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arXiv:2610.36365

Suppressing microwave crosstalk in superconducting quantum processors by designing parasitic drive capacitances

Anuj Aggarwal, Jorge Fernández-Pendás, Simon Pettersson Fors, Zakaria Mohamed, Halldór Jakobsson, Amr Osman, Janka Biznárová, Andreas Nylander, Anita Fadavi Roudsari, Marcus Rommel, Anton Frisk Kockum, Giovanna Tancredi

quant-phSubmitted Oct 26, 2026• Announced Oct 28, 2026• 0 citations
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Abstract

As superconducting quantum processors scale to large qubit counts, unwanted microwave drive crosstalk becomes an important source of coherent errors. Here, we investigate the role of parasitic coupling paths in the suppression of drive crosstalk in a processor with two fixed-frequency transmon qubits and a tunable coupler. To account for the many different coupling paths, we develop a detailed circuit model to capture the dependence of crosstalk on the coupler frequency. We experimentally demonstrate suppression of crosstalk, finding that the suppression does not originate solely from cancellation of the effective qubit-qubit coupling, but also is a result of interference between other coupling paths. Building on these insights, we show that drive crosstalk can be suppressed with the right choice of parasitic capacitances, which otherwise often are treated as unwanted design imperfections. Our results provide practical guidance for low-crosstalk device design of superconducting quantum processors and point toward a broader strategy in which parasitic coupling paths are engineered rather than simply minimized.

AI Summary

Microwave drive crosstalk can persist even when a tunable coupler cancels effective qubit–qubit exchange. This paper examines whether interference among drive pathways can explain and suppress that residual crosstalk, using two fixed-frequency floating transmons and a grounded tunable coupler. A circuit model includes drive-line capacitances to the target qubit, victim qubit, and coupler. Eliminating floating-qubit common modes, applying a first-order high-frequency expansion, and diagonalizing the quadratic Hamiltonian gives analytical dressed drive amplitudes and cancellation conditions. These predictions are compared with numerical Floquet simulations and measured self- and cross-Rabi rates versus coupler frequency. The qubits operate near 4.752 and 4.658 GHz; drive powers are referenced to 20 MHz self-Rabi rates with the coupler far detuned. Both cross-Rabi responses exhibit minima approximately 0.3 GHz above the estimated exchange-cancellation point at 3.665 GHz. Simultaneous fits to self- and cross-Rabi data require parasitic drive-to-coupler and drive-to-victim pathways. Parameter sweeps predict that engineering these capacitances can align the two directional crosstalk nulls, potentially at a low-$ZZ$ operating point. A two-level calculation predicts corresponding suppression of off-resonant victim excitation. Capacitance-engineered alignment and gate-fidelity improvements are not experimentally demonstrated. One measured minimum is shallower than predicted, attributed to additional microwave pathways absent from the model. The floating-versus-grounded design comparison depends on the chosen drive asymmetries rather than establishing a general architectural advantage. Off-resonant estimates assume weak driving and neglect small cross-Kerr shifts.

Little sign of AI-generated textIncremental

Error found: Both system-parameter tables give coupling constants with signs opposite to those required by the paper's circuit-to-Hamiltonian formula. Using the printed Hamiltonian and tabulated couplings, the effective qubit–qubit exchange is approximately +7.80 MHz at the claimed 3.665 GHz cancellation point, rather than zero, making that quantitative comparison inconsistent as written.

Written by gpt-6.1-sol from the paper's full text. Automated: it can be wrong.

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As superconducting quantum processors scale to large qubit counts, unwanted microwave drive crosstalk becomes an important source of coherent errors. Here, we investigate the role of parasitic coupling paths in the suppression of drive crosstalk in a processor with two fixed-frequency transmon qubits and a tunable coupler. To account for the many different coupling paths, we develop a detailed circuit model to capture the dependence of crosstalk on the coupler frequency. We experimentally demonstrate suppression of crosstalk, finding that the suppression does not originate solely from cancellation of the effective qubit-qubit coupling, but also is a result of interference between other coupling paths. Building on these insights, we show that drive crosstalk can be suppressed with the right choice of parasitic capacitances, which otherwise often are treated as unwanted design imperfections. Our results provide practical guidance for low-crosstalk device design of superconducting quantum processors and point toward a broader strategy in which parasitic coupling paths are engineered rather than simply minimized.
Microwave drive crosstalk can persist even when a tunable coupler cancels effective qubit–qubit exchange. This paper examines whether interference among drive pathways can explain and suppress that residual crosstalk, using two fixed-frequency floating transmons and a grounded tunable coupler.
A circuit model includes drive-line capacitances to the target qubit, victim qubit, and coupler. Eliminating floating-qubit common modes, applying a first-order high-frequency expansion, and diagonalizing the quadratic Hamiltonian gives analytical dressed drive amplitudes and cancellation conditions. These predictions are compared with numerical Floquet simulations and measured self- and cross-Rabi rates versus coupler frequency. The qubits operate near 4.752 and 4.658 GHz; drive powers are referenced to 20 MHz self-Rabi rates with the coupler far detuned.
Both cross-Rabi responses exhibit minima approximately 0.3 GHz above the estimated exchange-cancellation point at 3.665 GHz. Simultaneous fits to self- and cross-Rabi data require parasitic drive-to-coupler and drive-to-victim pathways. Parameter sweeps predict that engineering these capacitances can align the two directional crosstalk nulls, potentially at a low-ZZZZZZ operating point. A two-level calculation predicts corresponding suppression of off-resonant victim excitation.
Capacitance-engineered alignment and gate-fidelity improvements are not experimentally demonstrated. One measured minimum is shallower than predicted, attributed to additional microwave pathways absent from the model. The floating-versus-grounded design comparison depends on the chosen drive asymmetries rather than establishing a general architectural advantage. Off-resonant estimates assume weak driving and neglect small cross-Kerr shifts.
Crosstalk is a major concern in the implementation of large-scale quantum computation since it can degrade the performance of qubit addressing and cause gate errors. Finding the origin of crosstalk and separating contributions from different channels are essential prerequisites for figuring out crosstalk mitigation schemes. Here, by performing circuit analysis of two coupled floating transmon qubits, we demonstrate that, even if the stray coupling, e.g., between a qubit and the drive line of its nearby qubit, is absent, microwave crosstalk between qubits can still exist due to the presence of a spurious crosstalk channel. This channel arises from free modes, which are supported by the floating structure of transmon qubits, i.e., the two superconducting islands of each qubit with no galvanic connection to the ground. For various geometric layouts of floating transmon qubits, we give the contributions of microwave crosstalk from the spurious channel and show that this channel can become a performance-limiting factor in qubit addressing. This research could provide guidance for suppressing microwave crosstalk between floating superconducting qubits through the design of qubit circuits.
Microwave crosstalk poses a major challenge to scaling superconducting quantum devices as it introduces excess control errors. Although its magnitude and impact have been explored in various experimental settings, quantitative physical models capable of explaining measured crosstalk for a given device geometry remain scarce. Here, we address this gap by investigating microwave crosstalk in planar superconducting devices with crossovers. We identify two structures that can lead to strong crosstalk: a drive line routed in close proximity to another qubit, and a drive line crossing a qubit-qubit coupler using an air bridge. We design and characterize devices involving these structures and develop physical models that quantitatively explain the experimentally observed crosstalk. Based on these models, we discuss the design considerations for reducing microwave crosstalk. Our results provide practical guidance for low-crosstalk device layouts and establish a basis for the systematic investigation of weaker crosstalk mechanisms.
Single-qubit gates on superconducting quantum processors are typically implemented using microwave pulses applied through dedicated control lines. However, these microwave pulses may also drive other qubits due to crosstalk arising from capacitive coupling and wavefunction overlap in systems with closely spaced transition frequencies. Crosstalk and frequency crowding increase errors during simultaneous single-qubit operations relative to isolated gates, thus forming a major bottleneck for scaling superconducting quantum processors. In this work, we combine model-based qubit frequency optimization with pulse shaping to demonstrate crosstalk error mitigation in single-qubit gates on a 49-qubit superconducting quantum processor. We introduce and experimentally verify an analytical model of simultaneous single-qubit gate error caused by microwave crosstalk that depends on a given pulse shape. By employing a model-based optimization strategy of qubit frequencies, we minimize the crosstalk-induced error across the processor and achieve a mean simultaneous single-qubit gate fidelity of 99.96% for a 16-ns gate duration, approaching the mean individual gate fidelity. To further reduce the simultaneous error and required qubit frequency bandwidth on high-crosstalk qubit pairs, we introduce a crosstalk transition suppression (CTS) pulse shaping technique that minimizes the spectral energy around transitions inducing leakage and crosstalk errors. Finally, we combine CTS with model-based frequency optimization across the device and experimentally show a systematic reduction in the required qubit frequency bandwidth for high-fidelity simultaneous gates, supported by simulations of systems with up to 1000 qubits. By alleviating constraints on qubit frequency bandwidth for parallel single-qubit operations, this work represents an important step for scaling towards larger quantum processors.
Maintaining or even improving gate performance with growing numbers of parallel controlled qubits is a vital requirement for fault-tolerant quantum computing.For superconducting quantum processors, though isolated one-or two-qubit gates have been demonstrated with high fidelity, implementing these gates in parallel commonly shows worse performance.Generally, this degradation is attributed to various crosstalks between qubits, such as quantum crosstalk due to residual inter-qubit coupling.An understanding of the exact nature of these crosstalks is critical to figuring out respective mitigation schemes and improved qubit architecture designs with low crosstalk.Here we give a theoretical analysis of quantum crosstalk impact on simultaneous gate operations in a qubit architecture, where fixed-frequency transmon qubits are coupled via a tunable bus, and sub-100-ns controlled-Z (CZ) gates can be realized by applying a baseband flux pulse on the bus.Our analysis shows that for microwave-driven single-qubit gates, the dressing from the qubit-qubit coupling can cause non-negligible cross-driving errors when qubits operate near frequency collision regions.During CZ gate operations, although unwanted nearest-neighbor interactions are nominally turned off, sub-MHz parasitic next-nearest-neighbor interactions involving spectator qubits can still exist, causing considerable leakage or control error when one operates qubit systems around these parasitic resonance points.To ensure high-fidelity simultaneous operations, a request could be raised to figure out a better way to balance the gate error from target qubit systems themselves and the error from nonparticipating spectator qubits.Overall, our analysis suggests that towards useful quantum processors, the qubit architecture should be examined carefully in the context of high-fidelity simultaneous gate operations in a scalable qubit lattice.
Crosstalk refers to unwanted qubit addressing. This is particularly detrimental when scaling up quantum information systems because unintended interactions limit their overall performance. For superconducting qubits, tunable couplings and frequency tunability achieved through externally applied magnetic fluxes enable high-fidelity entangling gates; however, they also introduce crosstalk through unintended flux coupling. In this work, we investigate the impact of time-dependent external magnetic fluxes in quantized circuits on superconducting qubit couplings. We find that non-trivial cross-voltage driving emerges between capacitively linked qubits when the magnetic flux threading the SQUID loop of a qubit varies in time, in a manner analogous to Faraday's law of induction. Crucially, we show that this effect enables fast single qubit control through the coupler element in standard tunable-coupler architectures, potentially eliminating the need for individual microwave XYXYXY control lines.
Crosstalk between qubits fundamentally limits the scalability of quantum processors, necessitating physics-based models that can handle the complexity of large qubit arrays. Here, we develop a comprehensive theoretical and experimental framework that captures residual interactions between both adjacent and non-adjacent qubits in fixed-frequency transmon lattices. The model integrates the combined effects of exponential localization in banded capacitance matrices, suppression of virtual couplings through detuning products across intermediate modes, and evanescent decay of below-cutoff electromagnetic fields, yielding predictive scaling relations for coupling strength as a function of spatial separation and spectral detuning. Experimental characterization of a 4×44 \times 44×4 superconducting-qubit lattice with inductive shunt pillars reveals exponential spatial decay and frequency-dependent suppression consistent with theoretical predictions, achieving quantitative agreement for all nearest-neighbor couplings across the \qtyrange[range-phrase = --, range-units = single]{4}{6}{\giga\hertz} operating range. Our results show that standard dispersive Hamiltonian approximations systematically overestimate long-range coupling when spatial and spectral dependencies are neglected; these errors propagate into circuit simulation and design strategies. Our framework provides design guidance for crosstalk mitigation in larger-scale quantum processors under realistic fabrication constraints, addressing a bottleneck in scalability.