efficiency and small residual interaction over a wide range of detuning between the data transmons.

DOI
10.1103/physrevlett.130.260601

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All-microwave control of fixed-frequency superconducting quantum computing circuits is advantageous for minimizing the noise channels and wiring costs. Here we introduce a swap interaction between two data transmons assisted by the third-order nonlinearity of a coupler transmon under a microwave drive. We model the interaction analytically and numerically and use it to implement an all-microwave controlled-Z gate. The gate based on the coupler-assisted swap transition maintains high drive efficiency and small residual interaction over a wide range of detuning between the data transmons.
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.
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.
Entangling gates between neighboring physical qubits are essential for quantum error correction. Implementing them in an all-microwave manner simplifies signal routing and control apparatus of superconducting quantum processors. We propose and experimentally demonstrate an all-microwave controlled-Z (CZ) gate that achieves high fidelity while suppressing residual ZZ interactions. Our approach utilizes a fixed-frequency transmon coupler and multi-path coupling, thereby sufficiently reducing the net transverse interaction between data transmons to suppress residual ZZ interactions. The controlled phase arises from the dispersive frequency shift of the ∣gf⟩|gf\rangle∣gf⟩$\unicode{x2013}$∣eg⟩|eg\rangle∣eg⟩ transition between the coupler and one of the data transmons conditioned on the state of the other data transmon. Driving the transitions at the midpoint of two dispersively shifted resonance frequencies induces state-dependent geometric phases to achieve the CZ gate. Crucially, with this scheme, we can maintain a small net transverse interaction between two data transmons while increasing the coupling between the data and coupler transmons to accelerate the CZ-gate speed. Additionally, we measure the coupler state after the gate to detect a subset of decoherence-induced failures that occur during the gate operation. These events constitute erasure errors with known locations, enabling erasure-aware quantum error-correcting codes to improve future logical qubit performance.
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High-fidelity iSWAP\sqrt{i\text{SWAP}}iSWAP​ gates using a fixed coupler driven by two microwave pulses
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