Wigner negativity in Krylov space and emergent semiclassicality
Pith reviewed 2026-07-03 19:24 UTC · model grok-4.3
The pith
The Krylov basis makes dynamics in large-N many-body systems appear semiclassical, with Wigner negativity staying constant or growing only as the square root of time.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We propose that the Krylov basis gives a semiclassical representation of dynamics in general large-N, complex, many-body systems. As a probe of this semiclassicality, we study the growth of Wigner negativity under time evolution in Krylov space in several solvable models. In 2d CFTs, negativity remains an O(1) constant and does not grow at late times. In general one-cut matrix models, negativity grows as t^{1/2} at large O(1) times but does not scale with the Hilbert space dimension. In the double-scaled SYK model, an approximately classical phase at early times is followed by a semiclassical phase with t^{1/2} growth at late times. In all cases, negativity either remains constant or grows s
What carries the argument
The Krylov basis, generated by repeated application of the Hamiltonian starting from an initial state, used here as the representation in which Wigner negativity is computed to test for semiclassical dynamics.
If this is right
- In 2d CFTs with vacuum or thermofield-double initial states excited by primaries, Wigner negativity stays O(1) at all late times.
- In one-cut random matrix models, negativity grows as t^{1/2} yet remains independent of system size.
- Double-scaled SYK exhibits a constant-negativity phase at early times followed by t^{1/2} growth.
- Across all examined cases, the slow or absent growth of negativity indicates that Krylov-space dynamics remain approximately classical.
Where Pith is reading between the lines
- If the pattern holds beyond solvable models, classical simulation methods could be applied directly in Krylov space to approximate many-body evolution at large N.
- The result suggests checking whether other operator bases or subspaces also suppress Wigner negativity in the same systems.
- It would be useful to test the same negativity diagnostic in higher-dimensional field theories or lattice models where exact solutions are unavailable.
Load-bearing premise
The specific solvable models examined are representative of generic large-N many-body systems, and Wigner negativity serves as a reliable quantitative proxy for the difficulty of classical simulation.
What would settle it
A calculation in one of the models showing Wigner negativity that grows linearly with time or scales with Hilbert-space dimension in the Krylov basis would falsify the claim of emergent semiclassicality.
read the original abstract
We propose that the Krylov basis gives a semiclassical representation of dynamics in general large-$N$, complex, many-body systems. As a probe of this semiclassicality, we study the growth of Wigner negativity -- a measure of the complexity of classical simulation -- under time evolution in Krylov space in several solvable models. We begin with 2d CFTs, initially in either the vacuum or the thermofield double state on a line excited by a primary operator. In both cases, Wigner negativity remains an $O(1)$ constant and does not grow at late times, indicating approximately classical dynamics in the Krylov basis. We then study random matrix theory with the maximally entangled state between two copies as the initial state. For general one-cut matrix models, we argue that Wigner negativity in the Krylov basis grows as $t^{1/2}$ at large $O(1)$ times but does not scale with the Hilbert space dimension, thus indicating semiclassical dynamics in Krylov space. Finally, in the double-scaled SYK model, we find an approximately classical phase (constant negativity) at early times and a semiclassical phase ($t^{1/2}$ growth) at late times. In all these examples, Wigner negativity either remains constant or grows slowly, demonstrating emergent semiclassicality of dynamics in Krylov space.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript proposes that the Krylov basis provides a semiclassical representation of dynamics in general large-N, complex, many-body systems. This is probed by studying the growth of Wigner negativity under time evolution in Krylov space across three solvable models: 2d CFTs (vacuum or TFD + primary, where negativity remains O(1)), one-cut random matrix models (maximally entangled initial state, where negativity grows as t^{1/2} independent of Hilbert-space dimension), and the double-scaled SYK model (early-time constant negativity followed by late-time t^{1/2} growth).
Significance. If the central claim holds, the result would indicate emergent semiclassicality in Krylov space for large-N systems, with Wigner negativity serving as a quantitative proxy for the difficulty of classical simulation. The explicit calculations in the three solvable models constitute a concrete strength, yielding specific scaling predictions (O(1) or t^{1/2}) that are falsifiable within those models.
major comments (2)
- [§1] §1 (Introduction): The central claim that the Krylov basis yields semiclassical dynamics in 'general large-N, complex, many-body systems' extrapolates from explicit calculations in three models possessing special structure (conformal symmetry, random-matrix universality, all-to-all maximal chaos) without a universality argument or additional generic non-integrable, non-random example to establish that negativity growth remains sub-linear and dimension-independent outside this class.
- [§3] §3 (Random matrix models): The argument that Wigner negativity grows as t^{1/2} at large O(1) times but does not scale with Hilbert space dimension for general one-cut matrix models is tied to the maximally entangled initial state; it is unclear whether this independence from dimension follows from the model properties alone or requires additional assumptions that limit generality.
minor comments (1)
- Notation for the Krylov basis and Wigner negativity could be introduced with a brief self-contained definition in the main text rather than relying solely on references.
Simulated Author's Rebuttal
We thank the referee for their positive assessment of the significance of our calculations and for the constructive comments. We respond to each major comment below.
read point-by-point responses
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Referee: [§1] §1 (Introduction): The central claim that the Krylov basis yields semiclassical dynamics in 'general large-N, complex, many-body systems' extrapolates from explicit calculations in three models possessing special structure (conformal symmetry, random-matrix universality, all-to-all maximal chaos) without a universality argument or additional generic non-integrable, non-random example to establish that negativity growth remains sub-linear and dimension-independent outside this class.
Authors: The manuscript presents the statement as a proposal ('We propose that...') motivated by the explicit results across three distinct classes of large-N models. We do not provide a universality argument or additional examples. To address the concern, we will revise the introduction to frame the claim more explicitly as a conjecture supported by the studied models rather than implying a general result. revision: partial
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Referee: [§3] §3 (Random matrix models): The argument that Wigner negativity grows as t^{1/2} at large O(1) times but does not scale with Hilbert space dimension for general one-cut matrix models is tied to the maximally entangled initial state; it is unclear whether this independence from dimension follows from the model properties alone or requires additional assumptions that limit generality.
Authors: Section 3 explicitly uses the maximally entangled initial state between two copies. The t^{1/2} growth without dimension scaling is derived for one-cut models under this initial condition. We will revise the text to clarify the dependence on this choice of initial state and the associated assumptions. revision: yes
Circularity Check
No circularity; claims rest on explicit calculations in solvable models
full rationale
The paper's proposal that the Krylov basis yields semiclassical dynamics for general large-N systems is supported by direct computations of Wigner negativity in three specific models: 2d CFTs (vacuum or TFD + primary, negativity O(1)), one-cut random matrices (maximally entangled state, ~t^{1/2} growth independent of dimension), and double-scaled SYK (early constant, late t^{1/2}). These are presented as evidence rather than derived from fitted parameters renamed as predictions or from self-citations. No self-definitional steps, load-bearing self-citations, or ansatze imported via citation appear in the derivation chain. The extrapolation to generic systems is an interpretive claim, not a reduction by construction to the inputs.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Krylov basis is obtained by repeated application of the Hamiltonian starting from a reference state
- domain assumption Wigner negativity quantifies the complexity of classical simulation
Reference graph
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discussion (0)
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