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IEEE International Conference on Quantum Computing and Engineering (QCE)

Towards ZZ-Based Global Control in a Superconducting Quantum Computer

Marco Riccardi, Riccardo Aiudi, Aviv Glezer Moshe, Daniel B. Szombati, Marco Polini, Vittorio Giovannetti

Published Aug 30, 2025• 0 citations
Publisher

Abstract

Current superconducting quantum computing architectures are designed in such a way that each qubit is locally controlled with a dedicated drive and, sometimes, flux line. Scaling up these architectures to larger chips sizes and qubit counts is problematic, as the increased number of wires in the system would produce additional crosstalk and thermal load, as well as increasing costs. Recently, there have been a number of proposals to overcome this wiring bottleneck. These proposals rely on engineering a large$\mathbf{Z Z}$interaction between neighboring qubits in order to enter the so-called blockade regime, which enables the execution of logical operations with only three global drive lines regardless of the number of qubits on the chip. In these architectures, the entire dynamics is governed by the ZZ interaction, which needs to be the dominant energy scale to preserve the blockade regime. We present here preliminary results showing the possibility of retaining a large$\mathbf{Z Z}$coupling between two strongly detuned qubits, with an increase of several orders of magnitude with respect to what has been reported so far in the literature. In particular, we measure the$\mathbf{Z Z}$coupling between two capacitively-coupled transmon qubits and extract a ZZ strength of 22 MHz for a detuning of 2.4 GHz, placing our system well into the blockade regime. These findings provide an experimental validation of the working principle of several global quantum computing architectures and represent a first step towards their practical realization.

DOI
10.1109/qce65121.2025.10404

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Current superconducting quantum computing architectures are designed in such a way that each qubit is locally controlled with a dedicated drive and, sometimes, flux line. Scaling up these architectures to larger chips sizes and qubit counts is problematic, as the increased number of wires in the system would produce additional crosstalk and thermal load, as well as increasing costs. Recently, there have been a number of proposals to overcome this wiring bottleneck. These proposals rely on engineering a largeZZ\mathbf{Z Z}ZZinteraction between neighboring qubits in order to enter the so-called blockade regime, which enables the execution of logical operations with only three global drive lines regardless of the number of qubits on the chip. In these architectures, the entire dynamics is governed by the ZZ interaction, which needs to be the dominant energy scale to preserve the blockade regime. We present here preliminary results showing the possibility of retaining a largeZZ\mathbf{Z Z}ZZcoupling between two strongly detuned qubits, with an increase of several orders of magnitude with respect to what has been reported so far in the literature. In particular, we measure theZZ\mathbf{Z Z}ZZcoupling between two capacitively-coupled transmon qubits and extract a ZZ strength of 22 MHz for a detuning of 2.4 GHz, placing our system well into the blockade regime. These findings provide an experimental validation of the working principle of several global quantum computing architectures and represent a first step towards their practical realization.
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.
We propose a platform to implement a scalable universal quantum computer based on a 2D array of superconducting qubits. Notably, calculations in our scheme are enabled by a set of only three global driving signals, irrespective of the number of qubits, ensuring full scalability of the computation. In stark contrast with the existing literature, global control in our scheme is achieved by leveraging the longitudinal ZZ coupling between neighboring qubits. We show how this enables information flow, one-and two-qubit gates through a series of three global pulses that act together on different subsets of the physical qubits in our system. These findings provide an alternative route to the realization of scalable and fault-tolerant superconducting quantum computers with reduced wiring, bringing them one step closer to solving practical problems in our daily lives.
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.
Precise control of superconducting qubits is essential for advancing both quantum simulation and quantum error correction. Recently, transmon qubit systems employing the single-transmon coupler (STC) scheme have demonstrated high-fidelity single- and two-qubit gate operations by dynamically tuning the effective coupling between qubits. However, the integration of STCs increases the number of control lines, thereby posing a significant bottleneck for chip routing and scalability. To address this challenge, we propose a robust control line multiplexing scheme based on a double-transmon coupler (DTC) architecture, which enables shared coupler control lines to substantially reduce wiring complexity. Moreover, we experimentally verify that this multiplexed configuration efficiently suppresses undesirable static ZZZZZZ coupling while maintaining accurate control over two-qubit gate operations. We further demonstrate the feasibility of the architecture through two distinct gate implementations: a fast coupler ZZZ-control-based CZ gate and a parametric iSWAP gate. To validate the practical applicability of this multiplexing approach in quantum circuits, we prepare Bell and three-qubit GHZ states using the proposed scheme with fidelity exceeding 99% and 96%, respectively. This multiplexed DTC architecture offers significant potential to minimize wiring overhead in two-dimensional qubit arrays, thereby greatly enhancing the scalability of superconducting quantum processors.
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.
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.