In the model of approximate holographic codes introduced by Cao, Cheng, Karthikeyan, Li, and Preskill and analyzed perturbatively by Witten, the error in entanglement wedge reconstruction is smaller than the gravitational backreaction by a factor 1/d_2^2 at second order in the skewing parameter, where log d_2 plays the role of the area term. It was recently observed numerically that this hierarchy fails at fourth order for non-degenerate bulk spectra, through a coefficient growing as d_2^4. In the present work a closed form for this coefficient is proved: writing p = (p_1, ..., p_d1) for the Schmidt spectrum of the bulk state, the leading fourth-order term of the mean reconstruction error is epsilon^4 sigma_W^4 alpha(p) d_2^4, where alpha(p) = (1/2) d_1^2 (sum_i 1/p_i - d_1^2) coincides with (1/2) d_1^4 chi^2(u || p), the Neyman chi-squared divergence of the bulk spectrum from the uniform distribution u = (1/d_1, ..., 1/d_1). The mechanism is identified exactly: the ensemble mean of the second-order state perturbation is a decoherence drift of magnitude proportional to d_2^2 directed toward maximal mixing, and alpha is the Kubo-Mori quadratic form of this drift. The spectral-degeneracy protection of the hierarchy observed numerically is thereby explained as the equality case of Jensen's inequality. The formula, conjectured from five calibration spectra, was subsequently confirmed by a zero-parameter blind prediction at a previously unmeasured configuration (d_1 = 4, a new spectrum), which it matched to 0.08 per cent (-0.13 sigma). This is a corrected version of the preprint DOI 10.13140/RG.2.2.14852.69768. Three presentational defects are corrected and no result, proof, or measured value changes. First, the figure committed in advance for the blind test was 601,378, whereas the closed form proved in the paper gives 15552 alpha = 601,344 exactly; the committed figure corresponds to a rounded alpha = 38.6689 and exceeds the exact one by 34, a relative excess of 5.7e-5. Both figures are now stated with the deviation of the measurement from each, and the test is passed against either. Second, the fourth calibration spectrum was printed as (0.453, 0.333, 0.213), which does not sum to one; it is now given as the exact spectrum (34, 25, 16)/75 that was simulated, for which the stated alpha = 4.0202 is correct. Third, the blind test is described as zero-parameter rather than registered: the prediction uses no fitted quantity of any kind, but it was not deposited with a third party, so no external timestamp is claimed. The corrections are listed in the "Note on this version" on the first page. Simulation scripts and data: Zenodo DOI 10.5281/zenodo.21223051.
In the model of approximate holographic codes introduced by Cao, Cheng, Karthikeyan, Li, and Preskill and analyzed perturbatively by Witten, the error in entanglement wedge reconstruction is smaller than the gravitational backreaction by a factor 1/d_2^2 at second order in the skewing parameter, where log d_2 plays the role of the area term. It was recently observed numerically that this hierarchy fails at fourth order for non-degenerate bulk spectra, through a coefficient growing as d_2^4. In the present work a closed form for this coefficient is proved: writing p = (p_1, ..., p_d1) for the Schmidt spectrum of the bulk state, the leading fourth-order term of the mean reconstruction error is epsilon^4 sigma_W^4 alpha(p) d_2^4, where alpha(p) = (1/2) d_1^2 (sum_i 1/p_i - d_1^2) coincides with (1/2) d_1^4 chi^2(u || p), the Neyman chi-squared divergence of the bulk spectrum from the uniform distribution u = (1/d_1, ..., 1/d_1). The mechanism is identified exactly: the ensemble mean of the second-order state perturbation is a decoherence drift of magnitude proportional to d_2^2 directed toward maximal mixing, and alpha is the Kubo-Mori quadratic form of this drift. The spectral-degeneracy protection of the hierarchy observed numerically is thereby explained as the equality case of Jensen's inequality. The formula, conjectured from five calibration spectra, was subsequently confirmed by a zero-parameter blind prediction at a previously unmeasured configuration (d_1 = 4, a new spectrum), which it matched to 0.08 per cent (-0.13 sigma). This is a corrected version of the preprint DOI 10.13140/RG.2.2.14852.69768. Three presentational defects are corrected and no result, proof, or measured value changes. First, the figure committed in advance for the blind test was 601,378, whereas the closed form proved in the paper gives 15552 alpha = 601,344 exactly; the committed figure corresponds to a rounded alpha = 38.6689 and exceeds the exact one by 34, a relative excess of 5.7e-5. Both figures are now stated with the deviation of the measurement from each, and the test is passed against either. Second, the fourth calibration spectrum was printed as (0.453, 0.333, 0.213), which does not sum to one; it is now given as the exact spectrum (34, 25, 16)/75 that was simulated, for which the stated alpha = 4.0202 is correct. Third, the blind test is described as zero-parameter rather than registered: the prediction uses no fitted quantity of any kind, but it was not deposited with a third party, so no external timestamp is claimed. The corrections are listed in the "Note on this version" on the first page. Simulation scripts and data: Zenodo DOI 10.5281/zenodo.21223051.
In approximate holographic codes obtained by skewing an exact code with a Gaussian random perturbation, the accuracy of entanglement wedge reconstruction has been shown to depend strongly on the structure of the perturbation ensemble: a 1/d_2^2 error-to-backreaction hierarchy for the unitarily invariant ensemble at second order, a chi-squared-divergence floor at fourth order for generic bulk spectra, an unsuppressed second-order floor for local cross-cut perturbations, and exact protection for perturbations confined to the geometric sector. This paper unifies these behaviours. For an arbitrary centered Gaussian Hermitian ensemble, it is proved that the ensemble mean of the second-order state deformation equals the action of a Lindblad dissipator whose jump operators are the Karhunen-Loeve modes of the covariance, reduced to the code sector; and that the second-order mean relative entropies are explicit Kubo-Mori quadratic functionals of the per-mode commutator amplitudes. The reconstruction-error floor at fixed backreaction then decomposes into two mechanisms, a second-order term given by a ratio of these functionals and a quartic term given by the Kubo-Mori norm of the induced drift, from which the previously observed laws follow as corollaries, including a closed-form derivation of the coupling-fraction law that had been measured but not derived, and a new exact-protection class: any perturbation diagonal in the Schmidt basis of the bulk state leaves the reduced code state invariant to all orders. The theory is deterministic in the ensemble covariance and was subjected to a second-generation blind test: three zero-parameter predictions (a hierarchy ratio, a drift matrix, and a quartic drift coefficient) were fixed in advance for a chiral cross-cut family never previously simulated, and all three were confirmed within statistical errors. This is a corrected version of the preprint DOI 10.13140/RG.2.2.35666.93125. The word "registered" is replaced by "zero-parameter" throughout, and a footnote records the provenance of the committed predictions: they are computed from the ensemble covariance with no fitted quantity and were fixed before the corresponding measurements were run, but were not deposited with a third party, so no external timestamp is claimed. No result, proof, prediction, or measured value changes. Simulation scripts and data: Zenodo DOI 10.5281/zenodo.21230383.
If entanglement wedge reconstruction is exact, then (under certain assumptions) the area term in the RT formula is a c-number, indicating that the choice of a bulk quantum state does not influence the geometry. Recently Cao, Cheng, Karthikeyan, Li, and Preskill considered a generic perturbation away from exact entanglement wedge reconstruction. The optimal reconstruction was defined; based on this, an effective area function that depends nontrivially on the quantum state was defined and its properties were analyzed. Here we make one aspect of this picture more quantitative, by showing that if as expected the area term in the RT formula is of order 1/G while the bulk entropy is of order 1, then the corrections to entanglement wedge reconstruction are exponentially small (in G) relative to corrections to the area function. In the framework under discussion, there is an area function but no area operator; we discuss to what extent this is the expected behavior in holography.
A pure encoded state of an approximate quantum code is perturbed by exp(i eps W) with W drawn from a centered Gaussian ensemble, and D(eps) denotes the relative-entropy disturbance of a reduced state. This paper derives the complete fourth-order coefficient of the mean disturbance in closed form, and identifies what that coefficient is made of. The derivation has two stages, and they assume different things. The first assumes nothing about the noise beyond that it is centered and Gaussian: the coefficient splits into five channels, one disconnected drift term and four connected ones, each a finite sum over pairs of Karhunen-Loeve modes of the covariance contracted with exact confluent divided differences of the logarithm. For any concrete noise model the whole quartic disturbance is then computable deterministically, with no sampling. The second stage adds one hypothesis, that the mode family is complete and orthonormal, and evaluates every channel. The device throughout is to substitute the integral representation of the divided differences before performing the index sums, which turns each Kronecker delta into a coincidence of resolvent labels, and then to integrate by parts in the ring Q[d_1][t, m_1, ..., m_4] of resolvent traces. No coefficient is fitted and no index contraction survives. What survives is four scalar functions of the Schmidt spectrum: Xi, chi, S_w and T. The assembled coefficient is a function of d_1, d_2 and those four alone, and four is the right number. The five functions 1, Xi, chi, T, S_w are proved linearly independent for every d_1 >= 3, analytically: a degeneration lemma confines the kernel to two dimensions and an expansion about the degenerate spectrum closes it, with discriminant d_1 (d_1 - 2)^2 / 4, which vanishes exactly at d_1 = 2, where the two relations that do make the set reducible are exhibited. The four are not a list that happens to close. Each is a contraction of the Kubo-Mori metric and the Amari-Chentsov skewness tensor on two operator directions, the identity and the maximal-coherence operator of the Schmidt eigenbasis; and those directions are not chosen but read off the mode two-point function, which the Gaussian ensemble fixes identically in the spectrum and for every d_1. The two are Kubo-Mori orthogonal, so chi is the squared classical Fisher length of the spectrum and S_w - chi the squared coherent length, bounded below sharply by d_1^2 (d_1 - 1). Every channel is a rational-weighted sum of metric and skewness contractions, with d_2 the only number entering from outside the geometry, and the skewness tensor enters the whole coefficient exactly once: at order d_2^0 and with weight -1/6, which is precisely the weight the relative-entropy expansion assigns to its own cubic term. The paper is explicit about scope. A dedicated section separates what is proved about the ensemble alone, which is everything, from what the code supplies, which is the Schmidt spectrum and the reading of log d_2 as an area term, and from where the gap lies. On a family of Pauli strings satisfying the Knill-Laflamme conditions on a stabilizer code the first-order mode objects vanish identically, so three of the five channels are not small but exactly zero, and the entire Amari-Chentsov contribution goes with them: the mechanism is a property of the ensemble, not of approximate codes in general. The step from the coefficient to a physical reconstruction-error floor requires fixing the backreaction at a chosen fraction of its Page value, a modelling decision that this paper does not make. The limitations section states four further points, including one archived measurement at d_1 = 3, d_2 = 12 which is located at a definite configuration, quantified, and left unadjudicated with the two reasons given. Every closed form is certified. The checks run from the defining index sums, from the raw five-channel sum built from the mode objects on an explicit orthonormal basis, from the geometric identifications, and, beyond the invariant ensemble, against direct simulation on structured families and stabilizer codes. Two of the latter are retrodictions: coefficients measured and archived earlier in this programme, before the present theory existed, are reproduced to +0.4 and 0.0 standard errors, with a previously unexplained connected excess given exactly by the connected-channel formula. The appendix evaluates the eighteen connected contractions on which the connected channel rests, and the deposited code derives all eighteen of them from the Wick rule and then checks symbolically that they sum to the closed form. This paper supersedes and absorbs three earlier preprints of this programme: 10.5281/zenodo.23644208, 10.5281/zenodo.23684111 and 10.5281/zenodo.23726789, together with the contraction appendix of 10.5281/zenodo.23621356. Those records remain available and are not withdrawn; readers should cite the present paper for any of their results, which are proved here in full and without reliance on unrefereed material. Version 3 corrects the presentation of the appendix table of eighteen connected contractions. Up to version 2 the nine word pairs were listed in five grouped rows, on the ground that the relations T_3 = T_1^dagger and conjugation with u <-> v relate the members of each row; those relations preserve a pair's d_2-degree but not its value, and for three pairs the grouped entry is therefore not the pair's own. The table is now listed pair by pair, the collected sum before the kernel contraction is displayed, and the deposited script derives all eighteen entries from the Wick rule rather than taking them as given. No result of the paper changes: the two matchings of a word pair are added together, and sum_uv w_uv p_u = sum_uv w_uv p_v by the symmetry of w, so the collected sum and everything downstream of it are unaffected. Version 3 also corrects the wording of the proof of the entropy-kernel lemma. The first-order term of the expansion of Tr f(sigma) is Tr[Delta log sigma_0] + Tr Delta, and it is removed by the second, linear term of the relative entropy rather than by Tr Delta = 0 as the earlier versions said; the conclusion of the lemma is unchanged and is verified in the deposited code. A fifth script, an independent review pass over the statements the other four do not cover, is added to the archive. The verification scripts that generate every entry of the certification table, together with the derivation and symbolic verification of the appendix table, are included in the attached archive under the MIT licence.
We study how much of the proto-area response can be inferred from the bulk von Neumann entropy. In the perturbative holographic code setting considered by CCKLP and Witten, we keep the recovery frame fixed and analyze the response averaged over the GUE at second order in the encoding perturbation. Near the maximally mixed bulk spectrum, the response is fixed by the entropy through quadratic order. For d1≥3, spectral information first enters at cubic order through the third centered moment, so states with the same entropy can have different responses. We determine the resulting response range on small equal-entropy level sets and obtain its sharp leading D3/2 dependence, including the coefficient. The corresponding minimax error for prediction from entropy alone is one half of this range. For d1=2, the entropy instead determines the response exactly.
In the context of quantum theories of spacetime, one overarching question is how quantum information in the bulk spacetime is encoded holographically in boundary degrees of freedom.It is particularly interesting to understand the correspondence between bulk subregions and boundary subregions in order to address the emergence of locality in the bulk quantum spacetime.For the AdS/CFT correspondence, it is known that this bulk information is encoded redundantly on the boundary in the form of an error-correcting code.Having access only to a subregion of the boundary is as if part of the holographic code has been damaged by noise and rendered inaccessible.In quantum-information science, the problem of recovering information from a damaged code is addressed by the theory of universal recovery channels.We apply and extend this theory to address the problem of relating bulk and boundary subregions in AdS/CFT, focusing on a conjecture known as entanglement wedge reconstruction.Existing work relies on the exact equivalence between bulk and boundary relative entropies, but these are only approximately equal in bulk effective field theory, and in similar situations it is known that predictions from exact entropic equalities can be qualitatively incorrect.We show that the framework of universal recovery channels provides a robust demonstration of the entanglement wedge reconstruction conjecture as well as new physical insights.Most notably, we find that a bulk operator acting in a given boundary region's entanglement wedge can be expressed as the response of the boundary region's modular Hamiltonian to a perturbation of the bulk state in the direction of the bulk operator.This formula can be interpreted as a noncommutative version of Bayes's rule that attempts to undo the noise induced by restricting to only a portion of the boundary.To reach these conclusions, we extend the theory of universal recovery channels to finite-dimensional operator algebras and demonstrate that recovery channels approximately preserve the multiplicative structure of the operator algebra.
Bulk reconstruction is a central problem in AdS/CFT, and entanglement wedge reconstruction is its subregion version. We argue that this subregion statement should be separated from the stronger holographic quantum error correction interpretation, in which one region-independent logical bulk operator has code-preserving representatives in several boundary regions. A simple locality argument shows that such a common reconstruction must commute with the code-preserving local algebras in the complementary regions. This is the mechanism realized in HaPPY-type codes: the erased regions are blind to a protected logical algebra. An ordinary finite N holographic CFT does not have such a protected invisible sector for supergravity fields. Its low-energy local observables, in particular, suitably smeared stress tensors, detect the physical support and gravitational dressing of ordinary bulk operators, up to possible center or superselection data. Thus, there is no such holographic quantum error correction and the N=∞ agreement of global and subregion HKLL formulae is a free-theory statement. What remains is entanglement wedge reconstruction without holographic quantum error correction, or subregion complementarity: each boundary region has its own code-preserving low-energy algebra and its own region-adapted bulk interpretation, rather than a shared logical operator.