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arxiv: 2606.31306 · v1 · pith:HUHLOCA2new · submitted 2026-06-30 · 🪐 quant-ph

Ferroelectric transmon

M. Donaire , A. Cano This is my paper

Pith reviewed 2026-07-01 05:41 UTC · model grok-4.3

classification 🪐 quant-ph
keywords ferroelectric transmonsuperconducting qubitsJosephson junctionanharmonicitycharge noisequantum computingnonlinear capacitor
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The pith

A ferroelectric capacitor shunting a Josephson junction supplies an extra parameter for tuning transmon anharmonicity while remaining insensitive to charge noise.

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

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.

Core claim

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.

What carries the argument

The ferroelectric or incipient-ferroelectric capacitor that shunts the Josephson junction, whose voltage-dependent permittivity alters the effective circuit parameters.

If this is right

  • 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.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • 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.

Load-bearing premise

A ferroelectric capacitor can be integrated with a Josephson junction without introducing new sources of decoherence or charge noise beyond those in conventional transmons.

What would settle it

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.

Figures

Figures reproduced from arXiv: 2606.31306 by A. Cano, M. Donaire.

Figure 1
Figure 1. Figure 1: FIG. 1. Effective circuit of the ferroelectric transmon [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Low-lying energy levels [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Relative energy ratio [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Low-lying energy levels [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗
read the original abstract

Superconducting qubits are a leading platform for quantum computing. However, simultaneously achieving low noise sensitivity to suppress decoherence and sufficient anharmonicity to enable fast gate operations remains a central challenge. Here, we introduce the concept of the ferroelectric transmon (FEmon), in which the Josephson junction is shunted by a ferroelectric, or incipient ferroelectric, capacitor. We show, in particular, that 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

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).

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 (2)
  1. 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.
  2. 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.
minor comments (1)
  1. Abstract: a single quantitative metric (e.g., expected anharmonicity improvement or comparison to a standard transmon) would clarify the claimed advantage.

Simulated Author's Rebuttal

2 responses · 0 unresolved

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.

read point-by-point responses

  1. Referee: 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.

    Authors: 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: yes

  2. Referee: 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.

    Authors: 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: yes

Circularity Check

0 steps flagged

No circularity: high-level conceptual proposal only

full rationale

The manuscript introduces the ferroelectric transmon as a design concept. The abstract and provided text contain no equations, no fitted parameters, no derivation chain, and no self-citations that bear load on a claimed result. The central statement is a qualitative suggestion that nonlinear C(V) supplies an extra tuning knob while preserving the E_J ≫ E_C regime; this is not obtained by reducing any input to itself via definition, fitting, or prior self-work. No steps match any enumerated circularity pattern, so the finding is the default non-finding of a self-contained proposal.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review; no equations or parameters are visible, so the ledger cannot be populated with concrete entries. The proposal implicitly assumes standard transmon circuit theory plus an uncharacterized nonlinear capacitor response.

pith-pipeline@v0.9.1-grok · 5599 in / 987 out tokens · 17393 ms · 2026-07-01T05:41:48.210300+00:00 · methodology

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Reference graph

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