{"id":"9f16b42b-c240-40aa-a596-2051a8b7c82d","arxiv_id":"2603.28526","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"A simulated scheme with synchronously tuned double-transmon couplers at both ends of a 25-cm cable is predicted to enable a high-fidelity remote controlled-Z gate between fixed-frequency qubits, with tunable nonlocal ZZ coupling.","lead":"This paper proposes connecting two fixed-frequency qubits in separate packages with a 25-cm cable and twin 'double-transmon' couplers, letting the cable work as a switchable two-qubit gate instead of just a signal pipe. Numerical simulations predict a controlled-Z gate fidelity above 99.99% with an interaction on/off ratio above one million.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Cable truncation to two modes without a convergence check is the load-bearing weakness: unmodeled nearby modes or cable loss could lift the idle ZZ and lower the reported 99.99% coherent fidelity.","rationale":"The reader's weakest_assumption correctly identifies the two-mode cable truncation and the neglect of loss/parasitics as the load-bearing issue. I agree because this assumption threatens both headline numbers: the on/off ratio depends on the idle ZZ being ~10^-5 MHz, and the coherent fidelity is computed in a Hilbert space that excludes nearby modes which could provide additional leakage or dispersive shifts. The open-system omission is a real supporting concern, but it is secondary because the abstract at least acknowledges the coherent qualifier and mentions decoherence qualitatively; the truncation is unaddressed. A convergence test with modes m=9-12 is the natural single check that would settle the matter. My verdict remains CONDITIONAL, the same as the reader's, so no verdict change is needed.","tokens_in":11523,"tokens_out":9183,"duration_ms":101308,"concrete_test":"Using the Table S1 parameters, repeat the static ZZ landscape and the optimized CZ pulse simulation with four retained cable modes (m=9,10,11,12), scaling couplings as sqrt(10/m) with alternating parity signs, and recompute the idle ZZ at Φ≈0.3 and the coherent gate fidelity. If the idle ZZ rises above 10^-4 MHz or the coherent fidelity drops by more than 10^-4 relative to the two-mode result, the two-mode truncation is invalid and the on/off ratio / fidelity claims must be revised.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central quantitative claims—on/off ratio >10^6 and coherent CZ fidelity 99.99%—rest on a cable model truncated to two harmonic modes (m=10,11), with couplings assigned as J46=+25 MHz and J47=−25 MHz. The paper states, 'we truncate this spectrum and explicitly retain the two discrete modes (specifically, m=10 and m=11) situated closest to the qubit resonance frequencies,' but it provides no convergence test for the neighboring modes m=9 and m=12, which are only ~440 MHz away and have comparable 1/√m coupling amplitudes. Because the idle point Φ≈0.3 is identified in a subsystem containing only modes 10 and 11, there is no evidence that residual ZZ from other cable modes cancels at that flux; the deep-blue 10^-5 MHz region in Fig. 3(b) could be a truncation artifact. The same truncation also removes cable photon loss and connector parasitics, even though the abstract itself says photon loss in retained cable modes is 'non-negligible' in the open-system simulations. These omissions are not internally inconsistent, but they leave the headline on/off ratio and fidelity without a demonstrated margin. Moreover, the 99.99% figure is a closed-system, leakage-adjusted number, while the main-text conclusion calls it 'average gate fidelity' without the coherent qualifier used in the abstract.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript proposes a modular superconducting architecture in which two fixed-frequency transmon qubits in separate packages are connected through a 25-cm coaxial cable, with each qubit coupled to the cable via a double-transmon coupler (DTC). By synchronously flux-biasing the two DTCs, the authors claim to activate a tunable nonlocal ZZ interaction on demand while suppressing residual static coupling, yielding an on/off ratio exceeding 10^6. Using a circuit Hamiltonian truncated to two cable modes (m=10,11) and QuTiP time evolution, they report a remote controlled-Z gate with 99.99% average gate fidelity after optimizing a truncated Fourier flux pulse. The abstract additionally refers to open-system simulations showing qubit decoherence and cable photon loss contributions, but these results do not appear in the visible main text.","tokens_in":11964,"tokens_out":2427,"duration_ms":26183,"significance":"If the reported performance holds, this would be a valuable primitive for modular superconducting processors using fixed-frequency qubits: it avoids state-transfer overhead, retains fixed-frequency qubit advantages, and provides a gate-native interconnect with simulated fidelity above common error-correction thresholds. The work uses the full circuit Hamiltonian rather than a simplified effective model, and the time-evolution solver is a standard, reproducible tool. The strengths are the architectural concept, the synchronous dual-DTC control scheme, and the detailed numerical approach to pulse optimization. However, the headline quantitative claims—on/off ratio >10^6 and 99.99% fidelity—rest on a two-mode cable truncation that is not convergence-tested, and the abstract promises open-system results that are absent from the main text. These issues must be addressed before the claims can be regarded as demonstrated.","major_comments":[{"comment":"The cable is truncated to two harmonic modes m=10 and m=11 with couplings J46=+25 MHz and J47=-25 MHz, and no convergence check is provided for neighboring modes m=9 and m=12, which lie only ~440 MHz away and have comparable 1/sqrt(m) coupling amplitudes. The idle-point ZZ suppression in Fig. 3(b) is computed within a subsystem containing only modes 10 and 11; omitted modes could contribute residual ZZ at the 10^-5 MHz level or higher, directly affecting the on/off ratio claim. Please provide a quantitative justification: e.g., include the nearest omitted modes in the spectrum and ZZ extraction, estimate their perturbative contribution, or argue from a symmetry that their cancellation is robust. This is load-bearing for the central claims.","section":"Distributed Circuit Architecture (p.3, cable truncation)"},{"comment":"The abstract states that 'Open-system simulations further indicate that... endpoint-qubit decoherence is the largest contribution... photon loss in the retained cable modes remains smaller but non-negligible', but the main text contains no Lindblad master equation, no noise parameters, and no open-system fidelity results. The only fidelity reported in the main text is a closed-system coherent fidelity extracted from the projected unitary, yet the conclusion calls it 'average gate fidelity'. This conflates two different quantities. Either include the open-system analysis with the model and parameters, or clearly and consistently label the 99.99% as a coherent, closed-system fidelity and avoid the unsubstantiated open-system claims in the abstract and conclusion.","section":"Abstract vs. Performance of the Remote CZ Gate (p.5-6)"},{"comment":"The average gate fidelity is computed from the actual evolution operator projected onto the computational subspace. If leakage is non-zero, the projected map is not trace-preserving and Eq. (7) can overestimate the process fidelity unless appropriately renormalized or supplemented with a leakage term. The paper reports leakage below 10^-4, but the formula as written does not show how leakage is included in the 99.99% number. Please specify how the projected U is normalized and whether leakage is treated as infidelity in Eq. (7), or use a process fidelity definition that includes leakage.","section":"Eq. (7), fidelity definition"}],"minor_comments":[{"comment":"Eq. (3) defines the cross-Kerr shift for a single cable mode but the text and Fig. 2 refer to two cable modes (Cb1, Cb2); clarify how the individual ζ values are extracted from the multi-mode spectrum. The Fig. 2 caption has a typo: 'Cp1A and while Cp1B'.","section":"Eq. (3) and Fig. 2 caption"},{"comment":"The colorbar in Fig. 3(b) spans values that appear inconsistent with the caption 'log10|ZZ|' and the quoted 10^-5 MHz idle value. Please check the scaling and labeling so that the on/off ratio is visually and numerically unambiguous.","section":"Fig. 3(b), color scale"},{"comment":"In the flux waveform, Φ_f is described as 'modulation amplitude' but the formula uses it as the final target flux. Define Φ_f explicitly and state the initial/final flux values used in the optimization.","section":"Eq. (6), pulse definition"},{"comment":"The gate duration T=350 ns is presented as fixed. Is this an optimized parameter or an imposed duration? A sensitivity analysis of fidelity versus T would strengthen the claim that the pulse is robust.","section":"Gate duration"},{"comment":"Several references to the experimental literature are appropriate, but the paper would benefit from a brief statement distinguishing the present scheme from the long-range ZZ interaction of Ref. [43] and the resonator-induced phase of Ref. [42], particularly regarding the role of the DTCs in achieving high on/off ratio.","section":"References and notation"}],"recommendation":"major_revision","confidential_remarks":"The central idea is promising, and the closed-system simulations appear internally consistent. The main obstacles are the lack of convergence testing for the two-mode cable truncation and the discrepancy between the abstract's open-system claims and the main-text coherent-only results. These are fixable within the manuscript's scope but need to be addressed before publication. The paper also should not describe the coherent fidelity as 'average fidelity' without qualification."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"This is a proposal for remote CZ gates between fixed-frequency qubits using two double-transmon couplers, one at each end of a 25-cm cable. The genuinely new piece is synchronously flux-tuning the two DTCs to switch a nonlocal ZZ interaction on and off. That is a reasonable extension of the DTC work and the remote-coupling experiments they build on, and it is not in the cited prior literature.\n\nThe closed-system simulation is done honestly: they use the full circuit Hamiltonian via QuTiP, track eigenstates through avoided crossings, optimize the pulse to suppress leakage and phase error, and report swap and leakage errors below 1e-4. Within their model, the idle point at flux ~0.3 suppresses ZZ to 1e-5 MHz, and the working point gives ~10 MHz, which is where the >1e6 on/off ratio comes from. The gate duration is 350 ns and the coherent fidelity is 99.99%.\n\nThe main soft spot is exactly where the stress-test points: the cable is truncated to two modes (m=10,11), with couplings assigned as J46=+25 MHz and J47=-25 MHz. The nearest neighboring modes, m=9 and m=12, are only ~440 MHz away and have comparable coupling strength. There is no convergence check. That means the idle-point suppression could be a truncation artifact. If nearby modes contribute even a small residual ZZ, the idling picture and the 1e6 on/off ratio change. This is not a hidden flaw—they state the truncation explicitly—but it is load-bearing for the central quantitative claims.\n\nTwo smaller issues: the abstract mentions open-system simulations with a Markovian noise model, but the main text never shows those results; and the conclusion calls the fidelity \"average gate fidelity\" without the coherent qualifier used in the abstract. The device parameters are only in Table S1, which is not in the visible text.\n\nThe paper is a solid simulation-based proposal with a clear geometry and honest methods. The missing convergence analysis and open-system details are fixable, and the idea is worth taking seriously.\n\nWho should read this: circuit QED people interested in modular architectures and remote entangling gates. I would bring it to a reading group to discuss the truncation question.\n\nMy recommendation: send it to peer review. A good referee should push for a convergence test on the cable modes and the full open-system results before the headline numbers are accepted.","headline":"A plausible simulation-based proposal for remote CZ gates that hinges on an unverified two-mode truncation of the cable; worth refereeing, but the headline numbers need a convergence check.","tokens_in":12426,"tokens_out":1929,"would_cite":false,"duration_ms":21626,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"This paper proposes that two synchronously flux-tuned double-transmon couplers bridged by a 25 cm coaxial cable create a switchable nonlocal ZZ interaction between remote fixed-frequency transmon qubits, enabling a simulated remote CZ gate","keywords":["modular superconducting processors","fixed-frequency transmon qubits","double-transmon coupler","nonlocal ZZ interaction","remote controlled-Z gate","coaxial cable interconnect","tunable coupling","circuit QED"],"falsifier":"Run the simulation with the complete cable mode ladder and a lossy transmission-line model, or measure the qubit–cable ZZ on a single package across flux at the idle point; if leakage or dephasing from neglected modes, cable loss, or connector reflections appears at the 10^-4 level during a 350 ns gate, or if the measured idle ZZ is orders of magnitude above 10^-5 MHz, the predicted 99.99% fidelity and 10^6 on/off ratio are falsified.","tokens_in":11452,"feed_emoji":"🔗","tokens_out":7775,"duration_ms":79939,"temperature":0.7,"pith_summary":"The paper aims to show that two fixed-frequency transmon qubits in separate packages can be entangled directly through a 25 cm superconducting cable, without a state-transfer protocol, by synchronously tuning a double-transmon coupler at each end. Each coupler mediates a flux-controlled cross-Kerr interaction between its local qubit and a cable mode; tuned together, the two couplers create a long-range ZZ coupling between distant qubits that can be switched from roughly 10^-5 MHz to above 10 MHz. The claimed payoff is a remote controlled-Z gate with a simulated 99.99% average closed-system fidelity, an on/off ratio exceeding 10^6, and no tunability or flux-noise burden on the fixed-frequency qubits themselves. If correct, this would give modular superconducting processors a gate-native interconnect and remove a significant scaling bottleneck.","feed_headline":"Remote CZ gate hits 99.99% fidelity across a 25 cm cable","feed_subtitle":"Synchronized double-transmon couplers turn a coaxial link into a switchable entangling channel for modular qubit chips.","key_machinery":"The load-bearing component is the double-transmon coupler (DTC): a superconducting loop containing two transmon modes and a coupling Josephson junction, threaded by external flux. Flux modulates both the coupler's nonlinearity and its coupling rate, so the same device can idle (qubit–cable ZZ ≈ 10^-5 MHz at flux 0.3) or activate (≳ 10 MHz at flux 0.5). The effective interaction strength is computed from the energy combination ζ = E1100 − E0100 − E1000 + E0000, and the global remote coupling emerges when identical DTCs at both cable ends are tuned synchronously. A second ingredient is the spatial parity of the retained cable modes: mode m=10 couples with the same sign to both ends, while mode","core_discovery":"The central claim is that a double-transmon coupler (DTC)—a flux-threaded superconducting loop containing two transmon modes and a coupling Josephson junction—can serve as a gain-controlled tap between a fixed-frequency qubit and a multimode cable. In the local subsystem, the qubit–cable cross-Kerr strength is extracted from the energy combination ζ = E1100 − E0100 − E1000 + E0000; tuning the DTC flux near 0.3 (in units of the flux quantum) suppresses this coupling to about 10^-5 MHz, while flux near 0.5 maximizes it. Because structurally identical DTCs sit at opposite ends of the same cable, synchronously biasing both converts the two local qubit–cable couplings into a nonlocal qubit–qubit","pith_inferences":["A natural next calculation is to add the full cable mode ladder and a lossy transmission-line model; if the neglected modes remain far from the qubit frequencies the scheme should survive, but the on/off ratio will be bounded by how well those modes are suppressed.","The same DTC-pair mechanism could be extended to a network of modules by adding switching or multiple couplers along the cable, turning the remote gate into a routing primitive for distributed quantum error correction.","Because the cable's free spectral range (about 440 MHz) and mode parities shift with cable length, the scheme may be adaptable to other inter-module distances, though each distance would require fresh optimization of flux points and mode assignment.","A single-module experiment measuring ζ versus flux on one DTC plus the cable would already test the core contrast claim before building a full two-module device."],"forward_implications":["Remote CZ gates can be executed directly between fixed-frequency qubits in separate packages, removing the overhead of converting a state-transfer protocol into a two-qubit gate.","The nonlocal ZZ interaction can be switched between residual coupling on the order of 10^-5 MHz and active coupling above 10 MHz, so the same cable link can idle almost silently and then perform a 350 ns entangling gate on demand.","The tunability lives entirely in the couplers, so the fixed-frequency qubits keep their coherence and control advantages while the architecture gains modular connectivity.","Under the paper's open-system noise model, endpoint-qubit decoherence is the largest infidelity source, with photon loss in the retained cable modes smaller but non-negligible—pointing future work at qubit coherence and cable loss.","The mode-parity sign structure (mode m=11 couples with opposite signs at the two ends) is part of what shapes the interaction, so gate performance is tied to the cable's mode spectrum and length."],"fun_headline_variants":["Tunable nonlocal ZZ interaction yields 99.99% remote CZ gate","Double-transmon couplers enable remote qubit gate at 99.99% fidelity","Coaxial cable turned into switchable entangling link for qubits","Synchronized couplers deliver 99.99% fidelity remote CZ over 25 cm"],"cache_read_input_tokens":2304,"weakest_assumption_plain":"The load-bearing premise, stated in the Distributed Circuit Architecture section, is that the 25 cm cable can be modeled as a lossless resonator with its spectrum truncated to the two modes (m=10 and m=11) closest to the qubit frequencies, and that all other cable modes, cable loss, connector parasitics, and impedance mismatches contribute below roughly 10^-4; if they leak or dephase at that level, the 99.99% fidelity and 10^6 on/off ratio would not hold.","fun_headline_variants_meta":{"raw":{"variants":["Tunable nonlocal ZZ interaction yields 99.99% remote CZ gate","Double-transmon couplers enable remote qubit gate at 99.99% fidelity","Coaxial cable turned into switchable entangling link for qubits","Synchronized couplers deliver 99.99% fidelity remote CZ over 25 cm"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.00027,"raw_usage":{"total_tokens":1482,"prompt_tokens":784,"completion_tokens":698,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":528,"completion_tokens_details":{"reasoning_tokens":607}},"tokens_in":528,"tokens_out":698,"duration_ms":6093,"temperature":1.0,"reasoning_tokens":607,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-02T17:06:05.327158+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Run the simulation with the complete cable mode ladder and a lossy transmission-line model, or measure the qubit–cable ZZ on a single package across flux at the idle point; if leakage or dephasing from neglected modes, cable loss, or connector reflections appears at the 10^-4 level during a 350 ns gate, or if the measured idle ZZ is orders of magnitude above 10^-5 MHz, the predicted 99.99% fidelity and 10^6 on/off ratio are falsified.","supporting_citations":[],"review_version":1}