Krylov winding emerges as a generic feature of quantum chaotic systems from the universal operator growth bound, producing size winding when a low-rank Krylov-to-size mapping exists and the chaos bound saturates.
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Krylov Complexity
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We introduce and review a new complexity measure, called `Krylov complexity', which takes its origins in the field of quantum-chaotic dynamics, serving as a canonical measure of operator growth and spreading. Krylov complexity, underpinned by the Lanczos algorithm, has since evolved into a highly diverse field of its own right, both because of its attractive features as a complexity, whose definition does not depend on arbitrary control parameters, and whose phenomenology serves as a rich and sensitive probe of chaotic dynamics up to exponentially late times, but also because of its relevance to seemingly far-afield subjects such as holographic dualities and the quantum physics of black holes. In this review we give a unified perspective on these topics, emphasizing the robust and most general features of K-complexity, both in chaotic and integrable systems, state and prove theorems on its generic features and describe how it is geometrised in the context of (dual) gravitational dynamics. We hope that this review will serve both as a source of intuition about K-complexity in and of itself, as well as a resource for researchers trying to gain an overview over what is by now a rather large and multi-faceted literature. We also mention and discuss a number of open problems related to K-complexity, underlining its currently very active status as a field of research.
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Defines tripartite complexity and complexity gap for three-subsystem states and reports that the gap has definite sign across holographic CV, Fisher-Rao, and Krylov measures, suggesting it as a building block for complexity inequalities.
Entropy injection from an environment enlarges the effective Hilbert space and enhances many-body chaos, demonstrated via analytical computation of relaxation and Lyapunov exponent in a solvable complex Brownian SYK model.
For analytic nonlinearities with fast Fourier decay, mean-field trajectories in random recurrent networks are uniquely determined by their continuous past, revealed through Krylov state space unfolding that bounds the Lyapunov exponent.
q-Askey deformations of DSSYK produce transfer matrices from basic orthogonal polynomials whose chord numbers map to ER bridge lengths and signal geometric transitions with discrete spectra in sine dilaton gravity.
The paper establishes a Lie-algebraic framework for exact Krylov dynamics in time-dependent quantum systems and introduces a quantum speed limit for complexity growth that retains its time-independent form but saturates only when the Hamiltonian commutes with itself at different times.
Mini-boson stars in AdS spacetime are proposed as holographic realizations of quantum scars, exhibiting chaotic spectra with integrable subsectors, anomalously low entanglement, and robust Krylov complexity revivals.
In double-scaled SYK, state-adapted dressed chord operators change the emergent algebra from Type II1 to Type I∞ and restore purity of KM states, unlike generic operators.
Holographic Krylov complexity for charged composite and extended probes retains universal leading large-time growth but acquires structure-dependent subleading corrections.
LogK complexity via replicas distinguishes genuine scrambling from saddle effects in quantum and classical systems and refines the measure for integrable cases.
A spectral-resolvent Krylov framework defines a distribution for quantum Fisher information and identifies universal exponential or algebraic convergence regimes based on the Liouville spectrum.
In the continuum limit the discrete Krylov chain becomes a Klein-Gordon field in AdS2, with Lanczos growth rate α identified as πT, recovering the maximal chaos bound and requiring the Breitenlohner-Freedman bound for consistency.
In the quadratic resonant level model, Lanczos coefficients of impurity operators can be tuned to arbitrary growth patterns via coupling choice, showing they do not reliably indicate integrability or chaos.
Krylov complexity remains nonsingular at SWSSB crossovers but shows a singular area-to-volume-law transition at genuine mixed-state SWSSB phase transitions in dephasing channels.
Holographic complexity measures show universal linear growth followed by late-time saturation, proven necessary and sufficient via pole structures in the energy basis using the residue theorem, arising from random matrix statistics.
In W3 CFTs, Lanczos coefficients b_N grow as N^2 for generalized Liouvillian with W generators, violating the universal linear growth bound and causing divergent Krylov complexity, with the same quadratic growth in the SL(3,R) subalgebra.
Recursion method extension to quench dynamics is limited by state-dependent quench coefficients c_n lacking universal structure, restricting accurate timescales except for favorable initial states.
Wigner negativity in Krylov space stays O(1) or grows as t^{1/2} (without Hilbert-space scaling) in 2d CFTs, one-cut matrix models, and double-scaled SYK, indicating emergent semiclassicality.
Photon rings around black holes saturate the quantum chaos bound via Lyapunov exponents of null geodesics and OTOCs in the near-ring region.
A non-Lorentzian scalar QFT with SU(1,1) symmetry obtained from N=4 SYM is finite at all orders in perturbation theory.
Memoryful open random quantum circuits sustain entanglement and magic growth like unitary circuits while memoryless ones show decaying entanglement but persistent magic, with memoryful dynamics approaching k-designs more effectively.
Generalized Krylov complexity predicts the minimum time to realize target operations in analog quantum simulators such as Rydberg atom arrays.
Brick-wall spectra in de Sitter space show long-range chaotic signatures via spectral form factor and Krylov complexity even when conventional level repulsion is absent.
In holographic 6d N=(1,0) SCFTs, generalized proper momentum of infalling particles grows linearly at late times, with early dynamics modified by SU(2)_R charge and quiver spreading.
citing papers explorer
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Krylov Winding and Emergent Coherence in Operator Growth Dynamics
Krylov winding emerges as a generic feature of quantum chaotic systems from the universal operator growth bound, producing size winding when a low-rank Krylov-to-size mapping exists and the chaos bound saturates.
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Complexity Inequalities for Quantum Subsystems
Defines tripartite complexity and complexity gap for three-subsystem states and reports that the gap has definite sign across holographic CV, Fisher-Rao, and Krylov measures, suggesting it as a building block for complexity inequalities.
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Enhancing Many-Body Chaos via Entropy Injection from Environment
Entropy injection from an environment enlarges the effective Hilbert space and enhances many-body chaos, demonstrated via analytical computation of relaxation and Lyapunov exponent in a solvable complex Brownian SYK model.
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Predictable Mean-Field Chaos in Random Recurrent Networks
For analytic nonlinearities with fast Fourier decay, mean-field trajectories in random recurrent networks are uniquely determined by their continuous past, revealed through Krylov state space unfolding that bounds the Lyapunov exponent.
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q-Askey Deformations of Double-Scaled SYK
q-Askey deformations of DSSYK produce transfer matrices from basic orthogonal polynomials whose chord numbers map to ER bridge lengths and signal geometric transitions with discrete spectra in sine dilaton gravity.
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Krylov Dynamics and Operator Growth in Time-Dependent Systems via Lie Algebras
The paper establishes a Lie-algebraic framework for exact Krylov dynamics in time-dependent quantum systems and introduces a quantum speed limit for complexity growth that retains its time-independent form but saturates only when the Hamiltonian commutes with itself at different times.
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Quantum scars from holographic boson stars
Mini-boson stars in AdS spacetime are proposed as holographic realizations of quantum scars, exhibiting chaotic spectra with integrable subsectors, anomalously low entanglement, and robust Krylov complexity revivals.
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Emergent States and Algebras from the Double-Scaling limit of Pure States in SYK
In double-scaled SYK, state-adapted dressed chord operators change the emergent algebra from Type II1 to Type I∞ and restore purity of KM states, unlike generic operators.
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Holographic Krylov Complexity for Charged, Composite and Extended Probes
Holographic Krylov complexity for charged composite and extended probes retains universal leading large-time growth but acquires structure-dependent subleading corrections.
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Towards a Refinement of Krylov Complexity: Scrambling, Classical Operator Growth and Replicas
LogK complexity via replicas distinguishes genuine scrambling from saddle effects in quantum and classical systems and refines the measure for integrable cases.
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Krylov Distribution and Universal Convergence of Quantum Fisher Information
A spectral-resolvent Krylov framework defines a distribution for quantum Fisher information and identifies universal exponential or algebraic convergence regimes based on the Liouville spectrum.
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Krylov Subspace Dynamics as Near-Horizon AdS$_2$ Holography
In the continuum limit the discrete Krylov chain becomes a Klein-Gordon field in AdS2, with Lanczos growth rate α identified as πT, recovering the maximal chaos bound and requiring the Breitenlohner-Freedman bound for consistency.
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Resonant level model from a Krylov perspective: Lanczos coefficients in a quadratic model
In the quadratic resonant level model, Lanczos coefficients of impurity operators can be tuned to arbitrary growth patterns via coupling choice, showing they do not reliably indicate integrability or chaos.
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Krylov Complexity and Mixed-State Phase Transition
Krylov complexity remains nonsingular at SWSSB crossovers but shows a singular area-to-volume-law transition at genuine mixed-state SWSSB phase transitions in dephasing channels.
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Universal Time Evolution of Holographic and Quantum Complexity
Holographic complexity measures show universal linear growth followed by late-time saturation, proven necessary and sufficient via pole structures in the energy basis using the residue theorem, arising from random matrix statistics.
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Violation of Universal Operator Growth Hypothesis in $\mathcal{W}_3$Conformal Field Theories
In W3 CFTs, Lanczos coefficients b_N grow as N^2 for generalized Liouvillian with W generators, violating the universal linear growth bound and causing divergent Krylov complexity, with the same quadratic growth in the SL(3,R) subalgebra.
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Recursion method for out-of-equilibrium many-body dynamics: strengths and limitations
Recursion method extension to quench dynamics is limited by state-dependent quench coefficients c_n lacking universal structure, restricting accurate timescales except for favorable initial states.
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Wigner negativity in Krylov space and emergent semiclassicality
Wigner negativity in Krylov space stays O(1) or grows as t^{1/2} (without Hilbert-space scaling) in 2d CFTs, one-cut matrix models, and double-scaled SYK, indicating emergent semiclassicality.
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Black Hole Photon Rings Saturate the Quantum Chaos Bound
Photon rings around black holes saturate the quantum chaos bound via Lyapunov exponents of null geodesics and OTOCs in the near-ring region.
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Finite scalar field theory with SU(1,1) spacetime symmetry from near-BPS limits of $\mathcal{N}=4$ SYM
A non-Lorentzian scalar QFT with SU(1,1) symmetry obtained from N=4 SYM is finite at all orders in perturbation theory.
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Resource generation and dynamical complexities in open random quantum circuits
Memoryful open random quantum circuits sustain entanglement and magic growth like unitary circuits while memoryless ones show decaying entanglement but persistent magic, with memoryful dynamics approaching k-designs more effectively.
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Bridging Krylov Complexity and Universal Analog Quantum Simulator
Generalized Krylov complexity predicts the minimum time to realize target operations in analog quantum simulators such as Rydberg atom arrays.
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Cosmological brick walls & quantum chaotic dynamics of de Sitter horizons
Brick-wall spectra in de Sitter space show long-range chaotic signatures via spectral form factor and Krylov complexity even when conventional level repulsion is absent.
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Complexity and Operator Growth in Holographic 6d SCFTs
In holographic 6d N=(1,0) SCFTs, generalized proper momentum of infalling particles grows linearly at late times, with early dynamics modified by SU(2)_R charge and quiver spreading.
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Deforming the Double-Scaled SYK & Reaching the Stretched Horizon From Finite Cutoff Holography
Deformations of the double-scaled SYK model via finite-cutoff holography produce Krylov complexity as wormhole length and realize Susskind's stretched horizon proposal through targeted T² deformations in the high-energy spectrum.
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Complexity of Quantum Trajectories
Intrinsic dimension of quantum trajectories serves as an unsupervised probe sensitive to chaos, integrability, and ergodicity breaking in dissipative quantum systems.
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Cosmological Entanglement Entropy from the von Neumann Algebra of Double-Scaled SYK & Its Connection with Krylov Complexity
Algebraic entanglement entropy from type II1 algebras in double-scaled SYK is matched via triple-scaling limits to Ryu-Takayanagi areas in (A)dS2, reproducing Bekenstein-Hawking and Gibbons-Hawking formulas for specific regions while depending on Krylov complexity of the Hartle-Hawking state.
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Toward Krylov-based holography in double-scaled SYK
Establishes a threefold duality linking Krylov complexity growth rate to wormhole velocity and proper momentum in DSSYK holography, with higher moments capturing replica wormholes and Krylov entropy equaling parent-geometry von Neumann entropy after tracing baby universes.
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Krylov Complexity Under Hamiltonian Deformations and Toda Flows
Certain Hamiltonian deformations preserve the Krylov subspace, yielding generalized Toda equations and allowing imaginary-time dynamics to be recast as real-time unitary evolution, with applications to thermodynamic states and supersymmetric systems.
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Krylov Complexity: Flat bands and Carroll breaking deformations
Krylov complexity growth distinguishes phase-dependent resilience of Carrollian sectors in all-bands-flat fermionic ladders against delocalizing perturbations and exhibits UV sensitivity in a continuum Carroll scalar field with gradient deformation.
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Krylov complexity has it all
Krylov complexity is equivalent to Lanczos coefficients, return amplitude, and spectral density for operator dynamics, via an explicit recursive algorithm from its t=0 Taylor expansion.
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Krylov Complexity in Periodically Driven CFTs and Critical Fermions
Arnoldi coefficients approach unity exponentially in heating phases of driven CFTs but oscillate in non-heating phases; lattice realizations show distinct spectral and graph signatures despite similar CFT Krylov growth.
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Krylov complexity for Lin-Maldacena geometries and their holographic duals
In the BMN matrix model and its holographic duals, Krylov basis states and Lanczos coefficients are uniquely fixed by the model's mass parameter.
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Probing the Chaos to Integrability Transition in Double-Scaled SYK
A first-order phase transition in the Berkooz-Brukner-Jia-Mamroud interpolating model causes chord number, Krylov complexity, and operator size to switch discontinuously from chaotic (linear/exponential) to quasi-integrable (quadratic) growth.
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Krylov Complexity for Open Quantum System: Dissipation and Decoherence
Krylov complexity saturates in the full high-temperature Caldeira-Leggett system, reproduces dissipative features when decoherence is suppressed, shows oscillations when dissipation is suppressed, and remains insensitive to decoherence onset because the Krylov basis differs from the conventional one
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Complexity of Quadratic Quantum Chaos
Hard-core boson two-body models with random interactions exhibit chaotic spectral statistics, operator growth, and eigenstate properties approaching those of random matrices and the SYK model.
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Quantum analogues of exponential sensitivity: from Loschmidt echo to Krylov complexity
This review surveys the Loschmidt echo, OTOCs, and Krylov complexity as quantum proxies for classical Lyapunov exponents in chaotic systems.
- Scrambling of Entanglement from Integrability to Chaos: Bootstrapped Time-Integrated Spread Complexity
- Long-Range Pairing in the Kitaev Model: Krylov Subspace Signatures