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Concentrating Partial Entanglement by Local Operations

Quantum Physics · Submitted 21 Nov 1995 · 12 references

Pith's verdict: The founding entanglement-concentration paper: the central claim is correct and has held for thirty years, with only minor exposition gaps in the standardization lemma and a couple of stray factors.

T0 · AI review

How this review was made: An AI referee (selfhosted/deepseek-v4.1-flash) read the paper on November 11, 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.

  1. T0 AI review An AI read the paper and answered a fixed list of questions.
  2. T1 Audit Automatic checks that the references and files the paper relies on exist and match.
  3. T2 Compute The paper's code or calculations re-run with the same results.
  4. T3 Formal statement The claims written out in a formal math language a computer can read.
  5. T4 Proof Those claims proved, and the proof checked by computer.

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· 1 major objection · 9 minor · Ask about this paper · The compiled record · Read the full review →
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arXiv IDquant-ph/9511030 (version 1)
Pith IDJEDVRFFA, Pith's permanent record of this paper and its review
CitationBenjamin Schumacher, Charles H. Bennett, Herbert J. Bernstein, Sandu Popescu. "Concentrating Partial Entanglement by Local Operations." arXiv:quant-ph/9511030 (1995). https://pith.science/paper/JEDVRFFA
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Claim paper

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Referee report
The paper

The paper claims

Entanglement can be concentrated: many partly entangled pairs yield nearly as many perfect singlets as their entropy allows.

The reading

The paper asks whether partly entangled pure states are a usable currency for quantum tasks, or whether only perfect singlets count. It shows that two separated observers who share $n$ identical copies of a pure bipartite state can, by local measurements and classical messages, concentrate the entanglement into a number of perfect singlets approaching $nE-O(��log n)$, where $E$ is the entropy of entanglement of one copy. The proposed Schmidt projection measures how many of the $n$ pairs occupy the excited Schmidt branch, leaving a maximally entangled state in a known subspace; a second batching step converts that state into standard singlets. The converse is also argued: singlets plus classical communication can prepare any desired bipartite pure or mixed state, so for pure states entropy of entanglement is the asymptotic exchange rate. The result matters because it turns entanglement into a quantitative resource that can be concentrated, diluted, and spent in teleportation or dense coding.

What carries the argument

Schmidt projection is the central mechanism: Alice measures the total number $k$ of excited Schmidt branches across all $n$ pairs, not which pairs, projecting onto the subspace with common coefficient $��cos^{n-k}��theta��sin^k��theta$; the residual state is maximally entangled in dimension $��binom{n}{k}$. Because the distribution of $k$ has entropy $O(��log n)$, almost all of the original $nE$ entanglement survives. The standardization stage multiplies dimensions $��binom{n}{k_j}$ over batches until the product lies just above a power of two, then projects onto a $2^��ell$-dimensional subspace; local unitary rotations turn that into $��ell$ singlets. The Procrustean filter handles one pair at a time by absorbing a fraction $��tan^2��theta$ of the more probable branch.

Extended reading notes

Core claim

For any bipartite pure state $|��(A,B)��=��_i c_i|��_i��|��_i��$, the entropy of entanglement $E=-��_i c_i^2��log_2 c_i^2$ is asymptotically conserved by local operations and classical communication: $n$ copies can be converted into $nE-O(��log n)$ maximally entangled pairs, and conversely singlets can be diluted into copies of any bipartite state at the same asymptotic rate. The paper establishes this by Schmidt projection, which projects the $n$-copy state onto a subspace of fixed excitation number $k$ and leaves a maximally entangled state of dimension $��binom{n}{k}$, followed by a standardization step that groups such states until their total dimension is close to a power of two. It also gives a single-copy Procrustean method that probabilistically converts one partly entangled pair into one perfect singlet by filtering the larger Schmidt coefficient.

Load-bearing premise

The central claim depends on the unproved standardization lemma that a random walk on the unit interval with steps drawn from the binomial-coefficient distribution will, with probability one, come within a factor $1+��epsilon$ of a power of two, a lemma the paper calls elementary and states for arbitrary step distributions even though that general form fails for commensurate steps.

Original abstract

If two separated observers are supplied with entanglement, in the form of $n$ pairs of particles in identical partly-entangled pure states, one member of each pair being given to each observer; they can, by local actions of each observer, concentrate this entanglement into a smaller number of maximally-entangled pairs of particles, for example Einstein-Podolsky-Rosen singlets, similarly shared between the two observers. The concentration process asymptotically conserves {\em entropy of entanglement}---the von Neumann entropy of the partial density matrix seen by either observer---with the yield of singlets approaching, for large $n$, the base-2 entropy of entanglement of the initial partly-entangled pure state. Conversely, any pure or mixed entangled state of two systems can be produced by two classically-communicating separated observers, drawing on a supply of singlets as their sole source of entanglement.

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, 3,208 input tokens.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.21 · human participants 0.02 · controlled comparison 0.02 · randomized assignment 0.01 · states quantitative result 0.80 · compares with prior results 0.09 · mentions code or data availability 0.01.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

If two separated observers are supplied with entanglement, in the form of $n$ pairs of particles in identical partly-entangled pure states, one member of each pair being given to each observer; they can, by local actions of each observer, concentrate this entanglement into a smaller number of maximally-entangled pairs of particles, for example Einstein-Podolsky-Rosen singlets, similarly shared between the two observers. The concentration process asymptotically conserves {\em entropy of entanglement}---the von Neumann entropy of the partial density matrix seen by either observer---with the yield of singlets approaching, for large $n$, the base-2 entropy of entanglement of the initial partly-entangled pure state. Conversely, any pure or mixed entangled state of two systems can be produced by two classically-communicating separated observers, drawing on a supply of singlets as their sole source of entanglement.

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 If two separated observers are supplied with entanglement, in the form of $n$ pairs of particles in identical partly-entangled pure states, one member of each pair being given to each observer; they can, by local actions of each observer, concentrate this entanglement into a smaller number of maximally-entangled pairs of particles, for example Einstein-Podolsky-Rosen singlets, similarly shared between the two observers. contribution 0.59 0.60 0.59 hedged possibility 0.55
s1 The concentration process asymptotically conserves {\em entropy of entanglement}---the von Neumann entropy of the partial density matrix seen by either observer---with the yield of singlets approaching, for large $n$, the base-2 entropy of entanglement of the initial partly-entangled pure state. result 0.99 0.80 0.40 asserted 0.99
s2 Conversely, any pure or mixed entangled state of two systems can be produced by two classically-communicating separated observers, drawing on a supply of singlets as their sole source of entanglement. result 0.83 0.44 0.19 asserted 1.00

What kind of paper the classifier reports this is: theoretical result 0.83 · evidence from proof or derivation 0.98.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
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

362 later papers cite it

Evidence: substantial. 362 of the citing papers share no author with it, and 31 of those carry a confident label from Pith's citation classifier.

Pith read 40 of the 499 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

Builds on it 1 of 4 papers with this label
Uses its method or tool 3 of 16 papers with this label
How the literature uses it

68 arXiv papers cite this in 79 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 whose main result relates to this one's

Found among the papers closest to this one by title and main claim. Pith's classifier compared each pair's main results and a second model family confirmed the relation; checked against two further families, shown conflicts agreed 118 times in 120 and shown same results 218 times in 239. The quoted sentence is the other paper's main claim.

States the same result 1 paper
  • If two parties, Alice and Bob, share some number, n, of partially entangled pairs of qubits, then it is possible for them to concentrate these pairs into some smaller number of maximally entangled states.

    Quantum Networks for Concentrating Entanglement · 0.94

Papers citing it · 38 sentences without a confident label

Builds on it 1 paper
  • Now let us consider an interpretation of pseudo entropy from the view point of quantum entanglement by extending the well-known argument [ref] for the entanglement entropy.

    Holographic Pseudo Entropy · Interpretation via Quantum Entanglement · 0.99

Uses its method or tool 2 papers
  • Now, the idea of forming $n$ approximate copies $ψ_A^'B^' ^ω$ is then the same as it is in [ref] : Alice prepares the state $ψ_A^'nB^'n^ω$ locally, Alice and Bob require beforehand a maximally entangled state of Schmidt rank no larger than $2^n[ H(B^')_ψ^ω+δ]$, and then they perform quantum teleportation [ref] to teleport the $B^'n$ system to Bob.

    Entanglement cost and quantum channel simulation · Upper bound on the entanglement cost of teleportation-simulable channels with bosonic Gaussian resource states · 0.91

  • The entanglement entropy $S_EE(r)$, determined by the Von Neumann entropy on the reduced density matrix of one of the subsystems, is defined by [ref] [equation] The required $2×2$ reduced density matrix of the state $|ψ_e(r)⟩$ can be expressed as [equation] where $λ_1(r)$ and $λ_2(r)$ are the two nonzero eigenvalues of $ρ_e(r)$.

    Probing the quantum speed limit and entanglement in flavor oscillations of neutrino-antineutrino system in curved spacetime · Entanglement in two-flavor oscillations of the neutrino-antineutrino system in curved spacetime · 0.90

Compares with it 1 paper
  • Now, in known dilution protocols, the amount of entanglement does not fluctuate, while the original entanglement concentration [ref] protocol, is one where the final amount of entanglement does fluctuate, but probabilistically rather than coherently.

    Entanglement fluctuation theorems · Probabilistic BLOCC transformations and full reversibility of entanglement dilution and concentration · 0.91

Cites it as background 35 papers

Discussion

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