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Entropy density agreement does not force local state agreement

Statement

For a finite Hermitian matrix D, write ∥D∥1 for its trace norm: the sum of the absolute values of its eigenvalues, counted with multiplicity.

For the canonical densities γN of Additive spectral Hamiltonian and normalized ensembles with q=2, e1=e2=0 and any fixed finite β∈R, there are finite density sequences whose dimensionless entropy per site has the same limit as γN while their one-site reduced states remain separated from its one-site marginal by a fixed trace norm.

Facts & Assumptions

[F1]

Dimensionless entropy of a finite density matrix: a density with positive eigenvalues λi has entropy −∑iλilog⁡λi.

[F2]

Subsystem expectations and reduced finite tensor densities: the one-site reduction is the partial trace over all remaining factors.

[F3]

Additive spectral Hamiltonian and normalized ensembles: for the stated zero levels, p1=p2=1/2, ZN=2N, and the canonical density is γN=(I/2)⊗N.

Proof

1.1givenF1F3algebra

On V=C2, the supplier [F3] identifies the stated canonical one-site density as γ=I/2 and its N-site density as γN=(I/2)⊗N. For each N≥1, set γN=(I/2)⊗N and τN=∣1⟩⟨1∣⊗(I/2)⊗(N−1). Both are positive of trace one, so are finite tensor densities. Their positive eigenvalues are respectively 2−N with multiplicity 2N and 2−(N−1) with multiplicity 2N−1. By [F1], sN(γN)=Nlog⁡2 and sN(τN)=(N−1)log⁡2; therefore ∣sN(τN)−sN(γN)∣/N=(log⁡2)/N→0.

2.1F2step 1.1algebra∎

By [F2], the one-site reductions are γ and ∣1⟩⟨1∣. Their difference in the ordered basis is diag⁡(1/2,−1/2), whose eigenvalues have absolute sum one. Thus ∥τN,1−γ∥1=1 for every N. Entropy-density agreement alone therefore does not imply local trace-norm convergence.

Depends on

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