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arxiv: 2604.19687 · v1 · submitted 2026-04-21 · 🪐 quant-ph

Spin Kerr-cat qubits

Pith reviewed 2026-05-10 03:08 UTC · model grok-4.3

classification 🪐 quant-ph
keywords spin Kerr-cat qubitsnuclear spin encodingquadrupolar nucleiclock transitioncoherence timeantimony donorssilicon qubitstwo-qubit gate
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The pith

Spin Kerr-cat qubits use clock transitions in quadrupolar nuclei to suppress dephasing noise to first order.

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

The paper introduces the spin Kerr-cat encoding for nuclear spins with I greater than or equal to 1, defining qubit states as the lowest two levels of a Z2-symmetric Hamiltonian that approximate spin cat states. This leverages a clock transition to cancel first-order sensitivity to noise that causes dephasing. Calculations based on measured parameters for antimony-123 donors in silicon project a coherence time of 100 seconds under a 1/f noise model, while also accounting for relaxation from charge-induced quadrupolar fluctuations. The authors further describe an electron-hopping two-qubit gate and project 99 percent fidelity once the quadrupolar splitting is increased by a factor of about 4. The goal is hardware-level protection of quantum information in nuclear spin systems.

Core claim

The spin Kerr-cat encoding defines qubit basis states as the two lowest levels of a Z2-symmetric nuclear-spin Hamiltonian for quadrupolar nuclei, which are well approximated by spin cat states and exhibit a clock transition providing first-order suppression of dephasing noise.

What carries the argument

The Z2-symmetric nuclear-spin Hamiltonian whose two lowest levels form the qubit basis states and realize a clock transition for noise suppression.

If this is right

  • A coherence time T2* of 100 seconds is projected for 123Sb donors in silicon under the modeled 1/f noise.
  • A two-qubit gate fidelity of 99 percent is estimated when quadrupolar splittings are enhanced by a factor of approximately 4.
  • The encoding supplies first-order protection against dephasing for any quadrupolar nucleus with spin I greater than or equal to 1.
  • Relaxation times are limited by charge-noise-induced breaking of the Z2 symmetry in the quadrupolar tensor.

Where Pith is reading between the lines

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

  • Nuclear spin qubits with this encoding could function as long-lived memories in hybrid electron-nuclear quantum processors.
  • The electron-mediated gate scheme might extend to other donor or quantum-dot platforms that allow controlled hopping.
  • Real devices would need to verify that the cat-state approximation remains accurate when noise spectra deviate from the assumed 1/f form.
  • Combining the encoding with dynamical decoupling or error correction could yield even longer effective lifetimes.

Load-bearing premise

The qubit states are well approximated by spin cat states, the 1/f noise and charge-noise quadrupolar fluctuation models capture the main errors, and quadrupolar splittings can be enhanced by a factor of 4 without adding uncontrolled decoherence.

What would settle it

Experimental measurement of the actual T2* dephasing time for an implemented 123Sb spin Kerr-cat qubit, or direct test of whether a fourfold increase in quadrupolar splitting preserves the predicted coherence without introducing extra errors.

Figures

Figures reproduced from arXiv: 2604.19687 by Daniel Loss, Z. M. McIntyre.

Figure 1
Figure 1. Figure 1: FIG. 1. (a) Spectrum of [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. (a) Fidelity [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Coefficient [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Schematic of an electron tunneling from a quantum [PITH_FULL_IMAGE:figures/full_fig_p015_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Notably, values of Fg = 99% (indicated by the dashed contour) could be reached for asymmetry param￾eters η ≳ 0.7 provided Q can be made sufficiently large. The fact that lower values of η do not attain this level of fidelity follows in part from the spin-cat approxima￾tion, which falls below F = 99% near η = 0.5 [ [PITH_FULL_IMAGE:figures/full_fig_p016_5.png] view at source ↗
read the original abstract

The use of noise-robust qubit encodings provides a way of extending the lifetime of quantum information at the hardware level. In this work, we introduce the spin Kerr-cat encoding, which leverages a clock transition in the spectrum of quadrupolar nuclei (having spin length $I\geq 1$) to achieve a first-order suppression of noise leading to qubit dephasing. The basis states of the spin Kerr-cat qubit are given by the two lowest levels of a $\mathbb{Z}_2$-symmetric nuclear-spin Hamiltonian and are well approximated by spin cat states. We compute the dephasing time of the spin Kerr-cat qubit under a model of $1/f$ noise, as well as relaxation of the qubit due to breaking of the $\mathbb{Z}_2$ symmetry by charge-noise-induced fluctuations of the quadrupolar tensor. Using measured parameters for antimony (${}^{123}\mathrm{Sb}$) donors in silicon, we estimate that a coherence time of $T_2^*=100$ s could be achieved with this encoding. We propose a two-qubit gate mediated by hopping electrons and estimate that with an enhancement of measured quadrupolar splittings by a factor of $\approx 4$, a gate fidelity of $99\%$ could be achieved for spin Kerr-cat qubits encoded in ${}^{123}\mathrm{Sb}$ nuclear spins, neglecting errors that impact the electron while it is being shuttled and read out.

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 paper introduces the spin Kerr-cat qubit encoding for quadrupolar nuclei (I ≥ 1), using the two lowest levels of a Z₂-symmetric nuclear-spin Hamiltonian (approximated by spin cat states) to achieve first-order dephasing suppression via a clock transition. Using measured parameters for ¹²³Sb donors in silicon, it estimates T₂* = 100 s under a 1/f noise model and relaxation from charge-noise-induced quadrupolar fluctuations. It also proposes an electron-hopping-mediated two-qubit gate and estimates 99% fidelity assuming quadrupolar splittings can be enhanced by a factor of ≈4, while neglecting electron-related errors during shuttling and readout.

Significance. If the estimates and approximations hold, the encoding offers a hardware-level route to substantially longer coherence for nuclear-spin donor qubits in silicon, a platform of interest for scalable quantum computing. The use of measured parameters for the T₂* estimate is a strength. However, the overall significance is limited by the conditional nature of the gate-fidelity result and the lack of explicit validation for the noise model, cat-state approximation, and enhancement feasibility.

major comments (2)
  1. [Abstract] Abstract (two-qubit gate fidelity): The 99% fidelity estimate is conditioned on enhancing measured quadrupolar splittings by a factor of ≈4. No mechanism for achieving this enhancement is specified, and no calculation is provided to show that the enhancement preserves the assumed 1/f noise spectrum and does not increase the amplitude of charge-noise-induced quadrupolar fluctuations or introduce new relaxation channels. This assumption is load-bearing for the fidelity claim.
  2. [Abstract] Abstract (T₂* = 100 s estimate): The coherence-time claim relies on the basis states being well approximated by spin cat states and on the 1/f noise model plus charge-noise quadrupolar fluctuations fully capturing the dominant errors. The abstract states the numerical result but supplies no derivations, error bars, or explicit validation of these approximations, undermining assessment of whether the 100 s figure is robust.
minor comments (1)
  1. [Abstract] The abstract notes that electron shuttling and readout errors are neglected but does not estimate their contribution to the overall error budget or discuss how they might be mitigated.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading and constructive comments on our manuscript. We address the major comments point by point below. Where the comments identify areas needing clarification or additional discussion, we have revised the manuscript accordingly.

read point-by-point responses
  1. Referee: [Abstract] Abstract (two-qubit gate fidelity): The 99% fidelity estimate is conditioned on enhancing measured quadrupolar splittings by a factor of ≈4. No mechanism for achieving this enhancement is specified, and no calculation is provided to show that the enhancement preserves the assumed 1/f noise spectrum and does not increase the amplitude of charge-noise-induced quadrupolar fluctuations or introduce new relaxation channels. This assumption is load-bearing for the fidelity claim.

    Authors: We agree that the enhancement of the quadrupolar splitting is a load-bearing assumption for the 99% fidelity estimate. In the revised manuscript we now specify a concrete mechanism: the application of uniaxial strain or static electric field gradients, which have been shown to tune the quadrupolar interaction in group-V donors in silicon. We add a short calculation showing that a uniform scaling of the quadrupolar tensor by a factor of four preserves the 1/f character of the noise provided the underlying charge-fluctuator density remains unchanged; the relative fluctuation amplitude then scales linearly with the enhanced splitting. We acknowledge that a complete enumeration of all possible new relaxation channels introduced by the enhancement lies beyond the present theoretical scope and requires further experimental input; we have therefore added an explicit caveat to the fidelity claim. revision: partial

  2. Referee: [Abstract] Abstract (T₂* = 100 s estimate): The coherence-time claim relies on the basis states being well approximated by spin cat states and on the 1/f noise model plus charge-noise quadrupolar fluctuations fully capturing the dominant errors. The abstract states the numerical result but supplies no derivations, error bars, or explicit validation of these approximations, undermining assessment of whether the 100 s figure is robust.

    Authors: The abstract is necessarily concise. The main text (Sections III and IV) and supplementary material contain the explicit derivation of T₂* under the 1/f noise model, the numerical validation that the two lowest eigenstates overlap with the ideal spin-cat states to >99%, and the incorporation of charge-noise-induced quadrupolar fluctuations. We have revised the abstract to include a one-sentence reference to these sections and to report the estimate as an order-of-magnitude value (∼100 s) rather than a precise figure, thereby signaling the underlying approximations. No error bars were originally provided because the input parameters are taken directly from published measurements whose uncertainties are already discussed in the text; we have now added a brief sensitivity analysis in the supplementary material. revision: yes

Circularity Check

0 steps flagged

No circularity; estimates use external measured parameters under explicit assumptions

full rationale

The paper introduces the spin Kerr-cat encoding from the Z2-symmetric nuclear-spin Hamiltonian and approximates its lowest levels as cat states. Dephasing times are computed from a standard 1/f noise model applied to measured antimony donor parameters in silicon; the T2* = 100 s figure is therefore a direct numerical evaluation of that model on external data, not a fit or self-definition. The two-qubit gate fidelity estimate explicitly conditions on an assumed factor-of-4 enhancement of quadrupolar splittings while holding the same noise model fixed; the enhancement itself is stated as an external requirement rather than derived from the paper's equations. No self-citation supplies a uniqueness theorem or ansatz that the central claims rest upon, and no fitted parameter is relabeled as a prediction. The derivation chain therefore remains independent of its own outputs.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 1 invented entities

The proposal rests on the cat-state approximation, a specific 1/f noise model, the assumption that symmetry breaking occurs only via quadrupolar fluctuations, and one adjustable enhancement factor; no independent evidence is given for the enhancement.

free parameters (2)
  • quadrupolar splitting enhancement factor = ≈4
    Chosen as approximately 4 to reach the stated 99% gate fidelity; not derived from first principles.
  • 1/f noise spectral parameters
    Used to compute the T2* estimate; taken from measured values but details of fitting not shown.
axioms (2)
  • domain assumption Basis states of the Z2-symmetric Hamiltonian are well approximated by spin cat states
    Invoked to justify the encoding and noise suppression.
  • domain assumption Z2 symmetry is broken solely by charge-noise-induced fluctuations of the quadrupolar tensor
    Used to model relaxation.
invented entities (1)
  • spin Kerr-cat qubit no independent evidence
    purpose: Noise-robust encoding of quantum information in nuclear spins
    Newly defined encoding whose performance is estimated in the paper.

pith-pipeline@v0.9.0 · 5539 in / 1560 out tokens · 56871 ms · 2026-05-10T03:08:24.000104+00:00 · methodology

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Reference graph

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