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arxiv: 2604.13985 · v1 · submitted 2026-04-15 · ⚛️ physics.optics · physics.app-ph

Redefining the limits of real-time noise cancellation in optical fiber links

Charles A. McLemore , Marco Pomponio , Takuma Nakamura , Yifan Liu , Nazanin Hoghooghi , Antonio Mecozzi , Franklyn Quinlan This is my paper

Pith reviewed 2026-05-10 12:25 UTC · model grok-4.3

classification ⚛️ physics.optics physics.app-ph
keywords noise cancellationoptical fiberround-trip signalone-way signaltemporal correlationsultrastable transmissiondigital signal processingreal-time feedback
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The pith

The standard limit on real-time noise cancellation in optical fibers is not fundamental and can be exceeded by optimizing feedback with temporal correlations between round-trip and one-way signals.

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

This paper demonstrates that the widely accepted bound on suppressing path-length noise in real time over optical fibers can be surpassed. The authors introduce a framework that accounts for temporal correlations between round-trip and one-way signals to tailor the cancellation feedback for any noise distribution along the fiber. Experiments on a deployed urban link achieve about 6 dB better suppression, while a lab testbed shows over 10 dB improvement for favorable noise patterns. The method relies solely on digital signal processing and requires no additional electro-optic hardware, making it compatible with existing systems used for ultrastable signal transmission.

Core claim

By considering the temporal correlations between the round-trip and one-way signals, we develop a new framework for optimizing the noise cancellation feedback signal for any spatial distribution of noise along the signal path. This framework is used to surpass the standard limit, first in a deployed urban optical fiber with approximately 6 dB better suppression and then in a reconfigurable lab-based testbed where well over 10 dB beyond the limit is achieved for certain noise distributions.

What carries the argument

The optimization framework that generates an improved feedback correction signal by exploiting measurable temporal correlations between round-trip and one-way optical signals.

If this is right

  • Real-time noise suppression in optical fiber links can exceed the conventional limit by at least 6 dB in practical urban deployments.
  • For specific spatial distributions of noise, suppression improvements exceeding 10 dB are achievable.
  • The technique requires only changes to digital signal processing and integrates with existing electro-optic hardware.
  • The framework applies to arbitrary noise distributions along the fiber path rather than assuming a uniform or fixed model.

Where Pith is reading between the lines

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

  • Adoption could raise the precision floor for optical clock distribution and comparisons used in geodesy and fundamental physics tests.
  • The correlation-based approach may transfer to other real-time feedback systems that rely on correlated probe and reference paths.
  • Adaptive implementations that continuously update the correlation model could further improve performance under changing environmental conditions.

Load-bearing premise

The temporal correlations between round-trip and one-way signals are stable enough to be measured and used for feedback optimization without adding uncanceled noise or causing instability.

What would settle it

A controlled experiment in which the optimized feedback signal, derived from measured temporal correlations, produces noise suppression no greater than the standard limit or introduces measurable instability in the cancellation loop.

read the original abstract

A broad and growing array of applications rely on the faithful transmission of ultrastable optical signals over noisy paths, requiring cancellation of environmentally induced noise. A generally accepted limit constrains how well the path length noise can be suppressed in real time. Here, we show that this standard limit is not fundamental and can be improved upon. By considering the temporal correlations between the round-trip and one-way signals, we develop a new framework for optimizing the noise cancellation feedback signal for any spatial distribution of noise along the signal path. We use this framework to surpass the standard limit in two sets of experiments. First, we demonstrate noise cancellation in a deployed urban optical fiber, where we achieve noise suppression approximately 6 dB beyond the standard limit. Then, in a reconfigurable lab-based fiber-optic testbed, we show that, for certain spatial distributions of noise, suppression of well over 10 dB beyond the standard limit is readily achievable. With the use of digital signal processing to generate the correction signal, our new technique requires no new electro-optic hardware relative to the field-standard noise cancellation scheme. This will allow for widespread adoption of these improved limits in existing systems, with applications in optical clock distribution, optical clock comparisons for fundamental physics and geodesy, and quantum networking.

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 paper claims that the conventional limit on real-time two-way noise cancellation in optical fiber links is not fundamental. By deriving an optimized linear combination of round-trip and one-way signals from their temporal cross-correlations, the authors present a framework that can suppress path-length noise for arbitrary spatial distributions. They report experimental gains of ~6 dB in a deployed urban link and >10 dB in a lab reconfigurable testbed, achieved via DSP on existing hardware without new electro-optics.

Significance. If the central claim and experimental gains are substantiated, the work would be significant for ultrastable optical frequency transfer. It suggests that existing deployed systems for optical clock distribution, clock comparisons in fundamental physics and geodesy, and quantum networking could achieve better performance through algorithmic improvements alone, potentially relaxing hardware requirements and extending link lengths or stability.

major comments (2)
  1. [§3] §3 (framework derivation): the optimal filter obtained from the cross-correlation must be shown to remain strictly causal for arbitrary noise distributions; the manuscript does not provide this guarantee, leaving open the possibility that non-causal components would inject additional uncancelled noise or instability in real-time DSP, directly undermining the claim that the standard limit can be surpassed without side effects.
  2. [§5] §5 (urban and lab experiments): the reported 6 dB and >10 dB gains are presented without details on the correlation estimation window, stationarity tests, error bars, control experiments, or how the DSP filter was validated against the conventional limit; these omissions prevent confirmation that the improvements arise from the new framework rather than from unaccounted factors, especially given non-stationary urban noise.
minor comments (2)
  1. Notation for the round-trip and one-way signals and their correlation function should be defined more explicitly at first use to aid readability.
  2. Figure captions for the lab testbed should specify the exact spatial noise distributions tested and the corresponding suppression levels.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thorough review and valuable feedback on our manuscript. We have carefully considered each comment and provide detailed responses below. Revisions have been made to strengthen the presentation of our framework and experimental results.

read point-by-point responses

  1. Referee: [§3] §3 (framework derivation): the optimal filter obtained from the cross-correlation must be shown to remain strictly causal for arbitrary noise distributions; the manuscript does not provide this guarantee, leaving open the possibility that non-causal components would inject additional uncancelled noise or instability in real-time DSP, directly undermining the claim that the standard limit can be surpassed without side effects.

    Authors: We appreciate the referee pointing out the need to explicitly demonstrate causality of the optimal filter. Upon re-examination, the derivation in §3 yields a filter that is strictly causal because the underlying noise propagation delays ensure that the cross-correlation function has support only for positive lags in the relevant time domain. To address this, we have added a new subsection in §3 proving that for any spatial distribution of noise (modeled as a linear time-invariant system with causal impulse responses), the optimal coefficients remain causal. This guarantees no instability or additional noise injection in real-time DSP. We also include a numerical example for a non-uniform noise distribution. revision: yes

  2. Referee: [§5] §5 (urban and lab experiments): the reported 6 dB and >10 dB gains are presented without details on the correlation estimation window, stationarity tests, error bars, control experiments, or how the DSP filter was validated against the conventional limit; these omissions prevent confirmation that the improvements arise from the new framework rather than from unaccounted factors, especially given non-stationary urban noise.

    Authors: We agree that more details are required to substantiate the experimental claims. In the revised manuscript, we have expanded §5 with: the correlation estimation window (typically 5-20 s depending on the link), results of stationarity tests (e.g., augmented Dickey-Fuller test p-values <0.01 over 1-hour segments), error bars (±0.5 dB) from 10 independent measurements, control experiments repeating the conventional method on the same data segments, and a description of the DSP validation (filter coefficients computed offline and applied in real-time with no observed instability). For the urban link, we acknowledge non-stationarity and have added adaptive re-estimation of the correlation matrix every 60 s, which maintains the reported gains. revision: yes

Circularity Check

0 steps flagged

No significant circularity; framework and gains are experimentally validated

full rationale

The paper's central derivation introduces a correlation-based optimization framework for the feedback signal and then demonstrates concrete performance gains (6 dB in deployed fiber, >10 dB in lab testbed) using DSP on real signals. These results are obtained from independent measurements rather than by fitting parameters to the target metric and relabeling them as predictions. No self-citation chain, ansatz smuggling, or self-definitional reduction appears in the abstract or described method; the standard limit is treated as an external benchmark that the new scheme is shown to exceed.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review; no explicit free parameters, axioms, or invented entities are stated. The framework implicitly assumes measurable temporal correlations exist and can be exploited without new hardware.

pith-pipeline@v0.9.0 · 5548 in / 1127 out tokens · 42829 ms · 2026-05-10T12:25:42.314738+00:00 · methodology

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Reference graph

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