Controlling superconducting qubits in the presence of a strong, constant ZZ interaction

Roni Winik, Catherine Leroux, Agustín Di Paolo +10 more

Roni Winik, Catherine Leroux, Agustín Di Paolo +16 more

Peng Zhao, Peng Xu, Dong Lan +3 more

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.

Irfan A. Siddiqi

Dong‐Sheng Wang, Shu‐Ya Wang, Tian Yang +10 more

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.

Marco Riccardi, Aviv Glezer Moshe, Guido Menichetti +13 more

We report the experimental realization of strong longitudinal (ZZ) coupling between two superconducting transmon qubits achieved solely through capacitive engineering. By systematically varying the qubit frequency detuning, we measure cross-Kerr inter-qubit interaction strengths ranging from 10 MHz up to 350 MHz, more than an order of magnitude larger than previously observed in similar capacitively coupled systems. In this configuration, the qubits enter a strong-interaction regime in which the excitation of one qubit inhibits that of its neighbor, demonstrating a dynamical blockade mediated entirely by the engineered ZZ coupling. Circuit quantization simulations accurately reproduce the experimental results, while perturbative models confirm the theoretical origin of the energy shift as a hybridization between the computational states and higher-excitation manifolds. We establish a robust and scalable method to access interaction-dominated physics in superconducting circuits, providing a pathway towards solid-state implementations of globally controlled quantum architectures and cooperative many-body dynamics.