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arxiv: 2605.08923 · v1 · submitted 2026-05-09 · 🪐 quant-ph

Entanglement increase from local interactions which lead to non-positive local reduced dynamics

Pith reviewed 2026-05-12 02:13 UTC · model grok-4.3

classification 🪐 quant-ph
keywords entanglementlocal interactionsnon-positive mapsreduced dynamicsbipartite systemquantum channelsentanglement increaselocal environments
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The pith

Local interactions can increase entanglement in a bipartite quantum system when reduced dynamics are non-positive maps.

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

A bipartite quantum system has each part interacting only with its own local environment. Normally this keeps entanglement from growing if the reduced dynamics are completely positive trace-preserving maps, that is, quantum channels. The paper shows that the reduced dynamics can instead take the form of a product of two local non-positive maps. In those cases entanglement between the parts can exceed its initial value. The work maps out the general conditions for such behavior and supplies a new general construction that produces the required non-positive maps.

Core claim

For a bipartite system AB with purely local interactions, the reduced dynamics on the system can be given by a map of the form Ψ_A ⊗ Ψ_B where each Ψ is a non-positive linear map rather than a quantum channel. Under this condition the entanglement between A and B can increase beyond its starting value, as first illustrated in the Jordan et al. example; the paper identifies the general circumstances under which this occurs and introduces a further systematic procedure that generates additional families of such local non-positive maps.

What carries the argument

The tensor-product map Ψ_A ⊗ Ψ_B formed from two local non-positive maps that nevertheless arise from purely local system-environment interactions.

If this is right

  • Entanglement can exceed its initial value in any system whose reduced dynamics match the Jordan et al. example.
  • General families of local non-positive maps exist that produce the same entanglement increase.
  • A new constructive procedure yields further local non-positive maps with the same property.
  • The usual expectation that local interactions cannot increase entanglement fails precisely when the reduced map is non-positive.

Where Pith is reading between the lines

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

  • Protocols that assume entanglement is non-increasing under local noise may need to verify that the actual reduced dynamics remain completely positive.
  • Distinguishing physical local evolutions from abstract non-positive maps becomes experimentally relevant for entanglement control.
  • The same mechanism could be checked in open multipartite systems to see whether non-positive local reductions produce similar growth.

Load-bearing premise

The overall evolution can still be produced by local interactions with separate environments even when the reduced dynamics on the system are given by a product of non-positive maps.

What would settle it

An explicit model of local interactions on a bipartite system whose reduced dynamics turn out to be completely positive trace-preserving maps, together with a numerical check that entanglement never exceeds its initial value.

Figures

Figures reproduced from arXiv: 2605.08923 by Iman Sargolzahi.

Figure 1
Figure 1. Figure 1: FIG. 1. One way for performing physically the non-positive localized map [PITH_FULL_IMAGE:figures/full_fig_p006_1.png] view at source ↗
read the original abstract

Consider a bipartite quantum system S=AB such that each part interacts only with its local environment. Under such circumstances, one expects that the entanglement between parts A and B does not exceed its initial value during the time evolution. In fact, this is the case if the reduced dynamics of the system is given by $\mathcal{E}_{A}\otimes \mathcal{E}_{B}$, where $\mathcal{E}_{A}$ and $\mathcal{E}_{B}$ are quantum channels, i.e., completely positive trace-preserving maps. But, the reduced dynamics of the system may be given by a map as $\Psi_{A}\otimes \Psi_{B}$, where $\Psi_{A}$ and $\Psi_{B}$ are local non-positive maps. Then, the entanglement between A and B can exceed its initial value, as was shown in the case studied by Jordan et al. [Phys. Rev. A 76, 022102 (2007)]. In this paper, we first explore the general circumstances under which one can find such cases as they found. Next, we introduce another general procedure which leads to local non-positive maps that cause entanglement exceeding.

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 considers a bipartite system AB where each subsystem interacts locally with its own environment. It argues that while completely positive trace-preserving reduced dynamics (quantum channels) preserve the no-increase property for entanglement, the reduced dynamics can instead take the form of a tensor product of non-positive maps Ψ_A ⊗ Ψ_B. Under this condition entanglement between A and B can exceed its initial value. The authors first characterize the general circumstances in which such non-positive local maps arise from local interactions (generalizing the Jordan et al. 2007 example), and then present an explicit general construction that produces families of such maps leading to entanglement growth.

Significance. If the central construction is valid and the resulting maps are shown to arise from unitary evolution on the full system-environment space with only local interactions, the result supplies a systematic method for generating counter-examples to the expectation that local open-system dynamics cannot increase bipartite entanglement. This would be relevant to the study of non-Markovianity, initial system-environment correlations, and the boundary between positive and completely-positive maps in open quantum systems.

major comments (2)
  1. [§3] §3 (general procedure): the claim that the reduced dynamics is given by a fixed linear map Ψ_A ⊗ Ψ_B that can be applied to arbitrary initial ρ_AB is not demonstrated. When the non-positivity originates from initial A-E_A (B-E_B) correlations, the partial trace after the local unitaries U_A(t) ⊗ U_B(t) yields an output that depends on the specific form of those correlations; hence the map is not a dynamical map on the system alone. The entanglement-increase calculation must be performed on the actual physical reduced state rather than by formally applying the non-positive map to an arbitrary input.
  2. [§2.2] §2.2 (characterization of Jordan-type cases): the conditions under which the reduced map becomes non-positive are stated in terms of the initial joint state, but no explicit verification is given that the resulting time-evolved ρ_AB(t) indeed violates the entanglement bound while remaining consistent with a global unitary evolution on AB E_A E_B. A concrete numerical or analytic example with the full Hilbert-space evolution would be required to confirm the claim.
minor comments (2)
  1. [Introduction] Notation for the non-positive maps is introduced as Ψ_A, Ψ_B without an explicit statement of the domain (Hermitian operators, trace-preserving or not) and without a comparison to the standard definition of positive maps used in the literature.
  2. [Abstract] The abstract states that the reduced dynamics “may be given by” Ψ_A ⊗ Ψ_B; this phrasing should be clarified to indicate under which initial conditions the equality holds.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and for the constructive major comments. We agree that additional clarifications and explicit verifications will strengthen the presentation. We address each point below and will incorporate the suggested revisions in the next version of the manuscript.

read point-by-point responses
  1. Referee: [§3] §3 (general procedure): the claim that the reduced dynamics is given by a fixed linear map Ψ_A ⊗ Ψ_B that can be applied to arbitrary initial ρ_AB is not demonstrated. When the non-positivity originates from initial A-E_A (B-E_B) correlations, the partial trace after the local unitaries U_A(t) ⊗ U_B(t) yields an output that depends on the specific form of those correlations; hence the map is not a dynamical map on the system alone. The entanglement-increase calculation must be performed on the actual physical reduced state rather than by formally applying the non-positive map to an arbitrary input.

    Authors: We agree with the referee that the effective maps Ψ_A ⊗ Ψ_B arising from initial system-environment correlations are not universal dynamical maps that can be applied to arbitrary initial states ρ_AB. In the manuscript, these maps are derived specifically for the class of initial joint states that encode the relevant A-E_A and B-E_B correlations. The entanglement increase is shown for states belonging to this class. To address the concern, we will revise §3 to explicitly clarify that the construction yields effective (non-positive) maps valid only for the considered family of initial conditions, and we will recompute the entanglement measures directly from the reduced states obtained via the global unitary evolution on the full AB E_A E_B space rather than by formal application of the maps to arbitrary inputs. This will ensure physical consistency is transparent. revision: yes

  2. Referee: [§2.2] §2.2 (characterization of Jordan-type cases): the conditions under which the reduced map becomes non-positive are stated in terms of the initial joint state, but no explicit verification is given that the resulting time-evolved ρ_AB(t) indeed violates the entanglement bound while remaining consistent with a global unitary evolution on AB E_A E_B. A concrete numerical or analytic example with the full Hilbert-space evolution would be required to confirm the claim.

    Authors: We thank the referee for highlighting this point. While §2.2 provides a general characterization of the conditions leading to non-positive reduced maps in the Jordan-type setting, we acknowledge that an explicit verification with the full Hilbert-space evolution would make the result more convincing. In the revised manuscript we will add a concrete numerical example (with small-dimensional Hilbert spaces) that specifies the initial joint state of A B E_A E_B, the local unitaries, the global unitary evolution, the resulting reduced ρ_AB(t), and direct computation of an entanglement monotone (e.g., negativity) showing an increase, while confirming that the global evolution remains unitary. This will explicitly demonstrate consistency with the physical setup. revision: yes

Circularity Check

0 steps flagged

No circularity: derivation relies on external reference and explicit construction

full rationale

The paper's core argument starts from the standard fact that local CPTP maps preserve entanglement bounds and then contrasts this with the possibility of non-positive local maps arising from initial system-environment correlations. It cites Jordan et al. (external, 2007) for the concrete example and proceeds by exploring general conditions and constructing another procedure. No quantity is defined in terms of the target entanglement increase, no fitted parameter is relabeled as a prediction, and no load-bearing step reduces to a self-citation or ansatz smuggled from the authors' prior work. The derivation chain remains self-contained against the external benchmark of open-system quantum mechanics.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The work rests on standard quantum mechanics of open systems and the definition of reduced dynamics; no free parameters, ad-hoc axioms, or new invented entities are introduced in the abstract.

axioms (2)
  • domain assumption Reduced dynamics of a bipartite system interacting locally with separate environments can be expressed as a tensor product of local maps.
    Invoked in the abstract when stating that the reduced dynamics may be given by Ψ_A ⊗ Ψ_B.
  • standard math Completely positive trace-preserving maps cannot increase entanglement.
    Standard result in quantum information used as the baseline expectation.

pith-pipeline@v0.9.0 · 5495 in / 1085 out tokens · 32412 ms · 2026-05-12T02:13:03.698209+00:00 · methodology

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Reference graph

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