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Distributed Entanglement

Quantum Physics · Submitted 13 Jul 1999 · 20 references

Pith's verdict: The CKW paper: the derivation is correct, the tightness family really does cover the whole boundary, and the flaws are typographic rather than mathematical.

T0 · AI review

How this review was made: An AI referee (selfhosted/deepseek-v4.1-flash) read the paper on October 29, 2026 and answered a fixed list of questions about it. T0 is the starting level every paper gets. The levels describe how deeply Pith has checked the paper, never the authors.

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arXiv IDquant-ph/9907047 (version 2)
Pith IDV32LWRWU, Pith's permanent record of this paper and its review
CitationJoydip Kundu, Valerie Coffman, William K. Wootters. "Distributed Entanglement." arXiv:quant-ph/9907047 (1999). https://pith.science/paper/V32LWRWU
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Referee report
The paper

The paper claims

For three qubits, the amount of entanglement one qubit shares with a second plus what it shares with a third can never exceed what it shares with the pair — and the leftover is a genuinely three-way quantity.

The reading

Entanglement is a resource that cannot be handed out freely: if qubit A is perfectly entangled with qubit B, it has nothing left over for qubit C. This paper turns that folklore into an exact accounting rule for three qubits. Using a measure called the tangle — the square of the concurrence, which is a monotone function of the entanglement of formation — the authors prove that $τ_{AB} + τ_{AC} ≤ τ_{A(BC)}$, where $τ_{A(BC)} = 4$ det $ρ_A$ is the tangle between A and the pair BC treated as a single two-level object. They then show the bound cannot be improved: a one-excitation family of states saturates it for every allowed triple of values, and any alternative measure that also saturates it must coincide with the tangle. The difference between the two sides is the part of A's entanglement with BC that is not owed to B or C separately, and it turns out to be symmetric in all three qubits, so it deserves the name three-way tangle.

What carries the argument

The tangle, $τ_{AB} = [max{λ_1 - λ_2 - λ_3 - λ_4, 0}]^2$, built from the square roots of the eigenvalues of $ρ_{AB} ρ̃_{AB}$ with $ρ̃_{AB} = (σ_y ⊗ σ_y) ρ^*_{AB} (σ_y ⊗ σ_y)$ the spin-flipped density matrix. Two identities carry the argument. First, for a pair inside a pure three-qubit state, $τ_{AB} ≤ Tr(ρ_{AB} ρ̃_{AB})$, since the trace equals $λ_1^2 + λ_2^2$ while the tangle is $(λ_1 - λ_2)^2$. Second, the two traces sum to $4$ det $ρ_A$, obtained by rewriting the amplitude sum using $ε_{nn'} ε_{j'j} = δ_{nj'} δ_{n'j} - δ_{nj} δ_{n'j'}$ and expressing the result as $2( $det $ρ_A + $det $ρ_B - $det $ρ_C)$ plus its symmetric partner. The residual, computed as the determinant of the restriction of $ρ_{AB} ρ̃_{AB}$ to its two-dimensional range, is the three-way tangle.

Extended reading notes

Core claim

The paper establishes a monogamy relation for three qubits in a pure state: $τ_{AB} + τ_{AC} ≤ τ_{A(BC)}$, with equality achievable for any consistent set of values. The proof runs by bounding each pairwise tangle from above by $Tr(ρ_{AB} ρ̃_{AB})$, where $ρ̃$ is the spin-flipped density matrix, and then evaluating the sum of the two traces in closed form as $4$ det $ρ_A$ — the tangle of A with the pair BC. Because the state of ABC is pure, the pair BC lives effectively in two dimensions, so the tangle between A and BC is well defined even though BC has a four-dimensional state space. The authors then identify the gap $τ_{A(BC)} - τ_{AB} - τ_{AC}$ as a residual that can be written explicitly in terms of the eight amplitudes $a_{ijk}$ as $4|d_1 - 2d_2 + 4d_3|$, where the three terms collect products of four amplitudes whose indices sit at the corners of a cube in three characteristic configurations. That residual is unchanged under permutations of the qubits, giving the symmetric decomposition $τ_{A(BC)} = τ_{AB} + τ_{AC} + τ_{ABC}$; for the GHZ state this reads $1 = 0 + 0 + 1$. A mixed-state version, $τ_{AB} + τ_{AC} ≤ τ^{min}_{A(BC)}$, follows by applying the pure-state inequality to each member of an optimal decomposition and using convexity of the tangle. The authors close with a conjecture that the analogous inequality holds for any number of qubits, backed by an exact equality for the $n$-qubit one-excitation states.

Load-bearing premise

The inequality itself is proved by algebra on the tangle, so its force as a statement about entanglement rests on the tangle — the squared concurrence — being a faithful stand-in for the entanglement of formation, an identification imported from earlier work rather than re-derived here; the mixed-state version additionally leans on the tangle being convex.

Editorial extensions

If this is right

  • Any protocol that requires qubit A to be entangled with two separate partners must budget against a hard ceiling set by A's entanglement with the pair, so a fully entangled pair leaves zero for a third party.
  • The residual $τ_{ABC}$ gives a permutation-invariant number that separates genuine three-way entanglement from pairwise entanglement: GHZ states score 1 with all pairwise tangles zero, while the one-excitation states of the form $α|100⟩ + β|010⟩ + γ|001⟩ score zero.
  • The entanglement of formation does not obey the same inequality — the paper's example gives $E_{AB} = E_{AC} = 0.601$ against $E_{A(BC)} = 1$ — so accounting in ebits and accounting in tangle give different pictures of what a single qubit can afford to share.
  • For mixed states the bound survives in convex-roof form, $τ_{AB} + τ_{AC} ≤ τ^{min}_{A(BC)}$, so the trade-off is not an artifact of purity.
  • The authors conjecture, with exact equality verified for $n$-qubit one-excitation states, that $τ_{12} + τ_{13} + ⋯ + τ_{1n} ≤ τ_{1(23…n)}$ holds for all pure states of $n$ qubits.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Read as a resource-accounting statement, the inequality says that entanglement with a group is the total budget and each individual partner draws from it — a picture that suggests treating the three-way tangle as the part of the budget that can only be spent on genuinely tripartite tasks, such as GHZ-correlated protocols, and never on bipartite ones.
  • The failure of the entanglement of formation to satisfy the same inequality while remaining a monotone function of the tangle is a concrete case where two measures that agree on ordering disagree on how a resource decomposes; it would be worth checking whether other convex-roof measures show the same split.
  • The permutation invariance of $τ_{ABC}$ means the quantity can be computed from any qubit's point of view, which makes it a candidate invariant for classifying three-qubit states under local unitary transformations, alongside the quantities catalogued in the invariant-theory literature the paper cites.
  • The natural test of the paper's conjecture is a numerical search over pure four-qubit states for a violation of $τ_{12} + τ_{13} + τ_{14} ≤ τ_{1(234)}$; the one-excitation family only confirms the equality case, not the inequality's general validity.
Original abstract

Consider three qubits A, B, and C which may be entangled with each other. We show that there is a trade-off between A's entanglement with B and its entanglement with C. This relation is expressed in terms of a measure of entanglement called the "tangle," which is related to the entanglement of formation. Specifically, we show that the tangle between A and B, plus the tangle between A and C, cannot be greater than the tangle between A and the pair BC. This inequality is as strong as it could be, in the sense that for any values of the tangles satisfying the corresponding equality, one can find a quantum state consistent with those values. Further exploration of this result leads to a definition of the "three-way tangle" of the system, which is invariant under permutations of the qubits.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 4,914 input tokens.

What the abstract itself states, as probabilities: claims first 0.04 · states limitation 0.16 · human participants 0.01 · controlled comparison 0.02 · randomized assignment 0.01 · states quantitative result 0.85 · compares with prior results 0.08 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v2. Results and the main claim are underlined; .

Consider three qubits A, B, and C which may be entangled with each other. We show that there is a trade-off between A's entanglement with B and its entanglement with C. This relation is expressed in terms of a measure of entanglement called the "tangle," which is related to the entanglement of formation. Specifically, we show that the tangle between A and B, plus the tangle between A and C, cannot be greater than the tangle between A and the pair BC. This inequality is as strong as it could be, in the sense that for any values of the tangles satisfying the corresponding equality, one can find a quantum state consistent with those values. Further exploration of this result leads to a definition of the "three-way tangle" of the system, which is invariant under permutations of the qubits.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 Consider three qubits A, B, and C which may be entangled with each other. background 0.99 0.06 0.02 hedged possibility 0.98
s1 We show that there is a trade-off between A's entanglement with B and its entanglement with C. result 0.87 0.94 0.81 asserted 1.00
s2 This relation is expressed in terms of a measure of entanglement called the "tangle," which is related to the entanglement of formation. method 0.61 0.41 0.06 asserted 1.00
s3 Specifically, we show that the tangle between A and B, plus the tangle between A and C, cannot be greater than the tangle between A and the pair BC. result 1.00 0.94 0.85 asserted 1.00
s4 This inequality is as strong as it could be, in the sense that for any values of the tangles satisfying the corresponding equality, one can find a quantum state consistent with those values. result 0.67 0.90 0.13 asserted 0.98
s5 Further exploration of this result leads to a definition of the "three-way tangle" of the system, which is invariant under permutations of the qubits. contribution 0.62 0.91 0.07 asserted 0.98

What kind of paper the classifier reports this is: theoretical result 1.00 · evidence from proof or derivation 1.00.

What was examined, and what was not

sources read
  • arXiv listing abstract v2
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

References
How later papers used it

319 later papers cite it

Evidence: substantial. 319 of the citing papers share no author with it, and 53 of those carry a confident label from Pith's citation classifier.

Pith read 39 of the 502 citing sentences, choosing only those whose citation markers point at this paper alone, and wrote the paraphrases from them. The summary, the counts, the evidence level, the links and the quotes come from code; the summary counts the papers listed below. A later paper is listed under a label when Pith's citation classifier gives one of its sentences that label at probability 0.8 or more (0.95 for support), Pith's reading finds that the citing paper itself did it, and a second check confirms the paraphrase against the sentence. This section lists uses only. It does not list criticism or disagreement, so their absence here says nothing about how the paper has been received. Papers that share an author with this one are listed apart, under "By the same authors". A quote is the citing authors' own sentence, shown when their paper's licence is CC BY, CC BY-SA or CC0. JSON

Uses its method or tool 7 of 33 papers with this label
Compares with it 1 of 9 papers with this label
  • Our results reveal the polygamous nature of distributed quantum incompatibility, in stark contrast to the monogamy of entanglement [ref] and coherence [ref] across subsystems of multipartite quantum states.

    Distributed quantum incompatibility (2023) · Introduction · 0.97

Cites it as support 2 of 8 papers with this label
How the literature uses it

70 arXiv papers cite this in 103 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers citing it · 63 sentences without a confident label

Contests it 1 paper
  • Let us also mention here that there is a well-known difficulty with the approach of Page and of the work reviewed in [ref] which is that, by monogamy [ref] , the late emitted particles cannot simultaneously purify the early emitted particles and also be entangled (as one might expect them to be on a semiclassical understanding) with their partner-quanta which fell through the horizon.

    The black hole information loss puzzle, matter-gravity entanglement entropy and the second law · Introduction · contest 0.71

Uses its method or tool 4 papers
  • For the final state in Eq. [ref], the three single-qubit reduced density matrices are defined by [equation] For a pure three-qubit state, the one-versus-rest concurrence between subsystem $X$ and the remaining two-qubit system $Y$ is [equation] We then define the three squared concurrences as [equation] and regard $a,b,c$ as the three sides of the concurrence triangle [ref] .

    Genuine Tripartite Entanglement Selects Gauge-Invariant Theories · Concurrence fill · 0.92

  • The properties and peculiarities of the three-body system can be analyzed by means of the monogamy inequality [ref] C_i(kj)^2 ≤C_ij^2+C_ik^2 and the genuine multiparticle entanglement quantified by the concurrence triangle given by [ref] F_3= 4/√(3) [ Q (Q-C_1(23)) (Q-C_2(13)) (Q-C_3(12)) ]^1/2 with $F_3$ takes values from 0 and 1.

    Quantum entanglement and Bell inequality violation at colliders · Three-body decays · 0.86

  • The W-type entanglement can be detected with the help of 3-tangle $τ_ABC$ introduced in [ref] .

    Entanglement Classification from a Topological Perspective · Tripartite entanglement · 0.90

Compares with it 4 papers
Cites it as support 3 papers
  • The two-body entanglement is not enough to describe the tripartite entanglement [ref] .

    Tripartite Entanglement and Quantum Correlation · Introduction · 0.97

  • The appearance of 3-body entanglement quantity implies that the 2-body entanglement quantities are not enough [ref] .

    Non-Locality$\neq$Quantum Entanglement · Entanglement Measures · 0.98

  • Given this distinguishing ability, it is perhaps unsurprising that D-CTC circuits are also capable of cloning arbitrary quantum states [ref] (though, of course, entanglement is not cloned as this is forbidden by monogamy [ref] ).

    Reality, Causality, and Quantum Theory · Overview of D-CTCs · 0.96

Cites it as background 23 papers

Discussion

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