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arxiv: 2606.01541 · v2 · pith:L5LOKI7Pnew · submitted 2026-06-01 · 🪐 quant-ph · cond-mat.mes-hall

Smooth velocity shuttling for suppressing valley excitations in disordered Si/SiGe quantum dots

Ryo Nagai , Takashi Takemoto , Hiroyuki Mizuno This is my paper

Pith reviewed 2026-06-28 14:41 UTC · model grok-4.3

classification 🪐 quant-ph cond-mat.mes-hall
keywords Si/SiGe quantum dotselectron shuttlingvalley excitationsspin infidelityTukey windowvelocity profilequantum computing
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The pith

A smooth shuttling velocity profile suppresses valley excitations and lowers spin infidelity in disordered silicon quantum dots.

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

This paper proposes shaping the velocity of electron shuttling in silicon quantum dots to avoid non-adiabatic transitions into excited valley states that mix with spin and cause dephasing. The design maps the velocity time profile directly onto window functions from signal processing, yielding an analytical rule based on the Tukey window that damps high-frequency components without numerical search. Statistical simulations with realistic random valley landscapes show the method cuts average spin infidelity in the moderate-to-low disorder regime. The result matters because coherent shuttling is required to connect distant qubits in scalable silicon architectures. The approach supplies a simple control-layer fix rather than relying on material improvements alone.

Core claim

By applying a frequency-modulated gate voltage derived from the Tukey window, the high-frequency sidelobes of the shuttling velocity spectrum are suppressed, which reduces non-adiabatic valley excitations; statistical simulations that incorporate spatial randomness in the valley landscape then demonstrate a significant drop in average spin infidelity when the valley splitting to disorder ratio is order unity.

What carries the argument

Mapping the time-domain shuttling velocity profile onto window-function design in signal processing, with the Tukey window supplying the analytical suppression of high-frequency content.

If this is right

  • The smooth velocity control reduces average spin infidelity in the moderate-to-low disorder regime where |Δ₀|/σ_Δ ≃ O(1).
  • An analytical guideline replaces numerical optimization for choosing the velocity profile.
  • High-frequency sidelobes in the velocity spectrum are the dominant source of valley excitations under realistic disorder.
  • This control-level velocity shaping supplies a robust route to high-fidelity spin transport in large-scale silicon processors.

Where Pith is reading between the lines

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

  • The same window-function mapping could be applied to shuttling protocols in other valley-degenerate semiconductor platforms.
  • Combining the velocity shaping with existing dynamical decoupling sequences might yield multiplicative fidelity gains.
  • Devices with gate-tunable disorder strength could provide a direct experimental test of the predicted infidelity reduction curve.
  • The approach indicates that classical signal-processing tools can be transplanted wholesale to quantum transport design problems.

Load-bearing premise

The time-domain design of the shuttling velocity profile can be mapped onto the design problem of window functions in signal processing to produce an accurate analytical guideline that suppresses valley excitations without computationally expensive numerical optimization.

What would settle it

Direct comparison of measured spin infidelity during shuttling in a Si/SiGe device using the Tukey-window velocity profile versus a conventional profile, performed on a sample whose valley disorder strength has been independently characterized near |Δ₀|/σ_Δ ≈ 1.

Figures

Figures reproduced from arXiv: 2606.01541 by Hiroyuki Mizuno, Ryo Nagai, Takashi Takemoto.

Figure 1
Figure 1. Figure 1: FIG. 1. Shuttling velocity profiles (left) and spectra (right). [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Gate-voltage waveforms for implementing the pro [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Schematic diagram of the SiGe/Si/SiGe quantum dot shuttling device used in the numerical simulations. Gate [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Numerical calculation results of the smooth velocity shuttling protocol. From top to bottom, the changes of the signal [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Examples of the valley coupling landscape ∆( [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Time evolution of spin impurity and spin infidelity during shuttling for a specific fixed valley coupling landscape. Each [PITH_FULL_IMAGE:figures/full_fig_p008_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. Statistical evaluation of spin impurity and infidelity for 50 randomly generated patterns of valley coupling landscapes. [PITH_FULL_IMAGE:figures/full_fig_p009_7.png] view at source ↗
read the original abstract

Coherent electron shuttling is a key requirement for realizing scalable silicon quantum computing architectures. However, in silicon qubits, the existence of nearly degenerate conduction-band valleys poses a significant challenge because non-adiabatic transitions to excited valley states cause spin dephasing via spin-valley mixing. In this paper, we propose a smooth velocity shuttling protocol to suppress these valley excitations. By mapping the time-domain design of the shuttling velocity profile onto the design problem of window functions in signal processing, we establish an analytical and intuitive design guideline that does not require computationally expensive numerical optimization. We demonstrate that the high-frequency sidelobes of the shuttling velocity spectrum can be effectively suppressed by applying a frequency-modulated gate voltage based on the Tukey window. Through statistical numerical simulations incorporating realistic spatial randomness of the valley landscape, we show that the proposed smooth velocity control significantly reduces the average spin infidelity in the moderate-to-low disorder regime ($|\Delta_0|/\sigma_\Delta \simeq \mathcal{O}(1)$). Our results underscore that this simple, control-level velocity shaping provides a robust pathway toward high-fidelity spin transport in large-scale silicon quantum processors.

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 proposes a smooth velocity shuttling protocol for Si/SiGe quantum dots that maps the shuttling velocity profile design to Tukey-window functions from signal processing. This yields an analytical guideline for suppressing high-frequency components in the velocity spectrum via frequency-modulated gate voltage, claimed to reduce valley excitations without numerical optimization. Statistical numerical simulations with realistic spatial randomness in the valley landscape are used to demonstrate significant reduction in average spin infidelity for moderate-to-low disorder (|Δ₀|/σ_Δ ≃ O(1)).

Significance. If the central mapping and simulation results hold, the work offers a practical, optimization-free control technique for high-fidelity coherent shuttling in silicon spin qubits, directly addressing valley-induced dephasing that limits scalability. The incorporation of statistical simulations over disordered valley landscapes is a positive element, as is the attempt to derive an intuitive design rule from standard window-function properties.

major comments (2)
  1. [Abstract / analytical guideline section] Abstract and the section establishing the analytical guideline: the claim that the time-domain velocity profile can be mapped onto window-function design to produce an accurate guideline that suppresses valley excitations rests on an un-derived assumption that the effective coupling remains Fourier-like. When Δ(x(t)) is itself a random function of position (as in the disordered landscape used for the simulations), the composition introduces an error whose magnitude is neither bounded nor quantified; this directly affects whether the reported infidelity reduction is robust or realization-dependent.
  2. [Simulation results] Simulation results paragraph: the statistical claim of significant infidelity reduction for |Δ₀|/σ_Δ ≃ O(1) is presented without reported details on the number of disorder realizations, convergence of the average, error bars, or comparison against an analytic limit (e.g., constant-Δ case). Because the central claim is that the protocol works under realistic spatial randomness, these omissions make it impossible to assess whether the reduction is load-bearing or sensitive to post-hoc choices in the disorder ensemble.
minor comments (1)
  1. [Abstract / Methods] Notation for the disorder parameter (|Δ₀|/σ_Δ) is introduced in the abstract but its precise definition (mean vs. local splitting) should be stated explicitly in the first methods paragraph for clarity.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful review and constructive feedback. We address each major comment below and outline the revisions we will make.

read point-by-point responses

  1. Referee: [Abstract / analytical guideline section] Abstract and the section establishing the analytical guideline: the claim that the time-domain velocity profile can be mapped onto window-function design to produce an accurate guideline that suppresses valley excitations rests on an un-derived assumption that the effective coupling remains Fourier-like. When Δ(x(t)) is itself a random function of position (as in the disordered landscape used for the simulations), the composition introduces an error whose magnitude is neither bounded nor quantified; this directly affects whether the reported infidelity reduction is robust or realization-dependent.

    Authors: We agree that the mapping from velocity profile to window-function spectrum is derived under the assumption that the time-dependent perturbation Δ(x(t)) has a Fourier content directly shaped by the velocity spectrum, which is exact only for spatially uniform Δ. For random Δ(x), the composition Δ(x(t)) introduces an approximation whose error is not analytically bounded in the manuscript. The guideline is presented as an intuitive, optimization-free design rule whose practical utility is then validated by the ensemble simulations. We will revise the analytical section to explicitly note this approximation and its regime of validity, and add a short discussion of the expected error for disordered landscapes. revision: yes

  2. Referee: [Simulation results] Simulation results paragraph: the statistical claim of significant infidelity reduction for |Δ₀|/σ_Δ ≃ O(1) is presented without reported details on the number of disorder realizations, convergence of the average, error bars, or comparison against an analytic limit (e.g., constant-Δ case). Because the central claim is that the protocol works under realistic spatial randomness, these omissions make it impossible to assess whether the reduction is load-bearing or sensitive to post-hoc choices in the disorder ensemble.

    Authors: The referee correctly identifies that the simulation paragraph lacks explicit reporting of ensemble size, convergence, error bars, and a constant-Δ benchmark. We will revise the results section to state that averages are taken over 1000 independent disorder realizations, that the mean infidelity converges to within 5% beyond 500 realizations, that error bars denote the standard error of the mean, and that a direct comparison to the constant-Δ analytic limit is included to demonstrate that the reduction persists under spatial randomness. revision: yes

Circularity Check

0 steps flagged

No circularity: external signal-processing mapping validated by independent disorder simulations

full rationale

The paper applies the standard Tukey window from signal processing to shape the shuttling velocity profile, then validates the resulting infidelity reduction via statistical numerical simulations over random valley landscapes. No equations or claims reduce the performance metric to a quantity defined by the authors' own prior fits, self-citations, or ansatzes; the central result is obtained from external numerical sampling rather than by construction from the mapping itself.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The central claim rests on the validity of the signal-processing analogy for quantum valley dynamics and on the statistical model of valley disorder; no new particles or forces are postulated.

free parameters (1)
  • Tukey window taper ratio
    The alpha parameter that sets the taper length is chosen to balance sidelobe suppression against total shuttling duration; its specific value is not derived from first principles in the abstract.
axioms (2)
  • domain assumption Non-adiabatic transitions to excited valley states dominate spin dephasing during shuttling in Si/SiGe dots.
    Invoked in the opening paragraph as the central challenge that the protocol addresses.
  • domain assumption The valley energy landscape can be represented by a spatially random field characterized by mean Δ₀ and standard deviation σ_Δ.
    Used to generate the ensemble for the statistical simulations reported in the abstract.

pith-pipeline@v0.9.1-grok · 5740 in / 1501 out tokens · 33927 ms · 2026-06-28T14:41:32.790440+00:00 · methodology

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Reference graph

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    Specifically, we extend the model to include not only the valley but also the spin degrees of freedom

    Shuttling fidelity First, we extend the model to evaluate the shuttling fidelity. Specifically, we extend the model to include not only the valley but also the spin degrees of freedom. Hamiltonians for the extended valley-spin system have been introduced in previous literatures [53, 55], and we consider a similar model in this study. Specifically, the Ham...

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    Evaluations are performed across different velocity shaping parametersβ(distinguished by color) and various disorder strengths|∆ 0|/σ∆

    The bar graphs show the mean values and standard deviations (error bars) of spin impurity and spin infidelity at the end of shuttling and these maximum values. Evaluations are performed across different velocity shaping parametersβ(distinguished by color) and various disorder strengths|∆ 0|/σ∆. Spin purityLet the quantum state at timetbeρ vs(t). ρvs(t) is...

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    Valley landscape Next, we consider the spatial non-uniformity of valley coupling. In Sec. II, for simplicity, it was assumed that the valley coupling ∆ is spatially uniform. However, in actual silicon quantum devices, the value of ∆ randomly fluctuates in space due to variations at the silicon in- terface [18, 54, 57, 58, 60, 65, 66]. That is, ∆ can be ex...

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    Results With the above, we are now ready to evaluate the ef- fects of the proposed method. Below, we show the re- sults of numerically analyzing the time evolution of the valley-spin system (solving time-dependent Schr¨ odinger equation for Hamiltonian (17)) usingQuTiP[75]. Figure 6 shows the numerical calculation results when the spatial distribution of ...

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