REVIEW
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.
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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
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
- 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.
The abstract, sentence by sentence
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
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| 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 was examined, and what was not
- arXiv listing abstract v2
- full text
- figures and tables
- references and citing sentences
- relations to other papers
- P abstract
- S sections
- T body sentences
- C citations
- R cross-paper relations
References
How later papers used it
319 later papers cite it
10 later papers by other authors are listed below: 7 used its method or tool, 1 compared its own results with it and 2 cited it as support.
Uses its method or tool 7 of 33 papers with this label
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The tripartite entanglement is quantified by the minimum residual contangle $ℛ_τ^min$ that is defined as [ref] ,
Entanglement engineering in magnomechanical system via cross-Kerr interaction and mechanical parametric amplification (2026) · Quantification of tripartite entanglement · 1.00
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We measure the entanglement of individual spins using the tangle \(τ(x,t)\) [ref] : [equation] which is proportional to the purity \(τ_x = 2 - 2ρ_x^2\).
Entanglement dynamics and ergodicity breaking in a quantum cellular automaton (2022) · Chiral quasiparticles · 1.00
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Used the genuine multipartite entanglement measure originally introduced in this paper to quantify the entanglement in its system.
Strain induced coupling and quantum information processing with hexagonal boron nitride quantum emitters (2021) · 0.98 · paraphrased: the licence does not allow quoting
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Computed a quantum correlation measure based on the monogamy inequality of this paper to classify the generated states.
Manifestation of Rank-Tuned Weak Measurements Towards Featured State Generation (2022) · 0.97 · paraphrased: the licence does not allow quoting
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Following this paper, introduced the measure.
Light-pulse atom interferometry with entangled atom-optical elements (2022) · 0.97 · paraphrased: the licence does not allow quoting
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Chose the 3-tangle of this paper as the entanglement measure.
Entanglement Classification via Single Entanglement Measure (2021) · 0.88 · paraphrased: the licence does not allow quoting
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We start from the definition [ref] , write down all the terms and combine them to obtain the $f_i$s that are finally expressed as a function of the entangling phases.] The explicit formula is given by [equation] where [equation] where $p,q,r∈{1,2,3}$.
Distribution of quantum gravity induced entanglement in many-body systems (2023) · Pairwise entanglement and three-body correlations · 0.88
Compares with it 1 of 9 papers with this label
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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
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Said its RGNP algorithm implies little entanglement between A and B in the optimized state, possible due to the monogamy of entanglement of this paper.
Quantum mutual information redistribution by Number Partitioning algorithm (2023) · 0.97 · paraphrased: the licence does not allow quoting
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Noted that no entanglement will be present in the other teleportation channels due to the monogamy of entanglement of this paper.
Generalized Teleportation Fidelity and Singlet Fraction and their Relation for (In)-distinguishable Particles and Its Applications (2024) · 0.95 · paraphrased: the licence does not allow quoting
How the literature uses it
70 arXiv papers cite this in 103 sentences
Papers citing it · 63 sentences without a confident label
Contests it 1 paper
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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
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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
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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
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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
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Its persistence at large $r$—$h(A:B)→0.75$ for Bell and $0.16$ for GHZ—signals genuine three-party structure among $A,B,$, paralleling the fermionic finding that the Markov gap detects correlations invisible to the residual tangle [ref] .
Surviving correlations across a horizon: reflected entropy for bosonic fields in non-inertial frames and black hole spacetimes · Markov gap Results · 0.91
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In figure [ref] we show the correlation of the ladder $τ_3$ ([ref]) with the measure of triparite entanglement known as 3-tangle [ref] .
Connectomes and Properties of Quantum Entanglement · Measures of entanglement · 0.98
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We point out that <ref> (i) is reminiscent of the Coffman-Kundu-Wootters inequality [ref] .
Monogamy of correlations and entropy inequalities in the Bloch picture · Monogamy of Correlations from the Bloch Picture · 1.00
Cites it as support 3 papers
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The two-body entanglement is not enough to describe the tripartite entanglement [ref] .
Tripartite Entanglement and Quantum Correlation · Introduction · 0.97
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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
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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
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In quantum theory, quantum systems are currently modelled by density matrices $(ρ)$ and entanglement is recognized to be monogamous [ref] .
A paradox regarding monogamy of entanglement · Introduction · 0.95
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In this scenario, monogamy of entanglement [ref] typically has been discussed in terms of the paradox in [ref] , as commented on in [ref] .
Replica wormhole and island incompatibility with monogamy of entanglement · Introduction · 0.99
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The three-tangle [ref] is a genuine tripartite entanglement measure for three-qubit pure states [ref] .
Separability from Multipartite Measures · Tripartite Entanglement Measures · 0.94
Discussion
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