slug: quinta-andre-burchardt-zyczkowski-2019-m-resistant-states
bibkey: quinta2019resistant
doi: 10.1103/PhysRevA.100.062329
url: https://arxiv.org/abs/1908.10390v1
triage: theorem
motivation_gids:
  - D5/S3/Quantum/Entanglement/MResistantQuditExistence.result

M-resistant qudit states exist for every admissible loss parameter

Problem

Quinta, André, Burchardt and Życzkowski, Cut-resistant links and multipartite entanglement resistant to particle loss, Phys. Rev. A 100, 062329 (2019), arXiv:1908.10390v1, Section V, state:

Conjecture. For any $N$ and $m$, there exists an $m$-resistant $N$-qudit state in some local dimension $d$.

Their Section III definition says that an entangled state remains entangled after tracing out any $m$ subsystems and becomes separable after tracing out any $m+1$ subsystems. This dossier uses finite full separability for the separable predicate and the admissible range $N\ge 3$, $m+2\le N$.

Statement echo

The Lean claim quantifies all natural $N,m$ with $3\le N$ and $m+2\le N$, then supplies a natural local dimension $d$, a pure amplitude function on $(\mathrm{Fin}N)\to \mathrm{Fin}d$, unit norm by the finite sum of Complex.normSq, and IsMResistant m ψ. IsMResistant is the conjunction of initial failure of full separability, failure after every loss set of cardinality $m$, and full separability after every loss set of cardinality $m+1$. The pure-state convention is the frozen rankOneDensity declaration.

Motivation

The source leaves existence in some local dimension open for the full admissible range. Zhang, Han, Shi and Zhang, arXiv:2505.06567v1, Conjecture 1, restate the same existence question. Han, Zhang and Zhang, arXiv:2606.08561v1, still report only the range $m\le (N-2)/2$ as known. The frozen declaration D5/S3/Quantum/Entanglement/MResistantQuditExistence.result proves the source conjecture in its stated admissible range.

Gap

The kernel-checked construction in MResistantQuditExistence.result uses the full local carrier Finset (Fin N) × Bool, carried to Fin d, with $d=2^{N+1}$. The source conjecture asks only for existence in some dimension, so this closes its quantified claim. The restricted-label construction below proves in prose that dimension $2\left(\genfrac{}{}{0ex}{}{N}{m}\right)$ suffices. This is separate from the kernel-checked witness. The least possible local dimension is not determined here. Qubit constructions for $m>(N-2)/2$ and the minimal local dimension remain open.

The literature screen in preregistration #15010 is bounded to the cited source, Zhang et al. (2025), Han et al. (2026), and its recorded searches; it supports not-found-in-searched-scope and does not assert exhaustive worldwide priority. The escape audit is unfinished under issue #15589. Its concrete gap is a faithful dependent-family registration with full source reconstruction and equivalence bridge, actual-positive law, whole-family variation, independent sensitivity, and observational-dependence evidence. The delivered-head DTR reading is DTR-Unregistered (Observe) for separable_exchange_zero and result; no declared_validated registration is claimed.

Route

Let $k=N-m$. Encode each $k$-subset $S$ together with a bit. Every site records the common subset label, and the bit is recorded only at sites in $S$. The local carrier is Finset (Fin N) × Bool, and Fintype.equivFin carries it to a finite local basis of size $d=2^{N+1}$. Only $k$-subset labels occur in the state support. The state is the uniform superposition of the resulting distinct product-basis configurations.

A loss of $m+1$ sites meets every $k$-subset, so all off-diagonal terms disappear. The remaining diagonal is a finite convex mixture of products of local basis projectors and is fully separable. A loss of $m$ sites leaves exactly one intact flag. Its surviving coherence has a zero exchanged diagonal; separable_exchange_zero rules out full separability. The same exchanged-diagonal obstruction gives initial entanglement.

Restricted local dimension: proved in prose

Assume $N,m$ are natural numbers with $N\ge 3$ and $0\le m\le N-2$. Put $V=\{0,\dots ,N-1\}$, $k=N-m\ge 2$, and $\mathcal{E}=\{S\subseteq V:|S|=k\}$. Since $k\le N$, the number $s=|\mathcal{E}|=\left(\genfrac{}{}{0ex}{}{N}{k}\right)=\left(\genfrac{}{}{0ex}{}{N}{m}\right)$ is positive. Take the same local Hilbert space at every site, with orthonormal basis $\{|S,b⟩:S\in \mathcal{E},b\in \{0,1\}\}$. Its dimension is $d=2s=2\left(\genfrac{}{}{0ex}{}{N}{m}\right)$. For $S\in \mathcal{E}$ and $b\in \{0,1\}$, define the product-basis word

$$|w_{S,b}⟩=\underset{i\in V}{⨂}|S,b{1}_{i\in S}⟩,|{\psi }_{\mathcal{E}}⟩=\frac{1}{\sqrt{2s}}\sum _{S\in \mathcal{E}}\sum _{b=0}^1|w_{S,b}⟩.$$

1. Normalization and restriction. If $S\ne T$, the common subset labels distinguish $w_{S,b}$ and $w_{T,c}$ at every site. For fixed $S$, its two bit words differ at any site in the nonempty set $S$. Thus the $2s$ words are distinct orthonormal product-basis vectors, and $\Vert {\psi }_{\mathcal{E}}{\Vert }^2=(2s)/(2s)=1$. Let $H_{\mathrm{all}}$ have basis all $(S,b)$ with $S\subseteq V$, and let $U:H_{\mathcal{E}}\to H_{\mathrm{all}}$ send each restricted basis vector to the identically labelled full basis vector. It is an isometry. Every amplitude of the full-carrier flag state outside these words is zero, so $U^{\otimes N}{\psi }_{\mathcal{E}}$ is exactly that full-carrier state, before the bijective enumeration of its basis by $\mathrm{Fin}{2}^{N+1}$. Restriction retains every summand and its normalization coefficient; no projection with subsequent renormalization is needed.

2. Partial traces commute with this restriction. For any lost set $J\subseteq V$ and retained set $R=V\setminus J$, write ${\rho }_{\mathcal{E},J}={\mathrm{Tr}}_J|{\psi }_{\mathcal{E}}⟩⟨{\psi }_{\mathcal{E}}|$. Tracing a matrix unit $|w_{S,b}⟩⟨w_{T,c}|$ multiplies its retained matrix unit by ${\prod }_{j\in J}⟨T,c{1}_{j\in T}|S,b{1}_{j\in S}⟩$. The isometry preserves each such inner product. Hence, by linearity, the full-carrier marginal equals $U_R{\rho }_{\mathcal{E},J}{U}_R^{\ast }$, where $U_R={⨂}_{i\in R}U$; full-basis trace terms with a lost label outside $\mathcal{E}$ contribute zero. This identity also holds for $J=\varnothing$, when the empty product equals one.

3. Full separability is preserved in both directions. A finite convex mixture of products of local positive trace-one matrices on $H_{\mathcal{E}}$ maps under $U_R$ to such a mixture on $H_{\mathrm{all}}$, because each local map $\sigma \mapsto U\sigma U^{\ast }$ preserves positivity and trace. Conversely, suppose $U_R\rho U_R^{\ast }$ is fully separable and $\rho$ is a density matrix on $H_{\mathcal{E}}^{\otimes R}$. Write a finite product decomposition with weights $p_a\ge 0$ summing to one and local density matrices ${\sigma }_{a,i}$. Set $P=U{U}^{\ast }$ and $Q=I-P$. Its marginal at every site $i\in R$ is supported in $P$, so $0={\sum }_a{p}_a\mathrm{Tr}(Q{\sigma }_{a,i})$. All summands are nonnegative. For each $p_a>0$, therefore, $\mathrm{Tr}(Q{\sigma }_{a,i})=0$. Positivity implies ${\sigma }_{a,i}=P{\sigma }_{a,i}P$: the zero diagonal entries on an orthonormal basis of $Q$ force the corresponding rows and columns to vanish by the positive two-by-two minor inequality. Thus $U^{\ast }{\sigma }_{a,i}U$ is positive with trace one. Discarding zero-weight terms and compressing every remaining factor gives a finite fully separable decomposition of $\rho$. All marginals used below have at least one retained site. This proves that reducing the local carrier cannot change their full-separability status.

4. Every $(m+1)$-particle loss is fully separable. Assume $|J|=m+1$, so $J\ne \varnothing$ and $|R|=k-1$. Different subset labels are distinguished on any lost site. For equal subset labels $S$, the two bit branches are distinguished on a lost site as well: otherwise $S\subseteq R$, contradicting $|S|=k>|R|$. Consequently only terms with $(S,b)=(T,c)$ survive the trace, and

$${\rho }_{\mathcal{E},J}=\frac{1}{2s}\sum _{S\in \mathcal{E}}\sum _{b=0}^1|w_{S,b}^R⟩⟨w_{S,b}^R|.$$

Here $w_{S,b}^R=w_{S,b}{|}_R$ denotes the restriction of the word to retained sites. Each summand is a product of local basis projectors of trace one; the $2s$ nonnegative weights sum to one. This is an explicit fully separable decomposition, even if some retained words coincide.

5. Every $m$-particle loss is entangled. Assume $|J|=m$, so $|R|=k\ge 2$ and $R\in \mathcal{E}$. Let $x=w_{R,0}{|}_R$ and $y=w_{R,1}{|}_R$. A summand having retained word $x$ must have label $R$, and then branch zero; a summand having retained word $y$ must have label $R$, and then branch one. These two full words agree on every site of $J$ (their bit is zero there), so ${\rho }_{\mathcal{E},J}(x,y)=1/(2s)\ne 0$. Choose distinct $i,j\in R$. Exchange the $i$-th entries of $x$ and $y$, obtaining $u$ and $v$. Both exchanged words still have the common label $R$, but their bits at $i$ and $j$ differ. No supported word restricts to $u$ or $v$: its label would force $S=R$, whose branch bit is constant on $R$. Hence both exchanged diagonal entries are zero. The implication proved by FullySeparableExchangeObstruction.separable_exchange_zero applies to these finite local carriers: a fully separable matrix with a zero exchanged diagonal has zero original coherence. To see the implication directly, in a product decomposition each weighted exchanged-diagonal term is nonnegative; a zero sum forces each positively weighted product to have a zero local diagonal factor. Positivity annihilates that factor’s row or column, including the corresponding coherence factor. Thus all original coherence summands vanish, a contradiction. This proves nonseparability of the marginal. When $m=0$, $J=\varnothing$ and $R=V$; the same coherence and exchanged-diagonal computation applies without suppressing cross-label terms by a trace.

6. Initial entanglement. Choose any $S\in \mathcal{E}$ and set $x=w_{S,0}$, $y=w_{S,1}$. Their pure-density coherence is $1/(2s)$. Since $k\ge 2$, exchanging a site in $S$ gives words with unequal bits at two sites of $S$. Such words are absent from the support, so their pure-density diagonals vanish. The same full-separability implication used in step 5 proves that $|{\psi }_{\mathcal{E}}⟩⟨{\psi }_{\mathcal{E}}|$ is entangled. Together with steps 1, 4 and 5, this proves an $m$-resistant state in dimension $2\left(\genfrac{}{}{0ex}{}{N}{m}\right)$ for every stated admissible pair. Enumerating the restricted orthonormal basis by $\mathrm{Fin}d$ is a local unitary relabeling and preserves these properties.

Evidence kind: proved in prose. The restricted-dimension conclusion and the isometry argument above are not additional kernel-checked declarations. The kernel-checked MResistantQuditExistence.result retains its witness on the full carrier of dimension $2^{N+1}$. The prose construction supplies an upper bound $2\left(\genfrac{}{}{0ex}{}{N}{m}\right)$ on the least admissible local dimension; its optimal value and qubit existence for $m>(N-2)/2$ remain open.

Falsifier

A counterexample would be an admissible pair $(N,m)$ for which every finite local dimension fails to provide a normalized pure state satisfying the three resistance clauses, or a failure of one of the universal loss-set clauses for the displayed construction. The kernel-checked result covers all natural $N,m$ in the claim’s range. A qubit-only counterexample outside that range would not refute the settled claim.

Evidence

The supporting declaration D5/S3/Quantum/Entanglement/FullySeparableExchangeObstruction.separable_exchange_zero has declaration statement ID sha256:cc9db09ebb15605032c0bc389540ec1e76e4442c2d71fdc283cde133048236a2. The settling declarations have declaration statement IDs sha256:1327de9bec1f4dd508480f4407987fe29f8fc10c5d9ea9fb2b41f0ed887faa6b for IsMResistant, sha256:7d826234e720b0c2efc25eaac6761eee1912e871a92f4c99589597d27d7cd5ad for claim, and sha256:57e94cd6f0c4b6a84a1ae0d8066ee21cce8b203557c717e0a3f2ad90f98b2e6c for result. The settling module state pin is sha256:cc99e7d4b3509064bf7c9ba23b3a83bcdf0858f728cfe0ce70f3b5f31418998d.

The kernel axiom closure of every public declaration is contained in {propext, Classical.choice, Quot.sound}. The frozen prerequisites used by the settling proof are D5/S3/Quantum/PureState/PureStateHandshake.rankOneDensity, D5/S3/Quantum/Entanglement/LocalObservationPartialTraceEquivalence.partialTraceFirst, D5/S3/Quantum/Foundation/FiniteStateChannel.DensityState, and D5/S3/Quantum/Information/OrthogonalRecordEntropy.pointerState; the supporting obstruction is a same-delivery prerequisite.

The exact-rational reading is published at docs/reports/quinta-andre-burchardt-zyczkowski-2019-m-resistant-states/check.py in commit 664831fdc0f2267cd539af109024ede9e42826ab. The command python3 check.py exits 0, ends with ALL_OK, and checks $3\le N\le 6$ with exact rational entries. The script SHA-256 is c25ee3e55a0028bdabbeffd0e80e260e26022c08bd59b97cf359d319e9f1bb70.

Triage

What the settlement shows

The decisive mechanism is the shared subset label plus a localized branch bit. It works uniformly for every admissible natural $N,m$ and yields the kernel-witness dimension $2^{N+1}$. The proof extends the source’s known range $m\le (N-2)/2$ to the whole pure-state existence conjecture in arbitrary local dimension. The separate prose proof reduces the sufficient dimension to $2\left(\genfrac{}{}{0ex}{}{N}{m}\right)$. Neither proof settles qubit existence beyond that known range or determines the minimal local dimension. The source’s ordinary resistance statements remain compatible: this result supplies the existence premise in the full admissible range, while minimal dimension and qubit-specific questions remain separate.

The public result is the settling result for the named open problem and is admitted under open-problem-resolution (#15010; Proved). The escape audit remains unfinished under #15589 with the DTR-Unregistered reading above; no declared_validated registration is claimed.

ASSUMED-UNVERIFIED

The Lean kernel verifies the formal finite-dimensional statement and its standard axiom closure. The correspondence between the formal predicates and the source’s physical terminology, the quoted literature statements, and completeness of the bounded literature screen are external assumptions. The exact-rational experiment is evidence only for $3\le N\le 6$ and does not replace the uniform Lean proof.