Approximate FLM Formula in Holography

Updated 5 January 2026

The approximate FLM formula refines the classic RT prescription by incorporating subleading quantum corrections and higher-derivative effects, defining a more complete entanglement entropy framework.
It employs the replica trick and Hamilton–Jacobi variation to relate the extremal area operator to quantum corrections, ensuring consistency with bulk effective field theory.
This formulation is pivotal for entanglement wedge reconstruction and modular flow, providing explicit error bounds essential for holographic quantum error correction.

The approximate Faulkner-Lewkowycz-Maldacena (FLM) formula characterizes subleading quantum corrections to the Ryu-Takayanagi prescription for holographic entanglement entropy in AdS/CFT, extending the leading classical area/4G_N term to incorporate bulk quantum entanglement and higher-derivative contributions. This framework is essential for understanding entanglement wedge reconstruction, quantum error correction, and the detailed correspondence between bulk and boundary modular Hamiltonians.

1. Conceptual Overview and Formulation

The FLM formula refines the classical Ryu–Takayanagi (RT) prescription by expressing the entanglement entropy $S(\rho_R)$ for a boundary subregion $R$ as

$S(\rho_R)=\frac{\langle A[\gamma_R]\rangle}{4G_N}+S_{\rm bulk}(\Sigma_R)+S_{\rm Wald}^{(\rm HD)}(\gamma_R)+O(G_N),$

where $\gamma_R$ is the semiclassical extremal surface homologous to $R$ , $\langle A[\gamma_R]\rangle$ is the expectation value of the area operator, $S_{\rm bulk}(\Sigma_R)$ is the von Neumann entropy of bulk effective field theory (EFT) degrees of freedom in the entanglement wedge $\Sigma_R$ , and $S_{\rm Wald}^{(\rm HD)}$ refers to higher-derivative Wald-like corrections (Dong et al., 2019). The $O(G_N)$ terms represent higher-loop and non-perturbative corrections.

In the context of quantum error correction—where the AdS/CFT dictionary is formulated as a code subspace in the boundary CFT—the FLM formula is exact at leading and $O(G_N^0)$ order for small "code" spaces. Generalizations yield an approximate FLM formula for larger codes or when systematic coarse-graining is carried out (Dong et al., 1 Jan 2026, Dong et al., 25 Sep 2025).

2. Derivation Schemes: Replica Trick and Hamilton–Jacobi Variation

The derivation of the FLM formula hinges on the bulk replica trick, which relates Rényi entropies to the on-shell action of replica-symmetric saddles with conical singularities at the location of the extremal surface. In "One-loop universality of holographic codes" (Dong et al., 2019), the variation of the action with respect to the replica number $m=1/n$ yields a Hamilton–Jacobi relation: $\left.\frac{\partial}{\partial m} \tilde I_m[g_m]\right|_{m=1} = -\frac{A[\gamma_R]}{4G_N},$ establishing the area term as the conjugate quantity to replica number. Incorporating quantum corrections, the Renyi spectrum becomes flat at leading and one-loop order in the fixed-area code subspaces, and the effective entropy becomes

$S(\rho_R) = \mathrm{Tr}(\rho_W L_R) + S_W(\rho_W),$

where $L_R$ is the geometric entropy operator and $\rho_W$ is the bulk density matrix in the entanglement wedge.

3. Quantum Error Correction and Approximate FLM

In the quantum-corrected regime, each code subspace with fixed-area eigenvalue $A^\alpha$ realizes

$|S(\tilde\rho_R^\alpha) - (A^\alpha/(4G) + S(\rho_r^\alpha))| \leq \epsilon_{\rm FLM}^\alpha,$

with $\epsilon_{\rm FLM}^\alpha = o(G^0)$ (Dong et al., 1 Jan 2026). Large codes, built by direct sums over small codes, propagate the approximate FLM to the full state provided block-orthogonality and log-stability of the spectrum are maintained.

Table: Sources of FLM Approximation Error

Error Source	Scaling	Comment
Omitted quantum surfaces	$O(G_N)$	First correction from extremizing the full $S_{\rm gen}$
Isometry/nonorthogonality	$O(G_N^n)$ or $\exp(-{\rm const}/G_N)$	Subleading or nonperturbative
Area window width	$\sim G_N^\beta$ (β>1)	Finite window in area projections
Spectrum flatness	vanishing as $G_N \to 0$	Exact in fixed-area states

In the code subspace, the JLMS relation between the boundary/bulk modular Hamiltonians holds up to errors controlled by $\epsilon_{\rm FLM}$ , with the mapping

$K_R = \frac{A}{4G} + K_r + O(G_N) \qquad \text{on the code subspace},$

and approximate modular flow duality at finite $G_N$ (Dong et al., 2019, Dong et al., 1 Jan 2026).

4. Physical Regimes and Explicit Checks

The formula's regime of validity encompasses semiclassical bulk EFT with perturbative quantum corrections below a specified cutoff and dominance of replica-symmetric solutions (Dong et al., 2019). Direct computations in AdS $_3$ /CFT $_2$ for scalar and vector excitations verify the FLM formula: the leading $A/(4G_N)$ and bulk entanglement contributions precisely match with the boundary replica-trick calculations, with the first genuinely quantum correction arising at $(\pi x)^{8h}$ for light operator exchange (Chowdhury et al., 2024, Bhat et al., 17 Nov 2025).

Renormalization-group (RG) flows implemented via code coarse-graining adjust the area operator by integrating out high-energy bulk modes—shifting $G_N$ per the "Susskind–Uglum" picture—while retaining a generalized entropy of the form

$S(\rho_B) = \langle A_{\rm IR} \rangle_\rho + S(\rho_r),$

with $A_{\rm IR}$ embodying both IR area and entropy of integrated-out modes (Dong et al., 25 Sep 2025).

5. Extensions: Higher-Derivative and Gauge Contributions

For higher-derivative gravity, subleading corrections are given via Wald or Dong–Camps functionals, with explicit expressions arising from the derivative of the action as per the Hamilton–Jacobi analysis (Dong et al., 2019). In tensor network models for holographic codes with gauge fields, additional $\delta{\rm Area}/(4G_N)$ entropy corrections are localized to intersecting entangling surfaces and are coded by entanglement of edge degrees of freedom—structurally paralleling gauge theory results on lattice edge modes (Donnelly et al., 2016).

6. Connections to Quantum Extremal Surfaces and Limitations

At $O(G_N^0)$ , the FLM prescription agrees with the first term in the generalized entropy $S_{\rm gen}$ extremized for the quantum extremal surface. Corrections beyond one-loop necessitate adjusting the surface to extremize the full $S_{\rm gen}$ functional (Engelhardt et al., 2014). The approximate FLM formula fails to capture nonperturbative effects, breakdowns in subsystem orthogonality between area blocks, or replica symmetry breaking, implying that the formula is valid when the CFT state is semiclassical, code subspaces are "small," and the bulk is well-approximated by low-energy EFT.

7. Impact for Entanglement Wedge Reconstruction and Modular Flow

The validity of the (approximate) FLM formula is central to entanglement wedge reconstruction, modular flow duality, and the operator content of holographic codes. The precision of bulk-to-boundary operator mapping and modular Hamiltonian relations is directly quantified by the same $\epsilon_{\rm FLM}$ and associated error parameters (Dong et al., 1 Jan 2026, Dong et al., 2019). Only in the $G_N\to0$ limit does exact complementary recovery and sharp RT surfaces emerge; at finite $G_N$ , quantum-corrected codes achieve approximate error correction with boundable errors, permitting modular flow reconstruction up to order $G_N$ corrections.

The approximate FLM formula thus serves as a foundational tool for including quantum and RG effects in holographic entanglement entropy, yields explicit operator error bounds for AdS/CFT quantum information protocols, and links the geometric area operator directly to the quantum structure of holographic codes, modular flow, and generalized entropy (Dong et al., 1 Jan 2026, Dong et al., 2019, Dong et al., 25 Sep 2025, Engelhardt et al., 2014).