Zhang–Singh–Pati: strong subadditivity of subentropy

The question attributed to arXiv:1510.08859v2, §7.1 is recorded in zhang2017subentropy.

A similarly question can be asked for the strong subadditivity of subentropy: Q(ρ ABC) + Q(ρ B) ⩽ Q(ρ AB) + Q(ρ BC).

For every finite-dimensional tripartite density operator $\rho_{ABC}$, let $\rho_{AB}=\operatorname{Tr}C\rho{ABC}$, $\rho_{BC}=\operatorname{Tr}A\rho{ABC}$ and $\rho_B=\operatorname{Tr}{AC}\rho{ABC}$. The proposed assertion is

$$ Q(\rho_{ABC})+Q(\rho_B) \le Q(\rho_{AB})+Q(\rho_{BC}). $$

The formal statement quantifies over arbitrary finite coordinate types and the repository's DensityState; changing the parentheses of a product coordinate type only relabels matrix entries. The original assertion does not impose commutation, full rank, separability or purity.

Subentropy is a spectral information quantity whose subadditivity properties are distinct from those of von Neumann entropy. The target is preregistered in issue #18958. The existing finite density-state interface provides the common state domain.

The determinant-integral definition is connected to the positive distinct-spectrum formula, and continuity is proved on the entire positive trace-one domain, including zero and repeated eigenvalues. The witness uses the actual partial traces of one joint density state. Rectangular Gram invariance, pure-state vanishing and the two exact uniform values supply its subentropy evaluations.

For a positive trace-one matrix $A$, use the representation

$$ Q(A)=\int_0^\infty\left[ \frac{t}{1+t}-\frac{1}{\operatorname{Re}\det(I+t^{-1}A)} \right]dt. $$

On probability vectors, the kernel satisfies the bound $0\le K_t(p)\le(1-p_x)/(1+t)^2$. The integrable dominating function $(1+t)^{-2}$ supplies convergence and continuity. Partial fractions at positive distinct eigenvalues give the spectral formula in datta2014subentropy; continuity is needed at the counterexample's repeated and zero eigenvalues.

Sylvester's identity $\det(I+VW)=\det(I+WV)$ makes this integral invariant under rectangular Gram conversion and isometric embedding. In particular, pure states have value zero. These properties are proved directly for the integral representation, without a nonzero-spectrum or rank assumption.

Take $A=B=C=\mathbb C^2$ and

$$ |\Phi\rangle=\frac{|00\rangle+|11\rangle}{\sqrt2},\qquad \rho_{ABC}=|\Phi\rangle\langle\Phi|_{AB}\otimes I_C/2. $$

Direct partial trace gives $\rho_{AB}=|\Phi\rangle\langle\Phi|$, $\rho_B=I_2/2$ and $\rho_{BC}=I_4/4$. The isometry $Jv=\Phi\otimes v$ gives $Q(\rho_{ABC})=Q(I_2/2)$.

Writing $u_a(t)=(t+a)^{-1}$, the two kernels expand as

$$ K_t(I_2/2)=u_{1/2}(t)-u_1(t)-\tfrac14u_{1/2}(t)^2, $$

$$ K_t(I_4/4)=u_{1/4}(t)-u_1(t)-\tfrac38u_{1/4}(t)^2 +\tfrac1{16}u_{1/4}(t)^3-\tfrac1{256}u_{1/4}(t)^4. $$

Integrating reciprocal differences together, with $\int_0^\infty(u_a-u_b)=\log b-\log a$ and $\int_0^\infty(t+a)^{-k}dt=a^{1-k}/(k-1)$ for $a>0$, $k>1$, yields

$$ Q(I_2/2)=\log2-\tfrac12,\qquad Q(I_4/4)=\log4-\tfrac{13}{12}. $$

Consequently the left side minus the right side of the proposed inequality is

$$ 2(\log2-\tfrac12)-(\log4-\tfrac{13}{12})=\boxed{\tfrac1{12}}>0. $$

One valid tripartite density state with strictly positive displayed gap refutes the universal assertion. The Bell-pair product state above is the specified witness. Failure of any claimed marginal identity, spectral invariance, exact integral value or identification with the paper's subentropy would invalidate the refutation. Lean establishes the witness identities and exact integral values; the compiled identification theorem connects these evaluations to the paper's continuous-extension convention.

The formal refutation is StrongSubadditivityRefutation.result, whose type is literally ¬ conjecture. Its private witness_gap proves the left side minus the right side equals exactly $1/12$ for the Bell-pair product state. All three reduced states are obtained by the canonical partial-trace API, and the BC reassociation only relabels product indices.

Basic proves rectangular Gram invariance, isometric-embedding invariance and pure-state vanishing. UniformValues proves the exact values $\log 2-1/2$ and $\log 4-13/12$ for uniform spectra of cardinality two and four, using the improper integrals in UniformIntegrals.

SpectralFormula.matrixSubentropy_diagonal_eq_distinct_formula identifies the determinant integral with the paper's finite formula for positive pairwise distinct probabilities. DensityKernel proves the uniform kernel domination, absolute integrability on density matrices and continuousOn_matrixSubentropy on the full positive trace-one domain. The consumed private theorem subentropyLike_eq proves uniqueness for the family SubentropyLike: continuity on density states, unitary invariance and the positive distinct-spectrum formula identify every such family with subentropy. After diagonalization, mix the ordered eigenvalue vector with a positive strictly decreasing probability vector in the same ordering. Every positive mixture has distinct positive entries and sum one. Conjugate these diagonal approximations by the fixed eigenbasis and pass to the limit. The theorem consumes both the spectral formula and continuity. The closed conjecture quantifies over these families and every finite tripartite density state. canonicalSubentropy proves this family domain is inhabited, and result consumes it through the identification theorem before applying the exact witness gap.

SSA REFUTED in Lean, with exact gap $1/12$. The witness is a Bell pair tensored with a maximally mixed qubit. Its $AB$ marginal is pure; the $B$ and $C$ factors produce a strict product subadditivity gap.

Structural implication, proved here in prose and not Lean-certified. Let $F$ be invariant under isometric embeddings (hence under adjoining zero eigenvalues) and vanish on pure states. If $F$ satisfied the displayed SSA inequality for all finite tripartite states, it would satisfy product superadditivity $F(\sigma\otimes\tau)\ge F(\sigma)+F(\tau)$. Indeed, purify an arbitrary $\sigma_B$ to $|\psi\rangle_{AB}$ and apply SSA to $|\psi\rangle\langle\psi|_{AB}\otimes\tau_C$. Its $AB$ value is zero, its global value is $F(\tau)$ by the isometry $v\mapsto\psi\otimes v$, and its other two relevant values are $F(\sigma)$ and $F(\sigma\otimes\tau)$. This proves the implication. For subentropy, $\sigma=\tau=I_2/2$ violates that necessary consequence by $1/12$. Isometric invariance across dimensions is a needed hypothesis; dependence on a dimension-specific full spectrum alone does not supply it.

OPEN — Datta et al. Conjecture 1, bipartite subadditivity, $Q(\rho_{AB})\le Q(\rho_A)+Q(\rho_B)$, not addressed in this delivery. The counterexample's product spectra satisfy strict subadditivity, so this refutation gives no counterexample to Conjecture 1. No assertion about its present literature-wide status follows from this local open proof obligation.

The source quotation, version and section are supplied locators without an independently available original source. The journal DOI, expanded author names and bibliographic title have not been independently verified; see the library note. Searches of the arXiv identifiers, “subentropy subadditive”, “strong subadditivity” with subentropy, and the titles found no answer; citation indexes were not exhaustively checked. These bibliographic and literature-status boundaries are independent of the formal mathematical statements listed above.

Escape registrations for Basic.matrixSubentropy_rectangular_gram, Basic.matrixSubentropy_isometric_embedding, Basic.matrixSubentropy_pure and DensityKernel.continuousOn_matrixSubentropy have the verdict unclassified_form:source.descriptor_actual. Their Lean proofs compile, but the compiled registration descriptor has not been accepted as matching the actual source-family realization. The missing evidence is an accepted four-slot binding for each of these source theorems; the attempted registration is not declared_validated. See issue #18958.