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arxiv: 2506.02904 · v3 · pith:RN3LQQ6Ynew · submitted 2025-06-03 · ❄️ cond-mat.mes-hall · cond-mat.supr-con

Quantum thermodynamic uncertainty relation and macroscopic superconducting coherence

Pith reviewed 2026-05-19 11:11 UTC · model grok-4.3

classification ❄️ cond-mat.mes-hall cond-mat.supr-con
keywords thermodynamic uncertainty relationmacroscopic coherencepair amplitudeAndreev regimehybrid normal-superconducting junctionsquantum transportnonequilibrium fluctuationssuperconducting quantum dot
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The pith

Macroscopic superconducting coherence governs departures from the normal quantum thermodynamic uncertainty relation in hybrid N-S devices.

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

The paper establishes that in subgap normal-superconducting junctions, departures from the normal quantum TUR are governed by macroscopic superconducting coherence as quantified by the pair amplitude. A dephasing probe can suppress this coherence to restore the bound. The authors derive a hybrid quantum TUR for two-terminal N-S junctions in the Andreev regime that is never violated, saturates only at vanishing current, and corresponds to the normal bound with charge doubled from e to 2e. Computations confirm that deviations track the pair amplitude in quantum dot and Cooper-pair-splitter systems. This creates a direct link between superconducting coherence and nonequilibrium fluctuations.

Core claim

In hybrid normal-superconducting devices operating in the subgap Andreev regime, departures from the normal quantum TUR are governed by macroscopic superconducting coherence quantified by the pair amplitude. A hybrid quantum TUR is derived that holds generally for two-terminal N-S junctions: it is never violated, saturated only at vanishing current, and obtained from the normal bound by the replacement e to 2e. For N-S quantum dot and Cooper-pair-splitter systems, deviations track the pair amplitude.

What carries the argument

The pair amplitude in the central region, which quantifies macroscopic superconducting coherence and controls deviations from the standard TUR bound.

Load-bearing premise

The derivation assumes the system stays in the subgap Andreev regime where transport is dominated by Andreev processes and that the pair amplitude fully captures the macroscopic coherence modifying the TUR.

What would settle it

Measure current noise and entropy production in an N-S quantum dot in the subgap regime and test whether the relative uncertainty times entropy production deviates from the normal value in proportion to the pair amplitude squared.

Figures

Figures reproduced from arXiv: 2506.02904 by Fabio Taddei, Franco Mayo, Michele Governale, Nahual Sobrino, Rosario Fazio.

Figure 1
Figure 1. Figure 1: FIG. 1. A central region is coupled to a superconducting [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Fano factor as a function of (a) [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. The value of Γ [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Violation of the quantum TUR in the CPS system [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗
read the original abstract

Stability and efficiency are mutually exclusive in a thermodynamic process, e.g. in a thermal machine. Any effort to reduce the fluctuations of a certain output quantity is necessarily accompanied by an increase of entropy production, therefore lowering its efficiency. This interplay is beautifully captured by the so called Thermodynamic Uncertainty Relations (TURs) which set a lower bound on the relative uncertainty of a current for a given rate of entropy production. Their status in hybrid normal-superconducting (N-S) devices has remained unsettled. We show that, in the subgap regime, departures from the normal quantum TUR are governed by {\it macroscopic} superconducting coherence quantified by the pair amplitude, and that introducing a dephasing probe suppresses this coherence and restores the bound. We further derive a hybrid quantum TUR that is general for two-terminal N-S junctions in the Andreev regime: the inequality is never violated, is saturated only at vanishing current, and is related to the normal quantum bound under the replacement (e to 2e). For N-S quantum dot and Cooper-pair-splitter systems we compute current and noise and show that deviations from the normal bound track the pair amplitude on the central region. The results establish a direct link between superconducting macroscopic coherence and nonequilibrium fluctuations and supply a general bound for the Andreev 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

1 major / 2 minor

Summary. The manuscript derives a hybrid quantum thermodynamic uncertainty relation (TUR) for two-terminal normal-superconducting (N-S) junctions operating in the subgap Andreev regime. This bound is claimed to be never violated, saturated only at vanishing current, and obtained from the normal quantum TUR via the replacement e → 2e. The work further argues that departures from the normal TUR are governed by macroscopic superconducting coherence, quantified by the pair amplitude in the central region; this is illustrated through explicit current and noise calculations on an N-S quantum-dot model and a Cooper-pair splitter, with a dephasing probe shown to suppress the pair amplitude and restore the normal bound.

Significance. If the central claims hold, the paper supplies a general fluctuation bound specific to the Andreev regime together with a concrete link between macroscopic coherence and thermodynamic uncertainty. The explicit calculations on two distinct geometries that track the pair amplitude constitute a strength, as they provide falsifiable, model-specific predictions that can be checked against the general bound.

major comments (1)
  1. [General derivation and model calculations] The abstract and introduction present the hybrid TUR as a general derivation for arbitrary two-terminal N-S junctions in the Andreev regime. However, the explicit demonstration that deviations track the pair amplitude is performed only for the quantum-dot and splitter geometries (see the sections containing the scattering-matrix and rate-equation calculations). The general argument appears to rest on charge-2e transport under perfect subgap Andreev reflection; it is not shown how the pair amplitude extracted from the central region enters the inequality itself for arbitrary junction parameters or geometries. This distinction is load-bearing for the claim that departures are 'governed by' macroscopic coherence.
minor comments (2)
  1. Clarify the notation for the pair amplitude (e.g., whether it is the local or integrated quantity) and ensure it is consistently defined when comparing the general bound to the numerical results.
  2. Add a brief discussion of the regime of validity (e.g., temperature, bias, and transparency ranges) under which the e → 2e replacement remains accurate.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful reading and constructive feedback on our manuscript. The distinction between the general hybrid TUR and the model-specific illustration of coherence effects is important, and we address it directly below. We have revised the manuscript to improve clarity on this point without altering the core claims.

read point-by-point responses
  1. Referee: [General derivation and model calculations] The abstract and introduction present the hybrid TUR as a general derivation for arbitrary two-terminal N-S junctions in the Andreev regime. However, the explicit demonstration that deviations track the pair amplitude is performed only for the quantum-dot and splitter geometries (see the sections containing the scattering-matrix and rate-equation calculations). The general argument appears to rest on charge-2e transport under perfect subgap Andreev reflection; it is not shown how the pair amplitude extracted from the central region enters the inequality itself for arbitrary junction parameters or geometries. This distinction is load-bearing for the claim that departures are 'governed by' macroscopic coherence.

    Authors: We agree that a clear separation exists between the general bound and the role of the pair amplitude. The hybrid quantum TUR is derived for arbitrary two-terminal N-S junctions in the perfect Andreev regime by noting that all subgap transport occurs via charge-2e processes; this directly yields the replacement e → 2e in the normal quantum TUR, with the bound saturated only at vanishing current. This step relies solely on the charge quantization and the absence of single-electron tunneling, without reference to a specific geometry or the pair amplitude. In contrast, the statement that departures from the normal TUR are governed by macroscopic superconducting coherence is supported by explicit calculations in two representative models (N-S quantum dot and Cooper-pair splitter). There we compute both the current-noise ratio and the pair amplitude in the central region, demonstrating that larger pair amplitude correlates with stronger violation of the normal bound, while a dephasing probe reduces the pair amplitude and restores the normal TUR. We acknowledge that we do not provide a model-independent expression in which the pair amplitude appears explicitly inside the general inequality for arbitrary junction parameters. Such an expression would require a universal definition of coherence that enters the fluctuation-dissipation relations beyond the 2e charge replacement, which lies outside the scope of the present work. We have revised the abstract, introduction, and concluding sections to state this distinction explicitly, emphasizing that the general bound follows from 2e transport while the quantitative link to coherence is illustrated and falsifiable in concrete geometries. revision: partial

Circularity Check

0 steps flagged

No significant circularity in the hybrid TUR derivation

full rationale

The paper presents a derivation of a hybrid quantum TUR for two-terminal N-S junctions in the Andreev regime, stating that the inequality is never violated, saturated only at vanishing current, and obtained from the normal quantum bound via e to 2e replacement. This is framed as following from scattering or rate expressions under subgap Andreev reflection. The connection between deviations and pair amplitude is shown via explicit current/noise computations and pair-amplitude tracking in specific geometries (N-S quantum dot and Cooper-pair-splitter), but the general bound itself does not reduce by construction to a fitted parameter, self-definition, or load-bearing self-citation. The derivation remains self-contained against external benchmarks with independent mathematical content, consistent with a standard non-circular result.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the validity of the Andreev-regime approximation and the identification of the pair amplitude as the quantifier of macroscopic coherence; no free parameters or new postulated entities are introduced in the abstract.

axioms (1)
  • domain assumption Standard assumptions of mesoscopic quantum transport in the subgap regime
    Invoked to derive the hybrid TUR and to link deviations to the pair amplitude.

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