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| title | date | draft | tags | |||
|---|---|---|---|---|---|---|
| Research Paper: Fast Scrambling and Holographic Entanglement: SYK Dynamics and the Page Curve in Bipartite Quantum Graphs (Letter) | 2026-06-01T08:00:00Z | false |
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Abstract:
Introduction
Black hole evaporation models in discrete graphs often incorrectly rely on a dynamic shrinking of the physical Hilbert space dimension. Under global unitary evolution, the tensor product structure of the universe remains strictly fixed. The information paradox is resolved by the entanglement dynamics between fixed partitions, assuming the interior is a fast scrambler \cite{Hayden2007}. The SYK model provides an exactly solvable laboratory for such maximally chaotic dynamics \cite{Maldacena2016}.
The SYK Interior and Schwinger-Dyson Equations
Let the pure global state evolve in a fixed bipartite Hilbert space \mathcal{H}_{int} \otimes \mathcal{H}_{ext}. We model the interior using a maximally chaotic SYK Hamiltonian of N Majorana fermions \chi_i with all-to-all random couplings:
H_{SYK} = \sum_{1 \le i \lt j \lt k \lt l \le N} J_{ijkl} \chi_i \chi_j \chi_k \chi_l
The evaporation process is governed by a linear tunneling Hamiltonian H_{evap} = \sum_{i, k} V_{ik} \chi_i (\psi_k^\dagger + \psi_k).
In the large-N limit, the disorder-averaged dynamics on the Keldysh contour are governed by the Schwinger-Dyson equations for the Green's function G(\tau_1, \tau_2) = \frac{1}{N} \sum_i \langle T_c \chi_i(\tau_1) \chi_i(\tau_2) \rangle and the self-energy \Sigma:
G(i\omega_n) = \frac{1}{i\omega_n - \Sigma(i\omega_n)}, \quad \Sigma(\tau) = J^2 [G(\tau)]^3 + V^2 G_{bath}(\tau)
where G_{bath} is the Green's function of the exterior fermions. The physical dimensions \dim(\mathcal{H}_{int}) = 2^{N/2} remain strictly constant.
The Replica Trick and the Page Curve
Because the SYK interior maximally scrambles information, any fermion extracted by H_{evap} leaves behind highly scrambled entanglement. The exact calculation of the von Neumann entropy S(\mathcal{H}_{int}) requires the replica trick:
S(\mathcal{H}_{int}) = \lim_{n \to 1} \frac{1}{1-n} \log \text{Tr}(\rho_{int}^n)
Evaluating the path integral over n replicas introduces replica wormhole saddles \cite{Penington2020}. At early times, the disconnected saddle dominates, and the entanglement entropy grows linearly with the emitted radiation. Once the entanglement entropy reaches the maximal Page time t_{Page}, the replica wormhole saddle becomes dominant, actively purifying the early radiation. The generalized entropy S_{gen} perfectly traces the Page curve, peaking and returning to zero, despite the physical dimension of the graph remaining entirely static.
References
- [Hayden2007] P. Hayden and J. Preskill, Black holes as mirrors: quantum information in random subsystems, JHEP {\bf 09} (2007) 120.
- [Maldacena2016] J. Maldacena and D. Stanford, Remarks on the Sachdev-Ye-Kitaev model, Phys. Rev. D {\bf 94} (2016) 106002.
- [Penington2020] G. Penington, Entanglement Wedge Reconstruction and the Information Paradox, JHEP {\bf 09} (2020) 002.