Researchers at ETH Zurich have identified specific circuit layouts that contribute significantly to microwave crosstalk, a major obstacle to building larger superconducting quantum devices. The work pinpoints drive lines positioned near qubits and those crossing qubit-qubit couplers using air bridges as key sources of signal interference. By designing and characterizing devices with these structures, the team developed physical models that quantitatively explain observed crosstalk, providing practical guidance for future low-crosstalk device layouts and establishing a basis for investigating subtler interference mechanisms.

Planar Device Layouts Limit Qubit Scaling via Crosstalk

Drive lines positioned close to qubits represent a significant source of microwave crosstalk, limiting the scalability of superconducting quantum devices, according to new work at ETH Zurich. The research identifies specific geometric arrangements that amplify unwanted signal leakage, hindering efforts to build larger and more complex quantum processors. This crosstalk arises from stray capacitance between the control lines and the qubits themselves, creating excess control errors that degrade performance.

The team’s analysis further pinpoints air bridge qubit-qubit couplers as particularly vulnerable to crosstalk when intersected by a drive line. Researchers developed quantitative physical models that accurately predict crosstalk levels based solely on device geometry and packaging design, filling a gap where previous studies lacked predictive capability. These models were validated against experimental data from fabricated test structures, and also successfully explained the strongest crosstalk instances observed in a 17-qubit quantum processor.

The study also confirms that direct coupling between neighboring control lines is negligible, due to the typical spacing exceeding ten times the width of the lines themselves. Numerical simulations based on network parameters of the control lines in quasi-static conditions support this expectation. This finding focuses attention on proximity-induced crosstalk and crossover-induced crosstalk as the dominant mechanisms in planar superconducting quantum devices, allowing for targeted design improvements.

The analytical methods introduced can be applied to a broad range of superconducting quantum device applications, extending beyond quantum computing to areas like sensing, quantum optics, and fundamental quantum science. “We have fabricated test structures and characterized the crosstalk as a function of frequency,” the researchers state in their published work. The models developed offer practical guidance for low-crosstalk device layouts, enabling more reliable and scalable quantum systems.

The team’s analysis reveals that a drive signal can unintentionally address a nearby qubit through either stray capacitance or indirect coupling via the package volume. This understanding is important for optimizing device designs and minimizing unwanted interactions between qubits, a critical step towards realizing fault-tolerant quantum computation. The ability to quantitatively predict crosstalk based on layout and packaging represents an advancement in the field, providing a foundation for systematic investigation of weaker crosstalk mechanisms and further improvements in quantum device performance.

Proximity-Induced Crosstalk from Drive Line Placement

The distance between a drive line and a nearby qubit significantly impacts microwave crosstalk, according to new analysis of planar superconducting devices. The team’s work details how the amplitude of stray microwave fields diminishes with distance, suggesting that maximizing separation between drive lines and cross-coupled qubits is a viable mitigation strategy. Modeling the crosstalk at 5 GHz as a function of distance, they found that the combined capacitive and package-mediated crosstalk forms the basis of the proximity-induced crosstalk.

The simulations demonstrate a clear relationship between physical spacing and signal interference, providing quantitative data for future device designs. Specifically, the analysis shows that reducing the cross-capacitance, $C_{c r o s s}$ , from 2.2 fF to 0.6 fF is expected to result in an 11 dB reduction in crosstalk contribution from the air bridge crossover. This reduction in capacitance, the team suggests, could be achieved through advanced fabrication techniques like gray scale e-beam lithography.

The team’s calculations, based on equations combining multiple parameters, allow for the prediction of crosstalk levels based on device geometry. The work establishes a foundation for investigating even weaker crosstalk mechanisms.

Crossover-Induced Crosstalk via Air Bridge Structures

While air bridges are commonly used to isolate signal pathways, the research demonstrates that a drive line crossing these couplers introduces a significant capacitive coupling, potentially limiting the scalability of quantum processors. This crossover-induced crosstalk arises from the cross-capacitance, $C_{c r o s s}$ created between the drive line and the coupler. Measured crosstalk data aligns with the team’s physical models across a range of design parameters, validating their approach.

The researchers fabricated test structures with varying $C_{c, q b}$ and $C_{c r o s s}$ values to quantify the relationship between geometry and crosstalk. These experiments showed that crossover-induced crosstalk, despite being a second-order effect can be particularly prominent in planar devices due to the considerable cross-capacitance and large capacitor pads employed. The work demonstrates that accurately modeling microwave crosstalk requires accounting for both the proximate drive line and the package geometry, as both contribute to the overall signal interference. The study further establishes that reducing $C_{c r o s s}$ offers a direct path to mitigating crosstalk.

Modeling Microwave Crosstalk in Superconducting Devices

Researchers identified this configuration as a key contributor to unwanted signal transfer, a challenge hindering the scaling of quantum computing architectures. The team’s analysis moves beyond simply measuring crosstalk to developing quantitative physical models capable of predicting its magnitude for specific device designs. This predictive capability stems from a detailed investigation into two structures particularly prone to generating strong crosstalk. Beyond drive lines near qubits, the study highlights air bridge qubit-qubit couplers as hotspots when intersected by drive lines.

These bridges, intended to minimize interference, unexpectedly become pathways for microwave signals to couple between lines and qubits. The team’s models account for both proximity-induced crosstalk and this crossover-induced effect, offering a more complete picture of signal leakage. Accurate modeling, the researchers emphasize, requires considering the entire device package geometry.

In devices where a drive line crosses a qubit-qubit coupler with an air bridge, microwave signals can couple to the qubits connected by the coupler, adding to the crosstalk already present from nearby drive lines. The measured crosstalk, they found, aligns with calculations that incorporate all contributing factors. This approach differs from previous characterizations that often focused on isolated effects. The team’s work extends beyond identifying problematic structures to providing a framework for mitigating crosstalk.

This suggests a path toward designing layouts that minimize unwanted interactions between qubits, a critical step toward building larger, more stable quantum processors. The research also confirms that inter-line coupling is negligible when drive lines are spaced more than ten times their width, a finding validated through numerical simulations.

This allows designers to focus on the more dominant crosstalk channels, proximity and crossovers, without expending resources on mitigating less significant effects. This emphasis on the complete system, rather than isolated components, emphasises the complexity of managing signal integrity in superconducting quantum devices and the importance of comprehensive modeling for future advancements.

Quantifying Crosstalk Mechanisms with Device Fabrication

The research demonstrates that stray fields coupled into the sample package volume decay more slowly with distance than capacitive crosstalk, posing challenges for planar device scaling while maintaining acceptable error rates. This package-mediated effect complicates efforts to confine drive fields and minimize unwanted signal transfer between qubits. The team’s models quantitatively predict crosstalk based solely on device layout and sample package design, a departure from previous characterizations.

Strategies for suppressing microwave crosstalk include differential qubits and air-bridge crossovers with smaller footprints, the researchers found. Advanced fabrication techniques, such as through-silicon vias, inter-chip metallic bumps and modular devices using separate silicon pieces are expected to further reduce crosstalk by improving drive field confinement. These approaches address the challenge of scaling planar devices while minimizing spurious interactions, a critical step toward building larger, more reliable quantum systems.

The models developed account for the complete system, including the sample package geometry, which is important for accurately modeling microwave crosstalk. This contrasts with earlier studies that often focused on isolated components, potentially overlooking significant crosstalk channels. The team’s findings provide practical guidance for designing low-crosstalk device layouts and offer a basis for exploring even more subtle crosstalk mechanisms in future research.

Mitigation Strategies for Low-Crosstalk Architectures

Strategies to suppress microwave crosstalk center on both device geometry and fabrication techniques, as detailed in recent work. Maximizing the separation between drive lines and qubits represents a direct approach to reducing proximity-induced crosstalk, with simulations demonstrating crosstalk reduction as distance increases. Differential qubits offer an alternative architecture to mitigate crosstalk, making qubits susceptible only to gradients of spurious fields rather than their amplitude. This approach bypasses the limitations imposed by proximity-induced crosstalk, offering a pathway to more robust control.

Engineering the length of coplanar waveguide segments can create destructive interference between proximity-induced crosstalk and that stemming from crossovers, effectively canceling out the interference at specific drive line frequencies. Asymmetric qubit-qubit coupler pads can also suppress crosstalk-induced errors, particularly for qubits operating closer to the drive line’s frequency. Advanced fabrication methods are also important for minimizing unwanted signal interactions.

The research demonstrates that air-bridge crossovers, while often employed to avoid interference, can become crosstalk hotspots if not carefully designed. Markers in the study highlight that removing metal and optimizing the footprint of air bridges can significantly reduce crossover-induced crosstalk, denoted as X_cross.

The models developed provide a physically grounded and quantitative framework for understanding microwave crosstalk in planar superconducting devices, offering practical guidance for device layouts. “Based on these insights, we have discussed design strategies for suppressing the identified crosstalk mechanisms,” the researchers state, indicating the potential for further refinement and optimization of quantum circuit designs.

👉 More information
🗞 Microwave crosstalk in planar superconducting quantum devices
✍️ Yongxin Song, Dominic Hagmann, Kieran Dalton, Felix Henrich, Felix Wagner, Mohsen Bahrami Panah, Marek Pechal and Andreas Wallraff
🧠 DOI: http://link.aps.org/doi/10.1103/fdd3-sqxm

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