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arXiv:2512.20359

Hall's Exact Uncertainty Relation and the Geometry of Krylov-Space Operator Dynamics

Mohsen Alishahiha, Souvik Banerjee

quant-phhep-thSubmitted Dec 23, 2025• Announced Dec 24, 2025• 0 citations• Revised Oct 22, 2026 (v3)
v1 · Dec 23, 2025v2 · Feb 9, 2026v3 · Oct 22, 2026
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Abstract

Krylov representations of operator dynamics encode two inequivalent motions: probability transport along the discrete Krylov chain and motion of the normalized amplitude vector on the associated sphere. We lift Hall's exact uncertainty relation to Liouville space and show that, for the fixed Krylov-basis measurement, its classical Fisher information locally attains the quantum Fisher information. The amplitude vector therefore follows an exact constant-speed spherical trajectory. We construct its moving frame, identifying the first Lanczos coefficient with the speed and the higher coefficients with successive bending. Geodesic curvature controls the leading departure from Mandelstam-Tamm saturation. Separately, we compare cumulative and pointwise bounds on Krylov-index transport, which depends on the full Lanczos sequence.

Comments
6 pages + Appendixes; Major revisions, Journal version

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Krylov representations of operator dynamics encode two inequivalent motions: probability transport along the discrete Krylov chain and motion of the normalized amplitude vector on the associated sphere. We lift Hall's exact uncertainty relation to Liouville space and show that, for the fixed Krylov-basis measurement, its classical Fisher information locally attains the quantum Fisher information. The amplitude vector therefore follows an exact constant-speed spherical trajectory. We construct its moving frame, identifying the first Lanczos coefficient with the speed and the higher coefficients with successive bending. Geodesic curvature controls the leading departure from Mandelstam-Tamm saturation. Separately, we compare cumulative and pointwise bounds on Krylov-index transport, which depends on the full Lanczos sequence.
We establish a direct correspondence between the Krylov hierarchy of quantum Fisher information (QFI) and a Krylov-resolved decomposition of the symmetric logarithmic derivative (SLD). Building on the existing Krylov formulation of QFI, we show that the metrological information resolved at each Krylov level is exactly encoded in the incremental gain of the Lanczos hierarchy, giving the Krylov distribution a direct information-theoretic interpretation. A dual spectral formulation distinguishes the Hilbert--Schmidt geometry used to construct the computational Lanczos hierarchy from the state-dependent geometry in which the SLD and QFI are naturally defined, and relates their associated Liouville measures. This distinction clarifies the origin of different convergence regimes: we recover exponential convergence for gapped spectra and derive an exact algebraic hard-edge convergence law for the Jacobi class, including the exact finite-order error and the marginal case separating finite from divergent QFI.
Krylov complexity is a powerful diagnostic of quantum dynamics, with clear connections to other measures of quantum chaos and operator growth. One such measure is the Loschmidt amplitude, defined as the overlap of initially identical states evolved under two slightly different Hamiltonians. Its decay in certain systems is controlled by the classical Lyapunov exponent. Using the algebraic properties of the Krylov complexity operator, we express Krylov complexity as the derivative of a Loschmidt amplitude whose perturbation is parameterized by an angular variable φφφ. This formulation allows us to define a spectral propagator that encodes the entire complexity distribution, which we characterize for specific types of systems. We study the two-dimensional quantum geometry spanned by time and φφφ where the original and deformed trajectories reside, demonstrating that Krylov complexity is upper-bounded by its volume. We also express the time derivative of Krylov complexity in terms of a distinct Loschmidt amplitude. Depending on the growth of the Lanczos coefficients, the perturbation term in this amplitude can be truncated. We propose that the strength of this perturbation provides a classification scheme for Krylov complexity dynamics and relate it to the φφφ-derivative of the spectral propagator. Using this analytical framework, we derive general relations between the time-dependence of the survival amplitude and Krylov space measures.
Abstract Krylov complexity is considered to provide a measure of the growth of operators evolving under Hamiltonian dynamics. The main strategy is the analysis of the structure of Krylov subspace KM(H,η)\mathcal {K}_M(\mathcal {H},\eta )KM​(H,η) spanned by the multiple applications of the Liouville operator L\mathcal {L}L defined by the commutator in terms of a Hamiltonian H\mathcal {H}H, L:=[H,⋅]\mathcal {L}:=[\mathcal {H},\cdot ]L:=[H,⋅] acting on an operator η, KM(H,η)=span{η,Lη,…,LM−1η}\mathcal {K}_M(\mathcal {H},\eta )=\text{span}\lbrace \eta ,\mathcal {L}\eta ,\ldots ,\mathcal {L}^{M-1}\eta \rbraceKM​(H,η)=span{η,Lη,…,LM−1η}. For a given inner product ( ·, ·) of the operators, the orthonormal basis {On}\lbrace \mathcal {O}_n\rbrace{On​} is constructed from O0=η/(η,η)\mathcal {O}_0=\eta /\sqrt{(\eta ,\eta )}O0​=η/(η,η)​ by Lanczos algorithm. The moments μm=(O0,LmO0)\mu _m=(\mathcal {O}_0,\mathcal {L}^m\mathcal {O}_0)μm​=(O0​,LmO0​) are closely related to the important data {bn} called Lanczos coefficients. I present the exact and explicit expressions of the moments {μm} for 16 quantum mechanical systems which are exactly solvable both in the Schrödinger and Heisenberg pictures. The operator η is the variable of the eigenpolynomials. Among them six systems show a clear sign of ‘non-complexity’ as vanishing higher Lanczos coefficients bm = 0, m ≥ 3.
In this work we develop a real-time Schwinger-Keldysh formulation of Krylov dynamics that treats Krylov complexity as an in-in observable generated by a closed time contour path integral. The resulting generating functional exposes an emergent phase-space description in which the Lanczos coefficients define an effective Hamiltonian governing operator motion along the Krylov chain. In the semiclassical limit, exponential complexity growth arises from hyperbolic trajectories, and asymptotically linear Lanczos growth appears as a universal chaotic fixed point, with sub-leading deformations classified as irrelevant, marginal or relevant. Going beyond the saddle, the Schwinger-Keldysh framework provides controlled access to fluctuations and large deviations of Krylov complexity, revealing sharp signatures of integrability-chaos crossovers that are invisible at the level of the mean. This formulation reorganises Krylov complexity into a dynamical field-theoretic framework and identifies new fluctuation diagnostics of operator growth in closed quantum systems.
We develop a geometric approach to operator growth and Krylov complexity in many-body quantum systems governed by symmetries. We start by showing a direct link between a unitary evolution with the Liouvillian and the displacement operator of appropriate generalized coherent states. This connection maps operator growth to a purely classical motion in phase space. The phase spaces are endowed with a natural information metric. We show that, in this geometry, operator growth is represented by geodesics, and Krylov complexity is proportional to a volume. This geometric perspective also provides two novel avenues toward computation of Lanczos coefficients, and it sheds new light on the origin of their maximal growth. We describe the general idea and analyze it in explicit examples, among which we reproduce known results from the Sachdev-Ye-Kitaev model, derive operator growth based on SU(2) and Heisenberg-Weyl symmetries, and generalize the discussion to conformal field theories. Finally, we use techniques from quantum optics to study operator evolution with quantum information tools such as entanglement and Renyi entropies, negativity, fidelity, relative entropy, and capacity of entanglement.
In closed quantum systems, Krylov complexity admits a geometric description; operator growth is equivalent to Hamiltonian flow in an emergent phase space whose structure is fixed by the Lanczos coefficients. We show that this picture survives, albeit in a fundamentally altered form, once the system is coupled to an environment.Using a Schwinger-Keldysh formulation of the full counting statistics of the Krylov position, we derive an effective action for operator growth under Lindblad dynamics. Even for the minimal case of dephasing, the phase-space dynamics ceases to be Hamiltonian; environmental coupling generates diffusion in the variable conjugate to Krylov depth, converting deterministic trajectories in to stochastic ones. The hyperbolic mechanism underlying exponential complexity growth is therefore broadened and, beyond a parametrically controlled scale, destroyed.This identifies dissipation as a relevant perturbation of the chaotic Krylov fixed point and reveals operator growth in open systems as a problem of stochastic dynamics in an emergent phase space.