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REVIEW 3 major objections 5 minor 2 cited by

Low Crosstalk in a Scalable Superconducting Quantum Lattice

T0 review · 3 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read A 4x4 fixed-frequency transmon lattice demonstrates that parasitic crosstalk between non-neighboring qubits is negligible, while engineered nearest-neighbor couplings remain intact.

desk verdict Solid 4x4 lattice hardware paper with a central low-crosstalk claim that is more qualitative than the text admits; worth refereeing. read the letter →

arxiv 2505.22276 v1 pith:COH2SU2D submitted 2025-05-28 quant-ph

Mohammed Alghadeer , Shuxiang Cao , Simone D Fasciati , Michele Piscitelli , Paul C. Gow , James C. Gates , Mustafa Bakr , Peter J. Leek This is my paper
classification quant-ph PACS 85.25.Cp03.67.Lx
keywords superconductingqubitstransmoncrosstalk3D-integratedcircuitQEDfixed-frequencyZZcouplingrandomizedbenchmarkingquantumlattice
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper reports a 4x4 lattice of 16 fixed-frequency transmon qubits built in a tileable, 3D-integrated circuit architecture, and its central claim is that parasitic crosstalk between non-neighboring qubits is negligible while the engineered nearest-neighbor couplings remain intact. The authors measure nearest-neighbor exchange couplings in the range of about 0.4 to 1.06 MHz through static ZZ shifts and AC-Stark anticrossings, and in diagonal qubit pairs they see no long-range avoided crossings, with observed frequency fluctuations of 5 to 39 kHz that they attribute to intrinsic qubit frequency jitter. Simultaneous single-qubit randomized benchmarking gives error-per-gate values comparable to individual benchmarking despite the always-on qubit-qubit coupling, indicating that correlated spectator errors stay suppressed. If correct, the result validates a route to larger low-crosstalk superconducting lattices without tunable couplers.

What carries the argument

The central object is the tileable unit cell of the 3D-integrated circuit-QED architecture: a fixed-frequency transmon on one side of a silicon chip, capacitively coupled through the substrate to a readout resonator on the opposite side, with nearest-neighbor exchange couplings $J_{i,j}$ provided by lithographic capacitive arms and spurious enclosure modes suppressed by off-chip inductive shunting. The argument is carried by two measurement tools: the static ZZ shift, related to $J$ by the formula $\zeta \approx -2J^2(\alpha_i+\alpha_j)/((\Delta_{ij}+\alpha_i)(\alpha_j-\Delta_{ij}))$, and AC-Stark-shift Ramsey anticrossing spectroscopy, in which one qubit is tuned through resonance with another and the exchange coupling is extracted from the avoided-crossing splitting. For non-nearest-neighbor pairs the same measurement is used as a crosstalk diagnostic: the absence of a resolvable anticrossing is the paper's evidence that parasitic couplings are negligible.

What would settle it

For a diagonal pair such as Q5-Q11, repeat the AC-Stark sweep with longer integration and higher drive power, extract the full anticrossing fit parameters, and report the minimum resolvable splitting; if any non-nearest-neighbor exchange coupling is resolved at the level of tens of kilohertz or above, the claim that long-range crosstalk is negligible is falsified.

Watch

Extended reading notes

Core claim

On the paper's own terms, the discovery is that residual parasitic interactions in a 16-qubit square lattice can be pushed below the level of intrinsic frequency noise while retaining strong, uniform nearest-neighbor couplings. Direct AC-Stark anticrossing measurements on diagonal pairs such as Q6-Q2, Q6-Q10, Q5-Q11, Q8-Q10, and Q10-Q16 show no avoided crossings, and the authors interpret the observed frequency fluctuations (5 to 39 kHz) as evidence that long-range couplings are negligible. The nearest-neighbor couplings extracted from static ZZ shifts, ranging from 0.401 to 1.064 MHz, agree with those extracted from Stark-driven swap and anticrossing measurements, supporting the model that only designed couplings matter. Simultaneous randomized benchmarking errors are comparable to individual errors across the lattice, and two-qubit CZ gates are implemented via the siZZle technique with fidelities of 95.15% and 96.44% on two pairs, plus a three-qubit GHZ state with 83.88% fidelity. Together these results are offered as validation of a tileable 3D-integrated architecture in which off-chip inductive shunting suppresses enclosure-mediated crosstalk.

Load-bearing premise

The conclusion that parasitic crosstalk is negligible rests on treating the absence of visible level repulsions in a small set of diagonal qubit pairs as proof that all non-nearest-neighbor couplings are small, even though the reported numbers are frequency jitter, not direct upper limits on those couplings.

Editorial extensions

If this is right

  • Larger lattices can be formed by tiling the same unit cell, with the expectation that non-nearest-neighbor couplings remain near the intrinsic frequency-fluctuation floor rather than growing with qubit count.
  • Simultaneous single-qubit operations do not need extra crosstalk cancellation because simultaneous randomized benchmarking errors are comparable to individual errors despite the always-on ZZ coupling.
  • Fixed-frequency transmons without tunable couplers can still implement entangling gates, as demonstrated by CZ fidelities around 95 to 96 percent obtained with the siZZle technique on two qubit pairs.
  • Low qubit-frequency spreads of 0.5 percent and 1.5 percent for the two alternating groups, and a 0.6 percent anharmonicity spread, are achievable without post-fabrication junction annealing, simplifying fabrication targeting.
  • Correlated errors from spectator qubits are small enough that independent-error assumptions in quantum error correction become more plausible in this architecture.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A rigorous upper bound on parasitic coupling would follow from publishing the full anticrossing fit parameters ($A$, $B$, $C$) and the minimum resolvable splitting; that conversion of null observations into a quantitative crosstalk bound is an open next step.
  • A direct scaling test follows from the tileability claim: build an 8x8 version and check that diagonal and next-nearest-neighbor ZZ shifts remain below the noise floor.
  • The static crosstalk characterization could be extended to the driven regime, testing whether simultaneous Stark or gate drives activate parasitic couplings that are invisible at rest.
  • Comparing devices with and without the off-chip inductive shunts would clarify how much of the suppression comes from that design choice rather than from the lattice geometry itself.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 5 minor

Summary. This manuscript reports a 4x4 lattice of 16 fixed-frequency transmon qubits in a 3D-integrated, tileable circuit architecture with off-chip inductive shunting. It presents measured device parameters, coherence times, nearest-neighbor couplings extracted both from static ZZ shifts and from AC-Stark anticrossings, non-nearest-neighbor crosstalk measurements, individual and simultaneous single-qubit randomized benchmarking, and siZZle-based CZ gates with Bell and GHZ state demonstrations. The central claim is that parasitic long-range crosstalk is negligible, so that always-on nearest-neighbor coupling does not degrade simultaneous single-qubit gate errors.

Significance. If the low-crosstalk claim is established, the result would be significant for scalable fixed-frequency transmon lattices: it would show that a tileable 3D-integrated package can suppress enclosure-mode and long-range couplings without tunable couplers. The paper's strengths include the independent cross-check of nearest-neighbor J from static ZZ and AC-Stark measurements, detailed fabrication and coherence statistics, and an explicit comparison of individual versus simultaneous RB. The main weakness is that the central negative crosstalk result is presented as a set of frequency-fluctuation standard deviations rather than as calibrated upper bounds on parasitic couplings, and the simultaneous RB was performed on four-qubit sets rather than on the full 16-qubit device.

major comments (3)
  1. [Crosstalk Characterization, Table 2 and Fig. 5] The statement 'we observe no long-range couplings across the lattice' is not supported by a quantitative upper bound. Table 2 lists only five non-nearest-neighbor pairs and reports 'Std-Dev' values of 5-39 kHz, but a standard deviation of Ramsey frequency fluctuations is not an upper bound on the parasitic exchange coupling eJ. In the AC-Stark anticrossing measurement a nonzero eJ produces an avoided crossing with splitting of order 2eJ at resonance, while away from resonance the frequency shift scales as eJ^2/Delta; without the A, B, C fit parameters or a linewidth/sensitivity analysis, the null traces cannot exclude eJ values comparable to the nearest-neighbor couplings (0.401-1.064 MHz). For example, for Q6-Q10 with Delta=7.6 MHz, an eJ of about 0.5 MHz would produce a ZZ shift of roughly 5 kHz, below the reported 12 kHz standard deviation. Five pairs out of 96 non-nearest-neighbor pairs also do not support a global 'across the lattice' statement.
  2. [Single-Qubit Gate Errors, Tables 5 and 6] The claim that simultaneous single-qubit gate errors are comparable to individual errors is only partially supported. The simultaneous RB experiments were performed on four-qubit sets, so spectator crosstalk from the other twelve qubits was not tested; this should be stated explicitly when the abstract says 'simultaneous single-qubit gate errors across the device.' In addition, several qubits show simultaneous EPGs 1.7-2.1 times their individual values (Q4: 6.62e-3 to 1.11e-2; Q6: 1.56e-3 to 2.87e-3; Q14: 7.43e-4 to 1.31e-3; Q16: 7.42e-4 to 1.56e-3), while the text only flags Q3-Q4. The absolute errors are low, but the 'comparable' claim needs either a quantitative threshold or a discussion of these outliers.
  3. [Conclusion] The conclusion that 'inter-qubit couplings remain localized, with negligible long-range parasitic interactions' is stronger than the evidence in Table 2. The manuscript should either add a sensitivity analysis that converts each null measurement into an upper bound on eJ for the probed pair, or restrict the claim to the five measured pairs and to the frequency range probed by the AC-Stark scans.
minor comments (5)
  1. [Methodology, Eq. (3)] Equation (3) refers to δ_i and δ_j as the anharmonicities, but these symbols are not defined; the text should use α_i and α_j consistently.
  2. [Basic Device Parameters] The phrase 'measured with very low frequency spreads of0.5% and 1.5% MHz' is not grammatical; the percentages and the MHz unit should be separated or corrected.
  3. [Tables 1 and 2] Tables 1 and 2 have inconsistent column ordering for the qubit frequencies: for example, the Q6-Q2 row in Table 2 lists 4795.6 MHz and 4824.8 MHz, which appears to reverse the Q6 and Q2 frequencies relative to Table 4; please check the table formatting.
  4. [Throughout] There are several typos, including 'receptively' (Basic Device Parameters), 'oberving' (Two-Qubit Interactions), 'initail' and 'interctaion' (Supplementary Materials), and 'stark shifted' in the caption of Fig. 5; these should be corrected.
  5. [Single-Qubit Gate Errors] The RB description 'each experiment was performed with 16 different Clifford sequences with total sequence length of 1000 gates and with each sequence repeated for N=10 distinct Clifford gates' is confusing; please clarify the number of randomizations and how N relates to the reported EPG uncertainties.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the low-crosstalk conclusion rests on new multi-qubit measurements, and the Table 2 null result is a sensitivity limitation rather than a construction-level circular step.

full rationale

The central low-crosstalk claim is supported by new experimental data — static ZZ shifts, AC-Stark anticrossing traces, swap dynamics, and simultaneous randomized benchmarking — rather than by a parameter fitted to the target conclusion. Nearest-neighbour exchange couplings are extracted in two independent ways: from static ZZ shifts via Eq. 3 and from AC-Stark anticrossing fits, with the comparison shown in Table 1. This is a consistency check between two measurements of the same physical quantity, not a self-definitional reduction. The off-chip inductive-shunting architecture is motivated by prior same-group references (59, 60), but the present 16-qubit device is a new measurement and the cited prior work is externally testable experimental and modelling evidence; the argument does not reduce to an unverified self-citation. Table 2 does report frequency-fluctuation standard deviations rather than explicit upper bounds on parasitic exchange eJ, which is a genuine sensitivity and statistical limitation of the null crosstalk result, but it is not an instance of the conclusion being equivalent to its input by construction. No fitted parameter is renamed as a prediction, no uniqueness theorem is imported solely from the authors' prior work, and no ansatz is smuggled in through a self-citation. The paper therefore exhibits no significant circularity.

Assumptions & free parameters 1 free parameters · 4 assumptions · 0 invented entities

The central claims rest on standard transmon models plus several unverified measurement-sensitivity assumptions. No new physical entities are introduced; off-chip inductive shunting is a packaging feature, not a new particle or mediator. The main burden is the assumption that sparse null measurements can stand for the absence of crosstalk across the whole lattice.

free parameters (1)
  • A, B, C in anticrossing fit model = not reported
    The model Delta_f_AC = A J^2 / (B Delta + C) is used to extract J and to support the null crosstalk observations. Without numerical values, the mapping from measured frequency shifts to J cannot be independently checked.
assumptions (4)
  • domain assumption Equation 3, zeta approximately -2 J^2 (alpha_i + alpha_j) / ((Delta_ij + alpha_i)(alpha_j - Delta_ij)), relates static ZZ shift to exchange coupling J.
    Used to convert measured ZZ shifts into J values in Fig. 4. The formula is from cited literature (refs 66, 67) and is cross-checked with direct AC-Stark anticrossings.
  • domain assumption The absence of a visible avoided crossing in a Stark sweep implies the residual coupling eJ is negligible.
    Central to the low-crosstalk claim. Table 2 lists frequency fluctuation standard deviations (5 to 39 kHz) rather than an upper bound on eJ, so the measurement may not resolve small couplings.
  • domain assumption Off-chip inductive shunting suppresses enclosure-mediated parasitic modes.
    The paper does not compare devices with and without shunting; it relies on prior modeling by the same group (refs 59, 60).
  • domain assumption A single 16-qubit device is representative of the scalable tileable architecture.
    No yield statistics or multi-device reproducibility are reported. Scalability is inferred from the design principles rather than demonstrated statistically.

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Cite this review

Pith. "Pith review of Low Crosstalk in a Scalable Superconducting Quantum Lattice." pith.science (2026). https://pith.science/paper/COH2SU2D

@misc{pith2026250522276,
  author       = {Pith},
  title        = {Pith review of: Low Crosstalk in a Scalable Superconducting Quantum Lattice},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/COH2SU2D}},
  note         = {Machine review of arXiv:2505.22276}
}
read the original abstract

Superconducting quantum circuits are a key platform for advancing quantum information processing and simulation. Scaling efforts currently encounter challenges such as Josephson-junction fabrication yield, design frequency targeting, and crosstalk arising both from spurious microwave modes and intrinsic interactions between qubits. We demonstrate a scalable 4x4 square lattice with low crosstalk, comprising 16 fixed-frequency transmon qubits with nearest-neighbor capacitive coupling that is implemented in a tileable, 3D-integrated circuit architecture with off-chip inductive shunting to mitigate spurious enclosure modes. We report on the design and comprehensive characterization, and show that our implementation achieves targeted device parameters with very low frequency spreads and simultaneous single-qubit gate errors across the device. Our results provide a promising pathway toward a scalable, low-crosstalk superconducting lattice topology with high qubit connectivity for quantum error correction and simulation.

Figures

Figures reproduced from arXiv: 2505.22276 by the authors.

Figure 1
Figure 1. Design Schematics and images of the fabricated device including enclosure packaging. (a) 3D-integrated coaxial cQED architecture features a transmon qubit with a Josephson junction (JJ) and a readout resonator on the opposite side of the chip, enabling targeted couplings to off-chip coaxial ports for control and readout. (b) Extended design presented in this work consisting of 16 coupled qubits with capacitive arms,… view at source ↗
Figure 2
Figure 2. Device parameters. (a) Shows distinct resonator frequencies ranging from ~ 8.6 to 10 GHz, and (b) shows qubits with two-distinct, alternating, frequency pattern in a range between 4.8 and 4.9 GHz with most qubits operating in the straddling regime. This ensures that most qubits remain within a regime where their interactions can be effectively controlled for tuning up two-qubit interactions. Results and Discussion Q… view at source ↗
Figure 3
Figure 3. Relaxation and coherence times. (a) Energy relaxation times ⟨T1⟩400 averaged from measured traces following an exponential decay, fitted to S(∆t) = a+be−∆t/T1 . (b) Ramsey coherence times ⟨T2R⟩400 averaged from measured traces following a decaying oscillation, fitted to S(∆t) = a+bcos(2π∆f∆t +φ)e −∆t/T2R . (c) Spin-echo coherence times ⟨T2E⟩400 averaged from measured traces following an exponential decay, fitted to … view at source ↗
Figures from the paper (10 more)
Figure 4
Figure 4. Figure 4: Design of exchange coupling rate J and nearest-neighbor ZZ and J Measurements. (a) Modeling of the coupling J as a function of capacitive arms overlap obtained by combining a simple impedance formula66 with high-frequency structure simulator (HFSS) using terminal simul…
Figure 5
Figure 5. Figure 5: Nearest-neighbor coupling Ji, j and long-range crosstalk Jei, j . Both direct (a) and indirect (b) couplings are measured using AC Stark Ramsey between each pair of qubit in the three-qubit example. Each qubit is stark shifted with an additional AC tone until it is in …
Figure 6
Figure 6. Figure 6: Nearest-neighbor coupling Ji, j . Set of three-qubit in the lattice is considered to verify the measured coupling values through observing both dynamics of swap population (a) and anticrossing from AC Stark-shift Ramsey (b). In (a) one qubit is initially excited and th…
Figure 7
Figure 7. Figure 7: Single-qubit gate errors. (a) Individual and (b) simultaneous error-per physical gate errors (EPG) with median values across the device compared to coherence-limit physical gate errors (CLG) in (c). Measurements of EPG is done by randomized benchmarking on XY Clifford …
Figure 8
Figure 8. Figure 8: Two-qubit interactions and CZ gates Calibration. (a) Pulse sequence implemented to tune up two-qubit ZZ interactions based on the siZZle technique. Two-qubit ZZ-induced phase accumulation on target qubit for calibrating a CZ gate on Q2 (control) and Q7 (target) as func…
Figure 9
Figure 9. Figure 9: Schematic diagrams outlining the experimental setup and dilution refrigerator with wiring and cryogenic components. (a) Basic measurements for qubits and resonators are done using a Vector Network Analyzer (VNA) using the illustrated connections mainly for measuring re…
Figure 10
Figure 10. Figure 10: Additional Device Parameters. (a) Qint is internal Q factor of each resonator, (b) κext is external decay rate of each resonator, (c) α is the anharmonicity of each qubit, and (d) χ is the dispersive shift of each qubit. Single-Qubit Gate Calibration Randomized benchm…
Figure 11
Figure 11. Figure 11: Single-qubit gate fidelities. (a) Individual and (b) simultaneous error-per physical gate fidelities FEPG with median values across the device compared to coherence-limit physical gate fidelities FCL in (c). Measurements of each gate fidelity is done by randomized ben…
Figure 12
Figure 12. Figure 12: Two-qubit interactions and CZ gate Calibration. (a) Two-dimensional parameter sweep of Stark drive frequency and amplitude, showing the response of the control qubit used to infer interaction stability. (b) Corresponding measurement on the target qubit, capturing the …
Figure 13
Figure 13. Figure 13: Two-qubit interactions and CZ gate Calibration. States of the control qubit Q2 during ZZ-induced phase accumulation on target qubit Q7 during ⟨X⟩ measurements in (a) and ⟨Y⟩ measurements in (b), for calibrating a CZ gate between Q2 and Q7. See the corresponding ⟨X⟩ an…

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Forward citations

Cited by 2 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. Connectivity-induced surface-loss penalty in superconducting qubit-coupler lattices

    quant-ph 2026-07 conditional novelty 6.0 of 10

    Embedding a transmon in a flip-chip qubit-coupler lattice raises surface dielectric loss by factors of 1.3 (two couplers) and 1.8 (four couplers) relative to an isolated qubit.

  2. Intrinsic Multi-Mode Interference for Passive Suppression of Purcell Decay in Superconducting Circuits

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    Breaking the symmetry of a transmon capacitor activates multi-mode interference that can suppress Purcell decay, shown analytically, in simulation, and in one four-qubit device.

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Pith tools

Reviewed August 7, 2026 · model on record in the stance chip above.