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Advanced Quantum Technologies

Introducing Anharmonicity into Coupler for Suppressing ZZ Coupling: Toward Large‐Scale Scalable Superconducting Quantum Chip

Dong‐Sheng Wang, Shu‐Ya Wang, Tian Yang, Ben‐Zheng Yuan, Chuanbing Han, Xiaohan Yu, Bo Zhao, Weilong Wang, Huihui Sun, Fudong Liu, Xu‐Yan Qi, Yifan Hou, Zheng Shan

Published Dec 3, 2025• 0 citations
Publisher

Abstract

ABSTRACT The intrinsic parasitic longitudinal (ZZ) coupling between coupled qubits degrades the fidelity of quantum gate operations, becoming a major limiting factor for building large‐scale, scalable superconducting quantum chips. Therefore, it is essential to develop a coupling scheme that can flexibly control the ZZ interaction. This work proposes a simple coupling architecture that enables high‐fidelity quantum gate operations via tunable ZZ coupling. The scheme leverages a bus resonator with anharmonicity to effectively suppress the ZZ coupling between two frequency‐tunable Xmon qubits. Moreover, theoretical demonstrations show that the ZZ interaction can be tuned via qubit frequency adjustment, thus enabling two‐qubit controlled‐Z (CZ) gates. Finally, the fidelity of gate operations is evaluated using cross‐entropy benchmarking, realizing a CZ gate fidelity of (99.310.04)%. This work demonstrates flexible ZZ coupling control without increasing circuit complexity, offering a new paradigm for the development of large‐scale superconducting quantum processors with simplified control wiring.

DOI
10.1002/qute.202500762

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ABSTRACT The intrinsic parasitic longitudinal (ZZ) coupling between coupled qubits degrades the fidelity of quantum gate operations, becoming a major limiting factor for building large‐scale, scalable superconducting quantum chips. Therefore, it is essential to develop a coupling scheme that can flexibly control the ZZ interaction. This work proposes a simple coupling architecture that enables high‐fidelity quantum gate operations via tunable ZZ coupling. The scheme leverages a bus resonator with anharmonicity to effectively suppress the ZZ coupling between two frequency‐tunable Xmon qubits. Moreover, theoretical demonstrations show that the ZZ interaction can be tuned via qubit frequency adjustment, thus enabling two‐qubit controlled‐Z (CZ) gates. Finally, the fidelity of gate operations is evaluated using cross‐entropy benchmarking, realizing a CZ gate fidelity of (99.310.04)%. This work demonstrates flexible ZZ coupling control without increasing circuit complexity, offering a new paradigm for the development of large‐scale superconducting quantum processors with simplified control wiring.
For building a scalable quantum processor with superconducting qubits, the ZZ interaction is of great concert because of relevant for implementing two-qubit gates, and the close contact between gate infidelity and its residual. Two-qubit gates with fidelity beyond fault-tolerant thresholds have been demonstrated using the ZZ interaction. However, as the performance of quantum processor improves, the residual static-ZZ can also become a performance-limiting factor for quantum gate operations and quantum error correction. Here, we introduce a scalable superconducting architecture for addressing this challenge. We demonstrate that by coupling two superconducting qubits with opposite-sign anharmonicities together, high-contrast ZZ interaction can be realized in this architecture. Thus, we can control ZZ interaction with high on/off ratio for implementing two-qubit CZ gate, or suppress it during the two-qubit gate operations using XY interaction (e.g. iSWAP). Meanwhile, the ZZ crosstalk related to neighboring spectator qubits can also be heavily suppressed in fixed coupled multi-qubit systems. This architecture could provide a promising way towards scalable superconducting quantum processor with high gate fidelity and low qubit crosstalk.
To reduce circuit depth when executing Quantum algorithms, it is necessary to maximize qubit connectivity on a near-term quantum processor. While addressing this, we also need to ensure high gate fidelity, suppression of unwanted ZZ cross-talk, a compact layout footprint, and minimal control hardware complexity to support scalability. In current superconducting quantum chips, fixed coupling is used as it is easier to scale, but it is limited by unwanted static ZZ interaction during single qubit operations, which degrades system performance. To overcome these challenges, we have introduced a first-of-its-kind hybrid tunable-coupling architecture that connects four fixed-frequency transmon qubits using a single coupler. This hybrid coupler uses off-resonant Stark drives to tune ZZ strength between qubit pairs. Experimentally backed simulation results indicate that our proposed hybrid design maximizes the qubit connectivity while reducing control overhead. This design achieves a near 20% reduction in circuit depth compared to IBM's Heavy-Hexagonal layout, showing its potential for scalability.
For building a scalable quantum processor with superconducting qubits, ZZ interaction is of great concern because its residual has a crucial impact to two-qubit gate fidelity. Two-qubit gates with fidelity meeting the criterion of fault-tolerant quantum computation have been demonstrated using ZZ interaction. However, as the performance of quantum processors improves, the residual static ZZ can become a performance-limiting factor for quantum gate operation and quantum error correction. Here, we introduce a superconducting architecture using qubits with opposite-sign anharmonicity, a transmon qubit, and a C-shunt flux qubit, to address this issue. We theoretically demonstrate that by coupling the two types of qubits, the high-contrast ZZ interaction can be realized. Thus, we can control the interaction with a high on-off ratio to implement two-qubit controlled-Z gates, or suppress it during two-qubit gate operation using XY interaction (e.g., an iSWAP gate). The proposed architecture can also be scaled up to multiqubit cases. In a fixed coupled system, ZZ crosstalk related to neighboring spectator qubits could also be heavily suppressed.
High-fidelity quantum interconnection remains a critical challenge for scaling up superconducting quantum chips. Here, we realize a simple fixed coupling scheme to achieve high-fidelity two-qubit gate operation across separate superconducting quantum chips. In this scheme, qubit-qubit coupling between separate chips can be realized via exchange interaction mediated by a bus resonator. By merely tuning the frequency of a single qubit, the resonant mode of the bus resonator can be modulated, enabling high-quality and stable quantum interconnection across different chips. Consequently, the experimentally implemented two-qubit gate operations between separate chips exhibit excellent performance, with a maximum two-qubit CZ gate fidelity reaching 99.57%. This work lays a technical foundation for the realization of high-fidelity chip-to-chip quantum interconnection, which facilitates the development of large-scale superconducting quantum chips with high fidelity.
We propose a theoretical scheme of using two fixed-frequency resonator couplers to tune the interaction between two Xmon qubits. The indirect interaction between two qubits induced by two resonators can cancel each other, so direct qubit-qubit coupling is not essential for the switching off. So, we can suppress the static ZZ coupling with the weak direct qubit-qubit coupling and even eliminate the static ZZ coupling through the destructive interferences of the double-path couplers. The cross-Kerr resonance can induce additional poles for the static ZZ coupling which should be kept away during the two-qubit gates. The double-resonator couplers scheme could unfreeze some restrictions during the design of superconducting quantum chips and mitigate the static ZZ coupling, which might supply a promising platform for future superconducting quantum chips.
Controllable interaction between superconducting qubits is desirable for large-scale quantum computation and simulation. Here, based on a theoretical proposal by Yan et al. [Phys. Rev. Appl. 10, 054061 (2018)] we experimentally demonstrate a simply designed and flux-controlled tunable coupler with a continuous tunability by adjusting the coupler frequency, which can completely turn off adjacent superconducting qubit coupling. Utilizing the tunable interaction between two qubits via the coupler, we implement a different type of controlled-phase (cz) gate with ``dynamically decoupled regime,'' which allows the qubit-qubit coupling to be only ``on'' at the usual operating point while dynamically ``off'' during the tuning process of one qubit frequency into and out of the operating point. This scheme not only efficiently suppresses the leakage out of the computational subspace, but also allows for the acquired two-qubit phase being geometric at the operating point. We achieve an average cz gate fidelity of $98.3\ifmmode\pm\else\textpm\fi{}0.6\mathrm{%}$, which is dominantly limited by qubit decoherence. The demonstrated tunable coupler provides a desirable tool to suppress adjacent qubit coupling and is suitable for large-scale quantum computation and simulation.