The reconciliation of unitary quantum mechanics with the smooth geometry of General Relativity remains a central challenge in quantum gravity, epitomized by the Firewall Paradox. In this work, we investigate the emergent geometry of the black hole information manifold using a metric-preserving machine learning framework. We simulate a 1D quantum black hole using the Sachdev-Ye-Kitaev (SYK) model and reconstruct its bulk geometry via an autoencoder constrained by the Quantum Bures Metric. Our analysis reveals a strong negative correlation between the thermodynamic entropy and the box-counting dimension of the emergent manifold, with finite size scaling consistent with perfect anti-correlation in the thermodynamic limit. Crucially, control experiments reveal that this correspondence is chaos-dependent: integrable free fermion systems show dramatically weaker correlations, while spatially local Heisenberg chains exhibit intermediate behavior. This establishes chaos as a key requirement for strong entropy-topology correspondence, distinguishing maximally chaotic from integrable thermal quantum states. We observe no topological phase transitions across the thermal crossover, supporting the hypothesis that high-entropy chaotic thermalization drives geometric smoothing, consistent with the ER=EPR conjecture.
The reconciliation of unitary quantum mechanics with the smooth geometry of General Relativity remains a central challenge in quantum gravity, epitomized by the Firewall Paradox. In this work, we investigate the emergent geometry of the black hole information manifold using a metric-preserving machine learning framework. We simulate a 1D quantum black hole using the Sachdev-Ye-Kitaev (SYK) model and reconstruct its bulk geometry via an autoencoder constrained by the Quantum Bures Metric. Our analysis reveals a strong negative correlation between the thermodynamic entropy and the box-counting dimension of the emergent manifold, with finite size scaling consistent with perfect anti-correlation in the thermodynamic limit. Crucially, control experiments reveal that this correspondence is chaos-dependent: integrable free fermion systems show dramatically weaker correlations, while spatially local Heisenberg chains exhibit intermediate behavior. This establishes chaos as a key requirement for strong entropy-topology correspondence, distinguishing maximally chaotic from integrable thermal quantum states. We observe no topological phase transitions across the thermal crossover, supporting the hypothesis that high-entropy chaotic thermalization drives geometric smoothing, consistent with the ER=EPR conjecture.
This dissertation presents a comprehensive investigation of fermionic quantum matter into the intricate relationship between quantum chaos, thermalization, and their connection to gravitational duals, with particular emphasis on the Sachdev-Ye-Kitaev (SYK) model and its extensions. Through an integrated approach combining analytical solutions in the large-N limit with advanced numerical techniques, we explore fundamental questions in quantum many-body physics, including the interplay of chaos and ergodicity, universal transport in non-Fermi liquids, and the role of gravitational analogs in strongly correlated systems. We first demonstrate that coupled SYK lattices evade instantaneous thermalization despite weak charge fluctuations, governed by a discrete wave equation dependent solely on hopping strength, independent of on-site interactions. Thermodynamically, coupled SYK models exhibit critical exponents universal to van der Waals fluids and AdS black holes, yet diverge from the Hawking-Page transition at ultralow temperatures, revealing a continuum of first-order phase transitions. They simultaneously retain the quantum chaotic properties of both a single-dot SYK model and black holes. In extended SYK chains, we identify a universal DC conductivity bound across interaction regimes, identifying crossovers between insulating, Fermi liquid, strange metal, and bad metal phases with signatures of holographic insulator behavior. Quench dynamics in mixed SYK systems reveal rapid thermalization without prethermal plateaus, enabling direct observation of thermalization in closed quantum systems in the thermodynamic limit. To diagnose ergodicity-breaking, we introduce the Krylov variance, a probe of operator localization in Krylov space, validated through mass-deformed SYK and quantum East models. Finally, we uncover Page curve entanglement dynamics and temporal quantum phase transitions in entanglement Hamiltonians of interacting fermionic chains, where symmetry-sector crossings highlight limitations of perturbative calculations performed around initial times, such as in Hawking’s analysis of black hole evaporation. We provide a condensed matter analog of the holographic “island formula” — a mechanism invoking quantum extremal surfaces to resolve the black hole information paradox. Methodologically, this work integrates large-N-type expansions, Keldysh contour techniques, and numerical methods to bridge condensed matter physics, quantum information, and gravitational duality. The results establish SYK-type systems as a universal framework for studying non-Fermi liquids, quantum criticality, and black hole analogs, while paving the way for exploring bosonic extensions and generalizations to open quantum systems.
The realization of holographic duality on near-term intermediate-scale quantum (NISQ) hardware requires identifying sparse Hamiltonians that retain essential gravitational features. While previous approaches rely on machine-learning optimization to find such models, we demonstrate that simple magnitude-based pruning of the Sachdev-Ye-Kitaev (SYK) model preserves holographic transport at sparsities ≈5%. By mapping the phase diagram of sparsified SYK (N ≤ 12), we identify a distinct phase boundary where quantum chaos, diagnosed via the Spectral Form Factor (SFF), dissolves, even while traversable wormhole signatures persist. This decoupling suggests that information scrambling can occur without the emergent geometry required for gravity. We validate the gravitational phenomenon of the chaotic phase by observing a quantized Shapiro time delay in the teleportation signal with perturbation, consistent with the emergence of bulk geometry. Finally, we show that this robustness is unique to all-to-all connectivity. Imposing linear local geometry destroys the holographic phase, highlighting non-locality as critical for quantum gravity simulations.
The complex Sachdev-Ye-Kitaev (SYK) model, featuring fermions with all-to-all interactions, serves as a dual paradigm for understanding non-Fermi liquid behavior and the holographic nature of charged black holes. Two defining characteristics of the standard SYK model are its maximal chaos (Lyapunov exponent λL=2πT at temperature T), and its finite zero-temperature residual entropy. While previous studies have largely focused on couplings drawn from a zero-mean Gaussian distribution, we investigate a generalized model with a finite mean-to-standard-deviation ratio, g≡J0/δJ of the coupling distribution in order to get deeper insight into the evolution of chaos. We find that increasing g yields the following effects: (i) The system remains a fast scrambler with λL=AT, but with a suppressed coefficient A<2π. (ii) In the limit g→∞, out-of-time-ordered correlators (OTOCs) no longer exhibit exponential growth with λL≃0. (iii) The spectral correlations indicative of late-time chaos maintain Wigner-Dyson level spacing statistics for all values of g. (iv) The system preserves a finite residual entropy, albeit with reduced magnitude, for all g values. We conclude that in this generalized SYK model, there is a chaotic to non-chaotic crossover. Moreover different measures of chaos decouple, demonstrating that the presence of finite residual entropy does not strictly imply maximal chaos.
Quantum chaos remains a challenging concept because classical signatures of chaos do not directly transfer to quantum systems. This thesis investigates which indicators of chaos are shared between classical and quantum mechanics and how they can be used to characterise non-integrable many-body systems. We examine ergodicity and thermalisation by means of the Eigenstate Thermalisation Hypothesis (ETH) as signatures of non-integrability, and study the Out-of-Time-Ordered Correlator (OTOC) as a probe of information scrambling and as a measure of sensitivity to initial conditions. We briefly discuss the appearance of chaotic wavefunctions and observations such as quantum scars and quantum ergodicity in individual eigenmodes of the Bunimovich stadium potential. Using the OTOC, we discuss extensions of the Lyapunov exponent to quantum systems and apply these ideas to the Sachdev–Ye–Kitaev (SYK) model. In the large-N limit, and using the AdS/CFT correspondence, we find that the model might potentially be the dual of a black hole and that it is maximally chaotic. We conclude by discussing additional proposed signatures of quantum chaos and prospects for experimental realisations of the SYK model.
We investigate the bulk-boundary correspondence in the SYK model from a quantum information perspective. The SYK model describes a system of Majorana fermions with random all-to-all interactions, whose disorder average-typically taken over a Gaussian ensemble-admits a dual description in terms of JT gravity in the large-N, low-energy limit. This framework provides a minimal setting for exploring holography and emergent spacetime in nearly AdS2. We probe the holographic principle through diagnostics of quantum chaos and entanglement. In the early-time regime, the SYK model saturates the universal bound on the Lyapunov exponent, signaling maximal chaos consistent with semiclassical black hole dynamics. In the late-time regime, its spectral statistics are governed by random matrix theory, reflecting universal features of strongly chaotic quantum systems. These dynamical properties establish a concrete link between boundary quantum chaos and bulk semiclassical gravity. In parallel, we analyze quantum entanglement and the structure of operator algebras to investigate transitions in the associated von Neumann algebras and their implications for emergent geometry. To explore the robustness of these phenomena, we consider deformations of the SYK model through modified matter couplings and alternative random distributions. Our results clarify how quantum information-theoretic structures encode bulk gravitational dynamics and provide insight into the mechanism of spacetime emergence.
We review our recent work [arXiv:2009.10759] where we studied the chaotic property of the two coupled Sachdev-Ye-Kitaev systems exhibiting a Hawking-Page like phase transition. By computing the out-of-time-ordered correlator in the large N limit by using the bilocal field formalism, we found that the chaos exponent of this model shows a discontinuous fall-off at the phase transition temperature. Hence in this model the Hawking-Page like transition is correlated with a transition in chaoticity, as expected from the relation between a black hole geometry and the chaotic behavior in the dual field theory.