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arXiv:2602.06812

Hybrid Coupling Topology with Dynamic ZZ Suppression for Optimizing Circuit Depth during Runtime in Superconducting Quantum Processor

Uday Sannigrahi, Amlan Chakrabarti, Swapnil Saha, Shrinjita Biswas

quant-phSubmitted Feb 6, 2026• Announced Feb 9, 2026• 0 citations
arXivHTMLPDF

Abstract

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.

Comments
6 pages, 8 figures

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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.
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.
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.
Tunable coupling of superconducting qubits has been widely studied due to its importance for isolated gate operations in scalable quantum processor architectures.Here, we demonstrate a tunable qubit-qubit coupler based on a floating transmon device, which allows us to place qubits at least 2 mm apart from each other while maintaining over 50-MHz coupling between the coupler and the qubits.In the introduced tunable-coupler design, both the qubit-qubit and the qubit-coupler couplings are mediated by two waveguides instead of relying on direct capacitive couplings between the components, reducing the impact of the qubit-qubit distance on the couplings.This leaves space for each qubit to have an individual readout resonator and a Purcell filter, which is needed for fast high-fidelity readout.In addition, simulations show that the large qubit-qubit distance significantly lowers unwanted non-nearest-neighbor coupling and allows multiple control lines to cross over the structure with minimal crosstalk.Using the proposed flexible and scalable architecture, we demonstrate a controlled-Z gate with (99.81 ± 0.02)% fidelity.
As systems for quantum computing keep growing in size and number of qubits, challenges in scaling the control capabilities are becoming increasingly relevant. Efficient schemes to simultaneously mediate coherent interactions between multiple quantum systems and to reduce decoherence errors can minimize the control overhead in next-generation quantum processors. Here, we present a superconducting qubit architecture based on tunable parametric interactions to perform two-qubit gates, reset, leakage recovery, and to read out the qubits. In this architecture, parametrically driven multielement couplers selectively couple qubits to resonators and neighboring qubits, according to the frequency of the drive. We consider a system with two qubits and one readout resonator interacting via a single coupling circuit and experimentally demonstrate a controlled- Z gate with a fidelity of 98.30 ± 0.23 % , a reset operation that unconditionally prepares the qubit ground state with a fidelity of 99.80 ± 0.02 % and a leakage recovery operation with a 98.5 ± 0.3 % success probability. Furthermore, we implement a parametric readout with a single-shot assignment fidelity of 88.0 ± 0.4 % . These operations are all realized using a single tunable coupler and a statically decoupled resonator, demonstrating the experimental feasibility of the proposed architecture and its potential for reducing the system complexity in scalable quantum processors.
In recent years, several architectures have been proposed for implementing two-qubit operations on fluxonium superconducting qubits. A particularly promising approach, which was demonstrated experimentally by Refs. [1,2], employs a transmon superconducting qubit as a tunable coupler between the fluxonium qubits. These experiments have shown that the transmon coupler enables fast, high-fidelity two-qubit operations while suppressing unwanted ZZ crosstalk between the fluxonium qubits. In this work, we numerically study the scalability of this architecture. We find that, when trivially scaling this architecture, crosstalk from spectator qubits limits the gate fidelity to below 90%. We show that these spectator errors can be reduced to below 10−410^{-4}10−4 by reducing the coupling strength and by dynamically tuning transmons that are not used for a two-qubit operation to an off position. We further investigate the resilience of the operation to direct capacitive coupling between the transmon couplers and to microwave crosstalk.