{"id":"1a396009-4e0b-47b5-8002-2c8f90c9b313","arxiv_id":"2607.00688","paper_version":1,"verdict":"UNVERDICTED","confidence":"LOW","novelty_score":5.0,"correctness_risk":"unknown","formal_verification":"none","parameter_count":0,"one_line_summary":"Leakage hopping persists at 0.8-10 MHz even when couplers cancel single-excitation exchange or ZZ interaction due to transmon nonlinearity; frequency detuning localizes it, with 1-4 MHz next-nearest-neighbor spread needed to suppress longer hops, and two passive removal units are proposed.","lead":"This paper studies how leakage excitations move between transmon qubits linked by tunable couplers using numerical and analytical methods. The results give concrete rates and design rules for localizing or removing leakage to reduce errors in superconducting quantum processors.","discovery_kind":"extension","skeptic_critique":{"model":"grok-4.3","headline":"No significant objection identified","rationale":"The reader's weakest assumption (accuracy of numerical/analytical methods under realistic parameters) directly matches the only potential soft spot; because the full text supplies no evidence that this assumption fails, the UNVERDICTED verdict and low confidence are left unchanged.","tokens_in":1730,"tokens_out":273,"duration_ms":18281,"concrete_test":"Reproduce the leakage hopping rates for the two-qubit plus coupler system using the same device parameters (anharmonicity, coupler tunability range, and detunings) stated in the paper; if the extracted rates fall outside the reported 0.8-10 MHz window under the same coupler bias points that null the exchange or ZZ term, the persistence claim is affected.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim—that leakage hopping persists at 0.8-10 MHz due to nonlinearity even after canceling exchange or ZZ, while detuning localizes it except for possible NNN channels—rests on standard numerical diagonalization/time evolution plus analytical effective-rate derivations applied to a transmon-plus-tunable-coupler Hamiltonian. No internal inconsistency, hidden approximation that would invalidate the quoted rate range, or unsupported step in the localization argument is apparent from the description of the methods and conclusions.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.3","summary":"The manuscript studies leakage dynamics in transmon qubits coupled by tunable couplers using numerical diagonalization and analytical effective-rate derivations. It claims that leakage hopping rates remain in the 0.8–10 MHz range even after the coupler is tuned to null the single-excitation exchange or the ZZ interaction, owing to transmon nonlinearity; that typical frequency detuning localizes leakage excitations except possibly via next-nearest-neighbor channels; that a 1–4 MHz spread among next-nearest-neighbor frequencies is required to suppress longer-range tunneling; and that two passive leakage-removal units (one coupler-plus-pumped-transmon, one junction-readout) can be realized with realistic parameters.","tokens_in":1797,"tokens_out":352,"duration_ms":18822,"significance":"If the quoted rate ranges and localization conditions are borne out by the calculations, the work supplies concrete, architecture-level guidance for mitigating correlated leakage errors in superconducting processors and for engineering dedicated removal pathways. The dual numerical-plus-analytical approach and the explicit mapping onto experimental parameter regimes constitute a practical strength.","major_comments":[],"minor_comments":[{"comment":"Abstract: the clause 'The next-nearest-neighbor transmons can be still be near-resonant' contains a duplicated 'be'; correct to 'can still be near-resonant'.","section":"Abstract"},{"comment":"The manuscript should state the Hilbert-space truncation level and convergence tests used for the leakage-rate calculations, even if only in a methods paragraph, to allow readers to assess the quoted 0.8–10 MHz window.","section":"Methods"}],"recommendation":"minor_revision","confidential_remarks":null},"author_rebuttal":{"model":"grok-4.3","summary":"We thank the referee for their positive assessment of our manuscript, accurate summary of the key results, and recommendation for minor revision. No major comments were raised in the report.","responses":[],"tokens_in":1290,"tokens_out":53,"duration_ms":13364,"standing_objections":[]},"desk_editor":{"model":"grok-4.3","letter":"The main thing to know is that leakage hopping rates in tunable-coupler transmons remain 0.8-10 MHz even when the coupler cancels single-excitation exchange or ZZ, because of the transmon nonlinearity. Frequency detuning localizes most leakage in typical regimes, but next-nearest-neighbor qubits can still couple unless their frequency spread is kept to 1-4 MHz. The paper also sketches two passive removal units, one using a tunable coupler plus pumped transmon and one using a junction readout.\n\nWhat is new is the quantitative mapping of these rates and localization conditions specifically for tunable-coupler architectures, plus the removal proposals tied to realistic parameters. Earlier leakage studies are extended here with both numerical diagonalization/time evolution and analytical effective-rate derivations on the full Hamiltonian. That combination gives concrete numbers rather than just qualitative warnings, which is useful for error budgeting.\n\nThe work is straightforward and addresses a real scaling concern. The methods are standard for this system and the stress-test found no internal inconsistencies or hidden approximations that would break the quoted rate range. The claims rest on the Hamiltonian and the nonlinearity term, not on circular fitting.\n\nSoft spots are limited. Everything is numerical and analytical, so experimental checks would help pin down the exact rates, but that is normal for this stage. The 0.8-10 MHz window is wide, which probably reflects parameter sweeps, and the 1-4 MHz NNN condition is specific but follows directly from the localization argument. No load-bearing flaws stand out.\n\nThis paper is for hardware groups and theorists working on superconducting processors who need leakage mobility numbers for architecture choices. Readers focused on error mitigation or multi-qubit design will find the rates and removal ideas directly applicable. It deserves a serious referee because the topic matters for scaling and the results are specific enough to test or use.","headline":"Leakage hopping stays at 0.8-10 MHz after coupler cancellation due to nonlinearity, but detuning localizes it except for NNN channels that need 1-4 MHz spread; two passive removal units are proposed.","tokens_in":2280,"tokens_out":465,"would_cite":true,"duration_ms":20169,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"grok-4.3","headline":"Leakage hopping rates persist at 0.8-10 MHz in transmons even after tunable couplers cancel exchange or ZZ interactions due to nonlinearity.","keywords":["leakage mobility","transmon qubits","tunable couplers","leakage removal","superconducting circuits","quantum error mitigation","nonlinearity effects"],"falsifier":"Measure the actual leakage hopping rate between two transmons whose coupler is tuned to cancel both exchange and ZZ terms; a result outside 0.8-10 MHz would contradict the persistence claim.","tokens_in":2642,"feed_emoji":"⚛","tokens_out":733,"duration_ms":15164,"temperature":0.7,"pith_summary":"The paper examines how leakage excitations move between transmon qubits connected by tunable couplers. It shows that canceling the usual single-excitation exchange or ZZ coupling does not stop leakage hopping, which remains in the 0.8-10 MHz range because of the intrinsic nonlinearity of the transmons. Frequency detuning between qubits can localize the leakage in typical operating conditions, though next-nearest neighbors may still allow tunneling unless their frequencies differ by 1-4 MHz. The work uses these findings to outline two passive leakage removal units, one using a pumped transmon with a tunable coupler and another based on a junction readout scheme.","feed_headline":"Leakage hops at 0.8-10 MHz in transmons even when exchange and ZZ are canceled","feed_subtitle":"Nonlinearity keeps hopping alive; detuning localizes it and 1-4 MHz spreads block next-nearest tunneling, enabling passive removal units.","key_machinery":"Leakage hopping rates arising from transmon nonlinearity, which remain after tunable couplers null single-excitation exchange or ZZ coupling, and which can be localized by frequency detuning.","core_discovery":"Even if the couplers are tuned to cancel the single-excitation exchange or the ZZ interaction, the leakage hopping rates still persist in the range of 0.8-10 MHz due to transmon nonlinearity. In typical operation regimes, however, transmon frequency detuning localizes leakage excitations. The next-nearest-neighbor transmons can still be near-resonant opening leakage tunneling channels. To suppress longer-range hopping, the frequency spread of the next-nearest-neighbor transmons needs to be in the range of 1-4 MHz. Utilizing leakage mobility, two passive leakage removal units are proposed.","pith_inferences":["Architectures could deliberately engineer small frequency spreads between next-nearest neighbors to create controlled leakage pathways without active driving.","The same nonlinearity that enables unwanted hopping might be harnessed to route leakage to dedicated sinks without adding extra control lines.","If leakage localization holds across larger chains, error correlations from leakage migration could be reduced by simple frequency allocation rather than complex dynamical decoupling."],"forward_implications":["Leakage excitations become localized when transmon frequencies are detuned in normal operating ranges.","Next-nearest-neighbor leakage tunneling is suppressed only when those transmons have a frequency spread of 1-4 MHz.","Passive removal units can be built using a tunable coupler plus pumped transmon or a junction readout scheme.","Processor architectures can be designed to either mobilize leakage toward removal units or localize it to limit correlated errors."],"fun_headline_variants":["Leakage hops endure at 0.8-10 MHz due to transmon nonlinearity","Detuning localizes leakage excitations in transmon processors","1-4 MHz spread suppresses next-nearest leakage tunneling","Passive removal units exploit leakage mobility in tunable couplers"],"cache_read_input_tokens":2112,"weakest_assumption_plain":"Numerical and analytical models correctly describe leakage dynamics for the realistic device parameters used in the calculations.","fun_headline_variants_meta":{"raw":{"variants":["Leakage hops endure at 0.8-10 MHz due to transmon nonlinearity","Detuning localizes leakage excitations in transmon processors","1-4 MHz spread suppresses next-nearest leakage tunneling","Passive removal units exploit leakage mobility in tunable couplers"]},"model":"grok-4.3","cost_usd":0.003964,"raw_usage":{"total_tokens":2062,"prompt_tokens":738,"num_sources_used":0,"completion_tokens":69,"cost_in_usd_ticks":39637000,"prompt_tokens_details":{"text_tokens":738,"audio_tokens":0,"image_tokens":0,"cached_tokens":256},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":1255,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":738,"tokens_out":69,"duration_ms":9573,"temperature":1.0,"reasoning_tokens":1255,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-07-02T12:13:07.370053+00:00","model_set":{"reader":"grok-4.3"},"falsifier":"Measure the actual leakage hopping rate between two transmons whose coupler is tuned to cancel both exchange and ZZ terms; a result outside 0.8-10 MHz would contradict the persistence claim.","supporting_citations":[],"review_version":1}