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arxiv: 2510.16204 · v2 · pith:N5MHYB3Fnew · submitted 2025-10-17 · 🪐 quant-ph · cond-mat.dis-nn· physics.optics

Decoherence-free subspaces in the noisy dynamics of discrete-step quantum walks in a photonic lattice

Rajesh Asapanna , Cl\'ement Hainaut , Alberto Amo , \'Alvaro G\'omez-Le\'on This is my paper

Pith reviewed 2026-05-18 05:42 UTC · model grok-4.3

classification 🪐 quant-ph cond-mat.dis-nnphysics.optics
keywords quantum walksdecoherence-free subspacesphotonic latticeFloquet systemstopological statestemporal noisemaster equationbulk states
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The pith

Certain bulk states in quantum walks stay coherent under constant noise while topological edge states decohere.

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

This paper explores the impact of different temporal noise types on discrete-step quantum walks in a photonic lattice. It finds that noise constant within each Floquet period creates decoherence-free momentum subspaces in the bulk. Fully random noise leads to fast decoherence, and topological edge states decohere for all noise types. The authors derive a non-perturbative master equation to explain this and confirm it with an experiment in a double-fibre ring setup. The surprising result is that bulk states can be more robust to this noise than topological edge states.

Core claim

We find that in the bulk, temporal noise that is constant within a Floquet period leads to decoherence-free momentum subspaces, whereas fully random noise destroys coherence in a few time-steps. When considering topological edge states, we observe decoherence no matter the type of temporal noise. To explain these results, we derive a non-perturbative master equation to describe the system's dynamics and experimentally confirm our findings in a discrete mesh photonic lattice implemented in a double-fibre ring setup.

What carries the argument

Decoherence-free momentum subspaces in the bulk for noise constant within Floquet periods; this mechanism allows unitary evolution in those subspaces despite the noise.

If this is right

  • The bulk can host more robust states than topological edges under correlated temporal noise.
  • Non-perturbative methods are necessary to capture the exact dynamics of this system.
  • Photonic implementations allow direct observation of these protected subspaces.

Where Pith is reading between the lines

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

  • Designers of quantum devices might prefer bulk states over edge states for noise resilience in periodically driven systems.
  • Similar effects may appear in other periodically driven quantum systems beyond walks.
  • Experimentally varying the noise correlation time could map out the boundary of the decoherence-free regime.

Load-bearing premise

The noise stays constant during each entire Floquet period rather than fluctuating within it.

What would settle it

If bulk momentum states lose coherence as quickly as edge states even when noise is held constant within periods, that would contradict the claim.

Figures

Figures reproduced from arXiv: 2510.16204 by Alberto Amo, \'Alvaro G\'omez-Le\'on, Cl\'ement Hainaut, Rajesh Asapanna.

Figure 1
Figure 1. Figure 1: FIG. 1. (a) Scheme of the experimental setup with beam [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Measured light intensity in the [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Measured dispersions under different types of noise [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Measured (dots) averaged occupation probability [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. (a) Zoom on the first time steps of the measured time [PITH_FULL_IMAGE:figures/full_fig_p010_5.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. Numerically computed edge state return probability [PITH_FULL_IMAGE:figures/full_fig_p011_7.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. (a) Two-dimensional Fourier transform (2D-FT) of [PITH_FULL_IMAGE:figures/full_fig_p011_6.png] view at source ↗
read the original abstract

We study the noisy dynamics of periodically driven, discrete-step quantum walks in a one-dimensional photonic lattice. We find that in the bulk, temporal noise that is constant within a Floquet period leads to decoherence-free momentum subspaces, whereas fully random noise destroys coherence in a few time steps. When considering topological edge states, we observe decoherence no matter the type of temporal noise. To explain these results, we derive a non-perturbative master equation to describe the system's dynamics. We experimentally confirm our findings in a time multiplexed photonic lattice implemented in a double-fiber ring setup subject to laser pulse input states, in which we engineer different types of temporal noise.

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 / 2 minor

Summary. The manuscript studies the noisy dynamics of discrete-step quantum walks in a one-dimensional photonic lattice. It reports that temporal noise constant within each Floquet period produces decoherence-free momentum subspaces in the bulk, while fully random temporal noise destroys coherence rapidly; topological edge states decohere under both noise types. A non-perturbative master equation is derived to account for the dynamics, and the results are confirmed experimentally in a double-fibre ring photonic lattice. The central claim is that a class of bulk states can exhibit greater robustness to this specific noise than topological edge states.

Significance. If substantiated, the finding that bulk states can outperform topological edge states under temporally correlated noise offers a new perspective on noise resilience in Floquet-driven systems, potentially useful for photonic quantum information processing. The non-perturbative master equation and the experimental implementation in a fibre-ring setup are strengths that support the practical relevance of the work.

major comments (2)
  1. The derivation of decoherence-free momentum subspaces and the comparative robustness claim rest on the assumption that temporal noise remains constant within each Floquet period (as stated in the abstract and the noise-model section). This intra-period constancy is load-bearing: if the actual noise process has shorter correlation times, the subspaces disappear and the bulk-versus-edge robustness advantage does not hold. The manuscript should provide either an analytic extension or numerical checks showing the sensitivity of the result to violations of this assumption.
  2. Experimental confirmation section: the double-fibre ring implementation must demonstrate that the applied temporal noise satisfies the constant-within-period condition used in the theory. Without explicit verification of the noise correlation time relative to the discrete step or Floquet period, it is unclear whether the experiment actually probes the regime in which the decoherence-free subspaces are predicted to exist.
minor comments (2)
  1. Figure captions should explicitly label the different noise realizations (constant-within-period versus fully random) and the state types (bulk momentum versus edge) to improve readability.
  2. Notation for the master-equation parameters should be cross-checked against the experimental control parameters for consistency.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the thorough review and valuable feedback on our manuscript arXiv:2510.16204. We address the major comments point by point below and propose revisions to strengthen the presentation of our results on decoherence-free subspaces in noisy quantum walks.

read point-by-point responses

  1. Referee: The derivation of decoherence-free momentum subspaces and the comparative robustness claim rest on the assumption that temporal noise remains constant within each Floquet period (as stated in the abstract and the noise-model section). This intra-period constancy is load-bearing: if the actual noise process has shorter correlation times, the subspaces disappear and the bulk-versus-edge robustness advantage does not hold. The manuscript should provide either an analytic extension or numerical checks showing the sensitivity of the result to violations of this assumption.

    Authors: We agree with the referee that the assumption of temporal noise being constant within each Floquet period is central to the emergence of decoherence-free momentum subspaces. While our non-perturbative master equation is derived under this model, we acknowledge that shorter correlation times would alter the dynamics. In the revised version, we will add numerical checks by simulating noise with varying correlation times shorter than the Floquet period to illustrate the sensitivity and the regime where the subspaces persist. This will provide a clearer picture of the robustness of our findings. revision: yes

  2. Referee: Experimental confirmation section: the double-fibre ring implementation must demonstrate that the applied temporal noise satisfies the constant-within-period condition used in the theory. Without explicit verification of the noise correlation time relative to the discrete step or Floquet period, it is unclear whether the experiment actually probes the regime in which the decoherence-free subspaces are predicted to exist.

    Authors: We thank the referee for highlighting this important aspect of the experimental validation. In our double-fibre ring setup, the temporal noise is implemented by modulating the phase or amplitude in a controlled manner synchronized with the discrete steps. To address this, we will include in the revised manuscript explicit details on the noise generation process, including the measured or designed correlation time, which is engineered to be constant over the Floquet period (spanning multiple steps in the lattice). This verification ensures that the experiment operates in the predicted regime. revision: yes

Circularity Check

0 steps flagged

No circularity: derivation follows from explicit noise model and master equation without self-referential reduction

full rationale

The paper states that decoherence-free momentum subspaces appear in the bulk specifically when temporal noise is constant within each Floquet period, then derives a non-perturbative master equation to describe the dynamics under this condition and contrasts it with fully random noise. This modeling choice is presented as an input assumption rather than a fitted parameter or self-definition; the comparative robustness claim for bulk states versus topological edge states is obtained by applying the same derived equation to both cases. No load-bearing step reduces by construction to a prior self-citation, ansatz smuggled via citation, or renaming of a known result. The experimental implementation in the double-fibre ring setup supplies an independent check outside the analytic derivation, keeping the chain self-contained.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the modeling choice that noise is temporally constant within each Floquet period and on the validity of the non-perturbative master equation for the driven lattice.

axioms (1)
  • domain assumption Temporal noise is constant within each Floquet period for the decoherence-free subspace case.
    This condition is explicitly required in the abstract for the bulk decoherence-free subspaces to appear.

pith-pipeline@v0.9.0 · 5669 in / 1231 out tokens · 39868 ms · 2026-05-18T05:42:51.887185+00:00 · methodology

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