{"id":"61badcf9-6c8d-4852-8fa5-a74db95ad818","arxiv_id":"2606.31306","paper_version":1,"verdict":"UNVERDICTED","confidence":"LOW","novelty_score":7.0,"correctness_risk":"unknown","formal_verification":"none","parameter_count":0,"one_line_summary":"A ferroelectric shunt capacitor on a transmon qubit supplies an extra control knob for anharmonicity while remaining in the charge-insensitive regime.","lead":"The paper proposes the ferroelectric transmon (FEmon), a superconducting qubit design where a Josephson junction is shunted by a ferroelectric capacitor. This adds a tunable nonlinear response to help balance anharmonicity and charge-noise insensitivity in quantum computing hardware.","discovery_kind":"new_method","skeptic_critique":{"model":"grok-4.3","headline":"Nonlinear C(V) may make effective E_C state-dependent, potentially restoring charge dispersion despite E_J >> E_C","rationale":"The reader's weakest assumption correctly flags fabrication and decoherence risks, which are decisive for any practical device. However, the load-bearing theoretical step—whether the nonlinearity preserves charge insensitivity—is an internal modeling question that must be checked before the optimization claim can be accepted even conceptually. The two concerns are therefore related but not identical; the numerical diagonalization test directly probes the central claim without requiring material fabrication.","tokens_in":1566,"tokens_out":390,"duration_ms":17513,"concrete_test":"Construct the circuit Hamiltonian with C(V) taken from a standard ferroelectric model (e.g., C(V) = C0 / (1 + α V^2)), set E_J / E_C0 ≈ 80, and numerically diagonalize in the charge basis for n_g = 0 and n_g = 0.5; if the energy difference exceeds ~1 MHz while the anharmonicity gain is < 200 MHz, the insensitivity claim fails.","verdict_should_be":"CONDITIONAL","load_bearing_attack":"The central claim requires that the ferroelectric nonlinearity supplies an extra tuning knob for anharmonicity while the qubit remains in the charge-insensitive regime (E_J ≫ E_C). Because the shunt capacitance is voltage-dependent, the instantaneous charging energy becomes operator-valued (E_C(φ) or E_C(n) depending on representation). This can couple charge fluctuations to the nonlinear term, producing a charge dispersion that is not exponentially suppressed in the same way as the linear transmon. The abstract provides no indication that the Hamiltonian was diagonalized with a realistic ferroelectric C(V) (e.g., from Landau-Devonshire) to verify that the n_g = 0 to n_g = 0.5 splitting remains negligible at the operating point.","agreement_with_reader":"partial"},"referee_report":{"model":"grok-4.3","summary":"The manuscript proposes the ferroelectric transmon (FEmon), in which a Josephson junction is shunted by a ferroelectric or incipient-ferroelectric capacitor. The central claim is that the nonlinear voltage dependence of the ferroelectric capacitance supplies an additional degree of freedom for optimizing qubit anharmonicity while the device remains in the charge-noise-insensitive regime (E_J ≫ E_C).","tokens_in":1706,"tokens_out":404,"duration_ms":21610,"significance":"If the claim is substantiated by an explicit Hamiltonian treatment, the result would be significant for superconducting qubit design because it introduces a new tunable nonlinearity without sacrificing the exponential charge-noise suppression that defines the transmon architecture. The manuscript currently offers only a qualitative design suggestion with no equations, parameter values, or comparisons, so the practical advantage over existing anharmonicity-engineering approaches remains unquantified.","major_comments":[{"comment":"Abstract: the claim that the nonlinear ferroelectric response optimizes anharmonicity while preserving the charge-insensitive regime is stated at a high level but is unsupported by any derivation, effective Hamiltonian, or numerical diagonalization.","section":null},{"comment":"Main text (Hamiltonian discussion): because the shunt capacitance is voltage-dependent, the instantaneous charging energy is operator-valued. This can couple charge fluctuations to the nonlinear term and potentially restore charge dispersion that is not exponentially suppressed. The manuscript must diagonalize the Hamiltonian with a realistic C(V) (e.g., Landau-Devonshire) to confirm that the n_g = 0 to n_g = 0.5 splitting remains negligible at the operating point.","section":null}],"minor_comments":[{"comment":"Abstract: a single quantitative metric (e.g., expected anharmonicity improvement or comparison to a standard transmon) would clarify the claimed advantage.","section":null}],"recommendation":"major_revision","confidential_remarks":"The work is a conceptual proposal rather than a completed theoretical study; it may be better suited to a letter format once the required Hamiltonian analysis is supplied."},"author_rebuttal":{"model":"grok-4.3","summary":"We thank the referee for their detailed and constructive report on our manuscript proposing the ferroelectric transmon. The comments correctly identify that the current version remains largely conceptual and lacks the quantitative Hamiltonian analysis needed to substantiate the central claims. We address each point below and commit to a major revision that incorporates the requested derivations and numerical checks.","responses":[{"response":"We agree that the abstract presents the claim at a conceptual level without supporting calculations. The main text provides only a qualitative argument based on the voltage dependence of the shunt capacitance. In the revised manuscript we will add an explicit effective-Hamiltonian derivation that incorporates a nonlinear C(V) into the charging term and shows how the resulting anharmonicity can be tuned independently of the exponential charge-noise suppression. The abstract will be updated to reference this new section.","revision_made":"yes","referee_comment":"Abstract: the claim that the nonlinear ferroelectric response optimizes anharmonicity while preserving the charge-insensitive regime is stated at a high level but is unsupported by any derivation, effective Hamiltonian, or numerical diagonalization."},{"response":"This is a substantive and valid concern. Because C depends on voltage, the charging energy becomes an operator, and additional terms could in principle appear that affect charge dispersion. While we expect the transmon regime (E_J ≫ E_C) to keep dispersion exponentially small, a rigorous confirmation requires numerical diagonalization. We will add such a calculation in the revision, using a realistic Landau-Devonshire model for C(V) with parameters appropriate for an incipient ferroelectric (e.g., SrTiO3), and will explicitly report the n_g = 0 to n_g = 0.5 energy splitting at the chosen operating point.","revision_made":"yes","referee_comment":"Main text (Hamiltonian discussion): because the shunt capacitance is voltage-dependent, the instantaneous charging energy is operator-valued. This can couple charge fluctuations to the nonlinear term and potentially restore charge dispersion that is not exponentially suppressed. The manuscript must diagonalize the Hamiltonian with a realistic C(V) (e.g., Landau-Devonshire) to confirm that the n_g = 0 to n_g = 0.5 splitting remains negligible at the operating point."}],"tokens_in":1195,"tokens_out":481,"duration_ms":24198,"standing_objections":[]},"desk_editor":{"model":"grok-4.3","letter":"The main takeaway is that Donaire and Cano are floating the idea of shunting a Josephson junction with a ferroelectric or incipient-ferroelectric capacitor so its nonlinear dielectric response gives an independent handle on anharmonicity while the device stays in the usual transmon regime. That specific combination is not the standard approach in the literature.\n\nThe paper does a straightforward job laying out the usual trade-off between charge noise and anharmonicity and then pointing to the ferroelectric nonlinearity as a possible way around it. The framing is direct and the motivation is clear.\n\nThe soft spot is exactly the one flagged in the stress-test note. Once capacitance depends on voltage, the effective charging energy becomes operator-valued. That can feed charge fluctuations back into the spectrum and potentially restore a dispersion that is no longer exponentially small in E_J/E_C. The abstract contains no Hamiltonian, no Landau-Devonshire model, no diagonalization, and no comparison of n_g = 0 versus n_g = 0.5 splittings, so there is no evidence yet that the central claim survives. Without those steps the proposal remains a suggestion rather than a demonstrated mechanism.\n\nThis is for people already working on transmon variants and material integration who are looking for new capacitor choices. A reader who wants a concrete new device architecture will find the idea worth noting, but anyone needing numbers or a verified Hamiltonian will come away empty. It deserves a serious referee only if the full manuscript actually performs the required diagonalization with a realistic C(V); otherwise it is too thin for review.","headline":"The ferroelectric transmon is a clean conceptual proposal for an extra tuning knob, but the abstract supplies no calculation to show the voltage-dependent capacitance actually preserves exponential charge insensitivity.","tokens_in":2170,"tokens_out":392,"would_cite":false,"duration_ms":16587,"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":"A ferroelectric capacitor shunting a Josephson junction supplies an extra parameter for tuning transmon anharmonicity while remaining insensitive to charge noise.","keywords":["ferroelectric transmon","superconducting qubits","Josephson junction","anharmonicity","charge noise","quantum computing","nonlinear capacitor"],"falsifier":"Fabricate a device with a ferroelectric shunt capacitor and measure its charge dispersion and anharmonicity to check if anharmonicity rises while charge noise sensitivity stays low.","tokens_in":2457,"feed_emoji":"","tokens_out":542,"duration_ms":23163,"temperature":0.7,"pith_summary":"The paper introduces the ferroelectric transmon, or FEmon, in which a ferroelectric capacitor replaces the usual linear shunt across the Josephson junction. The nonlinear response of this capacitor creates an additional degree of freedom that can be used to increase the anharmonicity of the qubit energy levels. This increase supports faster gate operations without leaving the charge-noise-insensitive regime that protects conventional transmons from decoherence. A sympathetic reader would care because the design directly targets the central trade-off that limits the performance of superconducting qubits today.","feed_headline":"Ferroelectric shunt adds anharmonicity control to transmons","feed_subtitle":"Nonlinear capacitor response supplies an extra knob while keeping the device in the low charge-noise regime.","key_machinery":"The ferroelectric or incipient-ferroelectric capacitor that shunts the Josephson junction, whose voltage-dependent permittivity alters the effective circuit parameters.","core_discovery":"The nonlinear ferroelectric response of the capacitor provides an additional degree of freedom for optimizing qubit anharmonicity while preserving operation in the charge-noise-insensitive regime.","pith_inferences":["Materials scientists could search for ferroelectrics compatible with superconducting circuits to test the concept.","If successful, this might reduce the need for complex circuit designs to achieve high anharmonicity.","Incumbent transmon fabrication processes would need modification to incorporate the ferroelectric layer."],"forward_implications":["The anharmonicity can be increased without raising sensitivity to charge fluctuations.","Gate speeds can improve while coherence times remain comparable to standard transmons.","The design opens a new materials-based route to qubit optimization.","The same principle may apply to other superconducting qubit architectures that rely on shunt capacitors."],"fun_headline_variants":["Ferroelectric shunt tunes transmon anharmonicity","FEmon introduces nonlinear control to qubits","Nonlinear response optimizes transmon anharmonicity","Ferroelectric capacitor controls qubit anharmonicity","FEmon preserves low noise with anharmonicity tuning"],"cache_read_input_tokens":2112,"weakest_assumption_plain":"A ferroelectric capacitor can be integrated with a Josephson junction without introducing new sources of decoherence or charge noise beyond those in conventional transmons.","fun_headline_variants_meta":{"raw":{"variants":["Ferroelectric shunt tunes transmon anharmonicity","FEmon introduces nonlinear control to qubits","Nonlinear response optimizes transmon anharmonicity","Ferroelectric capacitor controls qubit anharmonicity","FEmon preserves low noise with anharmonicity tuning"]},"model":"grok-4.3","cost_usd":0.00604,"raw_usage":{"total_tokens":2761,"prompt_tokens":475,"num_sources_used":0,"completion_tokens":69,"cost_in_usd_ticks":60399500,"prompt_tokens_details":{"text_tokens":475,"audio_tokens":0,"image_tokens":0,"cached_tokens":256},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":2217,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":475,"tokens_out":69,"duration_ms":17067,"temperature":1.0,"reasoning_tokens":2217,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-07-01T05:41:48.210300+00:00","model_set":{"reader":"grok-4.3"},"falsifier":"Fabricate a device with a ferroelectric shunt capacitor and measure its charge dispersion and anharmonicity to check if anharmonicity rises while charge noise sensitivity stays low.","supporting_citations":[],"review_version":1}