Quantum information spreading in inhomogeneous spin ensembles
Pith reviewed 2026-05-10 12:50 UTC · model grok-4.3
The pith
Quantum information spreads in spin ensembles at rates set by the statistical distribution of resonance frequencies.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The Krylov construction applied to the Hamiltonian of an inhomogeneous spin ensemble generates an orthonormal basis in which the single-excitation dynamics can be solved exactly for asymptotically large systems. This yields closed-form expressions for the Lieb-Robinson velocity and quantum speed limit that vary with the chosen statistical distribution of spin frequencies. The same construction also supplies a concrete figure of merit, Krylov complexity, that quantifies how the excitation spreads.
What carries the argument
The Krylov space obtained by repeated application of the Hamiltonian to an initial state, which supplies an exact basis for single-excitation dynamics in arbitrarily distributed spin ensembles.
If this is right
- Exact Lieb-Robinson velocities become available for any chosen frequency distribution without further approximation.
- Quantum speed limits can be evaluated analytically for large inhomogeneous ensembles.
- Krylov complexity serves as a computable indicator of information spreading speed.
- Engineering of spin-based quantum components must account for the full statistical distribution of resonance frequencies to predict information flow.
Where Pith is reading between the lines
- Choosing a narrow or broad frequency distribution could be used to slow or accelerate quantum information transfer in spin arrays.
- The same Krylov approach may be applied to other disordered many-body systems to obtain bounds on correlation spreading.
- Experiments with tunable inhomogeneity in atomic or solid-state spin ensembles could test the predicted distribution dependence by varying the spread while keeping the mean fixed.
Load-bearing premise
The derivations assume an asymptotically large ensemble and restrict attention to the single-excitation subspace.
What would settle it
A direct measurement or numerical simulation of the time required for a single excitation to reach a distant spin, performed for several different frequency distributions, should match the predicted Lieb-Robinson velocity only if the distribution dependence is correct.
Figures
read the original abstract
We present a Krylov space based theoretical framework for modeling inhomogeneous spin ensembles with arbitrary distributions of spin frequencies and couplings. The framework is then used to asymptotically large spin ensemble. In the single-excitation subspace, the Krylov construction allows for to derive exact expressions for the Lieb-Robinson velocity and quantum speed limit, and figure of merit such as Krylov complexity. Our work reveals a strong dependence of the speed of information flow on the statistical distribution of resonance frequencies in the spin ensemble with immediate implications for the design of components for quantum technologies, realized for example with nitrogen vacancy centers, nuclear spins or ultracold atoms.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript develops a Krylov-subspace framework for quantum information spreading in asymptotically large inhomogeneous spin ensembles with arbitrary distributions of on-site frequencies and (position-dependent) couplings. In the single-excitation subspace the method is used to obtain exact expressions for the Lieb-Robinson velocity, quantum speed limit, and Krylov complexity, which are shown to depend strongly on the statistical properties of the resonance-frequency distribution P(ω). The results are motivated by applications to NV centers, nuclear spins, and ultracold atoms.
Significance. If the claimed exact, distribution-dependent formulas can be substantiated, the work supplies a concrete analytic handle on how disorder shapes information propagation speed in spin ensembles. This is relevant for quantum-technology design where control of information flow is desirable. The use of Krylov methods to reduce the many-body problem to an effective single-particle tight-binding chain with a spectral measure determined by P(ω) is a standard and potentially powerful reduction, provided the resulting expressions remain tractable.
major comments (2)
- [Abstract / §3] Abstract and the central derivation (presumably §3–4): the repeated claim of “exact expressions” for the Lieb-Robinson velocity and quantum speed limit for completely arbitrary P(ω) is not yet supported. The Lanczos recurrence coefficients are fixed by the moments of the effective spectral measure; for a generic distribution these moments lack closed analytic forms. The manuscript must therefore either (i) exhibit explicit closed-form expressions that hold without further regularity assumptions on P(ω), or (ii) state the precise class of distributions for which the expressions remain exact and analytic. Without this clarification the “exact” qualifier risks being overstated.
- [§4] §4 (or wherever the velocity and QSL are derived): the reduction to the single-excitation subspace is standard, but the passage from the Krylov chain to a concrete Lieb-Robinson bound or quantum-speed-limit expression must be shown explicitly. In particular, it is unclear whether the final formulas involve only the first few moments or require the full infinite set of Lanczos coefficients; the latter would generally necessitate numerical quadrature even for simple P(ω).
minor comments (2)
- [Abstract] Abstract: the sentence “The framework is then used to asymptotically large spin ensemble” is grammatically incomplete; “allows for to derive” should be “allows one to derive”.
- [Introduction] Notation: the symbol P(ω) for the frequency distribution is introduced without an explicit definition of its normalization or support; a short paragraph clarifying the measure would improve readability.
Simulated Author's Rebuttal
We thank the referee for the careful reading and constructive comments. We address the major points below, clarifying the scope of our exact results and indicating the revisions we will implement.
read point-by-point responses
-
Referee: [Abstract / §3] Abstract and the central derivation (presumably §3–4): the repeated claim of “exact expressions” for the Lieb-Robinson velocity and quantum speed limit for completely arbitrary P(ω) is not yet supported. The Lanczos recurrence coefficients are fixed by the moments of the effective spectral measure; for a generic distribution these moments lack closed analytic forms. The manuscript must therefore either (i) exhibit explicit closed-form expressions that hold without further regularity assumptions on P(ω), or (ii) state the precise class of distributions for which the expressions remain exact and analytic. Without this clarification the “exact” qualifier risks being overstated.
Authors: The Krylov construction yields an exact, non-perturbative mapping of the single-excitation dynamics onto a one-dimensional chain whose Lanczos coefficients are the moments of the spectral measure induced by P(ω). Consequently the Lieb-Robinson velocity and quantum speed limit are expressed exactly in terms of this measure (or its moment sequence) without additional approximations. For a completely arbitrary P(ω) these moments are defined by integrals and therefore the expressions remain exact but are not necessarily elementary. We will revise the abstract and §3 to state this distinction explicitly, specify that closed elementary forms exist only when the moment-generating function of P(ω) is known in closed form (e.g., Gaussian or Lorentzian distributions), and add an appendix containing the general moment formulas together with explicit analytic results for two representative distributions. revision: partial
-
Referee: [§4] §4 (or wherever the velocity and QSL are derived): the reduction to the single-excitation subspace is standard, but the passage from the Krylov chain to a concrete Lieb-Robinson bound or quantum-speed-limit expression must be shown explicitly. In particular, it is unclear whether the final formulas involve only the first few moments or require the full infinite set of Lanczos coefficients; the latter would generally necessitate numerical quadrature even for simple P(ω).
Authors: We agree that the explicit steps connecting the Krylov chain to the final bounds must be displayed. The Lieb-Robinson velocity is the supremum of the group velocity on the dispersion relation generated by the infinite tridiagonal Lanczos matrix; the quantum speed limit follows from the same spectral measure. While the full set of coefficients is formally required, useful analytic bounds can be obtained from the first few moments (variance and kurtosis of P(ω)). We will expand §4 to show the explicit mapping from the Lanczos matrix to the velocity bound and the QSL, including the integral representation over the spectral density. For concrete numerical evaluation of general P(ω) we indeed employ quadrature, but the underlying expressions remain exact prior to discretization. revision: yes
- We cannot exhibit elementary closed-form expressions valid for every conceivable P(ω) without regularity assumptions, as the Hamburger moment problem for a generic measure does not admit such forms.
Circularity Check
No circularity: standard Krylov application yields independent expressions
full rationale
The paper applies the established Krylov subspace construction to the single-excitation sector of an inhomogeneous spin Hamiltonian with arbitrary frequency distribution P(ω). The abstract states that this construction 'allows to derive exact expressions' for Lieb-Robinson velocity, quantum speed limit and Krylov complexity. No quoted step defines a quantity in terms of itself, renames a fitted parameter as a prediction, or imports a uniqueness theorem from the authors' prior work. The derivation chain therefore remains self-contained: the moments of the spectral measure are computed from the given distribution and couplings, and the resulting velocities and bounds follow directly from the Lanczos recurrence without tautological reduction.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption The single-excitation subspace is sufficient for modeling information spreading.
- domain assumption Krylov space construction yields exact expressions for large ensembles.
Reference graph
Works this paper leans on
-
[1]
Finite frequency range(−1≤q≤1) The corresponding orthogonal set of polynomials asso- ciated withP q(x) in this range areq-Hermite polynomials ∞X n=0 Hn(x|q) (q;q) n tn = 1 (teiθ, te−iθ;q) ∞ ,(32) wherex= cosθand (x;q) n =Qn−1 k=0(1−xq k) is theq- Pochhammer symbol and (x, y;q) n = (x;q) n(y;q) n. This gives the first two polynomials as H0(x|q) = 1 andH 1(...
-
[2]
Infinite frequency range(1< q <3) In this limit, the distribution function is often repre- sented by the Tsallis q-Gaussian function [61] Pq(ξ) = √β Cq 1−(1−q)βξ 2 1/(1−q) ,(38) 7 where, Cq = √πΓ 3−q 2(1−q) √q−1Γ 1 1−q .(39) The associated orthogonal polynomial set to this distri- bution is the relativistic Hermite polynomials [62], which is defined forξ=...
work page 2021
- [3]
-
[4]
E. Iyoda and T. Sagawa, Scrambling of quantum infor- mation in quantum many-body systems, Phys. Rev. A 97, 042330 (2018)
work page 2018
-
[5]
D. A. Abanin, E. Altman, I. Bloch, and M. Serbyn, Col- loquium: Many-body localization, thermalization, and entanglement, Rev. Mod. Phys.91, 021001 (2019)
work page 2019
-
[6]
D. J. Luitz and Y. Bar Lev, Information propagation in isolated quantum systems, Phys. Rev. B96, 020406 (2017)
work page 2017
- [7]
-
[8]
B. Swingle and D. Chowdhury, Slow scrambling in disor- dered quantum systems, Phys. Rev. B95, 060201 (2017)
work page 2017
-
[9]
S. Haroche and J.-M. Raimond,Exploring the Quantum (Oxford University Press, 2006)
work page 2006
-
[10]
J. M. Deutsch, Eigenstate thermalization hypothesis, Rep. Prog. Phys.81, 082001 (2018)
work page 2018
-
[11]
W. W. Ho and D. A. Abanin, Entanglement dynamics in quantum many-body systems, Phys. Rev. B95, 094302 (2017)
work page 2017
-
[12]
D. N. Matsukevich and A. Kuzmich, Quantum state transfer between matter and light, Science306, 663 (2004)
work page 2004
-
[13]
Z.-Y. Xu, S. Luo, W. L. Yang, C. Liu, and S. Zhu, Quan- tum speedup in a memory environment, Phys. Rev. A 89, 012307 (2014)
work page 2014
-
[14]
K. G. Paulson, S. Banerjee, and R. Srikanth, The effect of quantum memory on quantum speed limit time for CP- (in)divisible channels, Quantum Inf. Process.21(2022)
work page 2022
- [15]
-
[16]
G. Kurizki, P. Bertet, Y. Kubo, K. Mølmer, D. Pet- rosyan, P. Rabl, and J. Schmiedmayer, Quantum tech- nologies with hybrid systems, Proc. Natl. Acad. Sci. U.S.A.112, 3866 (2015)
work page 2015
-
[17]
A. A. Clerk, K. W. Lehnert, P. Bertet, J. R. Petta, and Y. Nakamura, Hybrid quantum systems with cir- cuit quantum electrodynamics, Nature Physics16, 257 (2020)
work page 2020
-
[18]
E. H. Lieb and D. W. Robinson, The finite group velocity of quantum spin systems, Commun. Math. Phys.28, 251 (1972)
work page 1972
-
[19]
M. B. Hastings, Locality in quantum and markov dynam- ics on lattices and networks, Phys. Rev. Lett.93, 140402 (2004)
work page 2004
-
[20]
L. Mandelstam and I. Tamm, The uncertainty relation between energy and time in non-relativistic quantum me- chanics, inSelected Papers(Springer Berlin Heidelberg, Berlin, Heidelberg, 1991) pp. 115–123
work page 1991
-
[21]
N. Margolus and L. B. Levitin, The maximum speed of dynamical evolution, Physica D: Nonlinear Phenomena 120, 188 (1998), proceedings of the Fourth Workshop on Physics and Consumption
work page 1998
-
[22]
Swingle, Unscrambling the physics of out-of-time- order correlators, Nat
B. Swingle, Unscrambling the physics of out-of-time- order correlators, Nat. Phys.14, 988 (2018)
work page 2018
-
[23]
D. E. Parker, X. Cao, A. Avdoshkin, T. Scaffidi, and E. Altman, A universal operator growth hypothesis, Phys. Rev. X9, 041017 (2019)
work page 2019
-
[24]
E. Rabinovici, A. S´ anchez-Garrido, R. Shir, and J. Son- ner, Krylov localization and suppression of complexity, J. High Energy Phys.2022(3)
work page 2022
-
[25]
D. P. Pires, M. Cianciaruso, L. C. C´ eleri, G. Adesso, and D. O. Soares-Pinto, Generalized geometric quantum speed limits, Phys. Rev. X6, 021031 (2016). 12
work page 2016
-
[26]
K. Takahashi and A. del Campo, Krylov subspace meth- ods for quantum dynamics with time-dependent genera- tors, Phys. Rev. Lett.134, 030401 (2025)
work page 2025
-
[27]
A. A. Nizami and A. W. Shrestha, Krylov construction and complexity for driven quantum systems, Phys. Rev. E108, 054222 (2023)
work page 2023
-
[28]
E. Rabinovici, A. S´ anchez-Garrido, R. Shir, and J. Son- ner, Operator complexity: a journey to the edge of krylov space, J. High Energy Phys.2021(62)
work page 2021
- [29]
- [30]
-
[31]
Entanglement and quantum coherence in krylov space dynamics, 2026
S. Choudhary, S. Mondkar, and U. Sen, Entanglement and quantum coherence in krylov space dynamics, arXiv (2026), 2603.26619 [quant-ph]
-
[32]
A. Bhattacharya, P. P. Nath, and H. Sahu, Speed lim- its to the growth of krylov complexity in open quantum systems, Phys. Rev. D109, L121902 (2024)
work page 2024
-
[33]
A. Gill and T. Sarkar, Speed limits and scrambling in krylov space, Phys. Rev. B111, 184307 (2025)
work page 2025
-
[34]
F. Mivehvar, F. Piazza, T. Donner, and H. Ritsch, Cavity QED with quantum gases: new paradigms in many-body physics, Adv. Phys.70, 1 (2021)
work page 2021
-
[35]
Y. Kubo, C. Grezes, A. Dewes, T. Umeda, J. Isoya, H. Sumiya, N. Morishita, H. Abe, S. Onoda, T. Ohshima, V. Jacques, A. Dr´ eau, J.-F. Roch, I. Diniz, A. Auffeves, D. Vion, D. Esteve, and P. Bertet, Hybrid quantum cir- cuit with a superconducting qubit coupled to a spin en- semble, Phys. Rev. Lett.107, 220501 (2011)
work page 2011
- [36]
-
[37]
D. Lachance-Quirion, Y. Tabuchi, A. Gloppe, K. Usami, and Y. Nakamura, Hybrid quantum systems based on magnonics, Applied Physics Express12, 070101 (2019)
work page 2019
-
[38]
I. C. Skogvoll, J. Lidal, J. Danon, and A. Kamra, Tunable anisotropic quantum rabi model via a magnon–spin-qubit ensemble, Phys. Rev. Appl.16, 064008 (2021)
work page 2021
-
[39]
A. O. Levchenko, V. V. Vasil’ev, S. A. Zibrov, A. S. Zi- brov, A. V. Sivak, and I. V. Fedotov, Inhomogeneous broadening of optically detected magnetic resonance of the ensembles of nitrogen-vacancy centers in diamond by interstitial carbon atoms, Applied Physics Letters106, 102402 (2015)
work page 2015
-
[40]
Y. Rosenzweig, Y. Schlussel, and R. Folman, Probing the origins of inhomogeneous broadening in nitrogen-vacancy centers with doppler-free-type spectroscopy, Phys. Rev. B98, 014112 (2018)
work page 2018
-
[41]
V. V. Dobrovitski, A. E. Feiguin, D. D. Awschalom, and R. Hanson, Decoherence dynamics of a single spin versus spin ensemble, Phys. Rev. B77, 245212 (2008)
work page 2008
-
[42]
J. Keeling, Quantum corrections to the semiclassical col- lective dynamics in the tavis-cummings model, Phys. Rev. A79, 053825 (2009)
work page 2009
-
[43]
M. A. Zeb, Analytical solution of the disordered tavis- cummings model and its fano resonances, Phys. Rev. A 106, 063720 (2022)
work page 2022
-
[44]
H. Sharma and H. S. Dhar, Quadratic power enhance- ment in extended dicke quantum battery (2025)
work page 2025
- [45]
-
[46]
J. H. Wesenberg, Z. Kurucz, and K. Mølmer, Dynamics of the collective modes of an inhomogeneous spin ensemble in a cavity, Phys. Rev. A83, 023826 (2011)
work page 2011
- [47]
-
[48]
Q. Ansel,Optimal control of inhomogeneous spin ensem- bles : applications in NMR and quantum optics, Theses, Universit´ e Bourgogne Franche-Comt´ e ; Technische Uni- versit¨ at (Munich, Allemagne) (2018)
work page 2018
-
[49]
B. A. Chase and J. M. Geremia, Collective processes of an ensemble of spin-1/2 particles, Phys. Rev. A78, 052101 (2008)
work page 2008
- [50]
- [51]
-
[52]
M. Tavis and F. W. Cummings, Exact solution for ann- molecule—radiation-field hamiltonian, Phys. Rev.170, 379 (1968)
work page 1968
-
[53]
C. Lanczos, An iteration method for the solution of the eigenvalue problem of linear differential and integral op- erators, J. Res. Natl. Bur. Stand. B45, 255 (1950)
work page 1950
-
[54]
S. Putz, D. O. Krimer, R. Ams¨ uss, A. Valookaran, T. N¨ obauer, J. Schmiedmayer, S. Rotter, and J. Majer, Protecting a spin ensemble against decoherence in the strong-coupling regime of cavity qed, Nature Physics10, 720 (2014)
work page 2014
-
[55]
D. O. Krimer, S. Putz, J. Majer, and S. Rotter, Non- markovian dynamics of a single-mode cavity strongly coupled to an inhomogeneously broadened spin ensem- ble, Phys. Rev. A90, 043852 (2014)
work page 2014
-
[56]
S. Deffner and S. Campbell, Quantum speed limits: from heisenberg’s uncertainty principle to optimal quantum control, J. Phys. A Math. Theor.50, 453001 (2017)
work page 2017
-
[57]
W. K¨ onig, Orthogonal polynomial ensembles in probabil- ity theory., Probability Surveys [electronic only]2, 385 (2005)
work page 2005
-
[58]
Favard, Sur les polynomes de Tchebicheff., C
J. Favard, Sur les polynomes de Tchebicheff., C. R. Acad. Sci., Paris200, 2052 (1935)
work page 2052
- [59]
-
[60]
A. P. Magnus and V. Pierrard, Formulas for recur- rence coefficients of orthogonal polynomials related to lorentzian-like weights, Journal of Computational and Applied Mathematics219, 431 (2008)
work page 2008
-
[61]
Gautschi, Orthogonal polynomials: applications and computation, Acta numerica5, 45 (1996)
W. Gautschi, Orthogonal polynomials: applications and computation, Acta numerica5, 45 (1996)
work page 1996
-
[62]
R. Koekoek, P. A. Lesky, and R. F. Swarttouw,Hyperge- ometric Orthogonal Polynomials and Their q-Analogues (Springer Berlin Heidelberg, 2010)
work page 2010
- [63]
-
[64]
Vignat, Old and new results about relativistic hermite polynomials, J
C. Vignat, Old and new results about relativistic hermite polynomials, J. Math. Phys.52, 093503 (2011)
work page 2011
-
[65]
C. Tsallis, S. V. F. Levy, A. M. C. Souza, and R. May- nard, Statistical-mechanical foundation of the ubiquity of l´ evy distributions in nature, Phys. Rev. Lett.75, 3589 (1995)
work page 1995
-
[66]
M. J. Stanley, C. Matthiesen, J. Hansom, C. Le Gall, C. H. H. Schulte, E. Clarke, and M. Atat¨ ure, Dynamics of a mesoscopic nuclear spin ensemble interacting with an optically driven electron spin, Phys. Rev. B90, 195305 (2014)
work page 2014
-
[67]
L. Lugiato, F. Prati, and M. Brambilla, Inhomogeneous broadening, inNonlinear Optical Systems(Cambridge University Press, 2015) p. 170–176
work page 2015
- [68]
-
[69]
D. O. Krimer, M. Zens, and S. Rotter, Critical phenom- ena and nonlinear dynamics in a spin ensemble strongly coupled to a cavity. i. semiclassical approach, Phys. Rev. A100, 013855 (2019)
work page 2019
-
[70]
S. Putz, A. Angerer, D. O. Krimer, R. Glattauer, W. J. Munro, S. Rotter, J. Schmiedmayer, and J. Majer, Spec- tral hole burning and its application in microwave pho- tonics, Nature Photonics11, 36 (2017)
work page 2017
-
[71]
H. A. R. El-Ella, A. Huck, and U. L. Andersen, Contin- uous microwave hole burning and population oscillations in a diamond spin ensemble, Phys. Rev. B100, 214407 (2019)
work page 2019
-
[72]
K. Sandner, H. Ritsch, R. Ams¨ uss, C. Koller, T. N¨ obauer, S. Putz, J. Schmiedmayer, and J. Majer, Strong magnetic coupling of an inhomogeneous nitrogen-vacancy ensemble to a cavity, Phys. Rev. A85, 053806 (2012)
work page 2012
-
[73]
E. P. Wigner, Random matrices in physics, SIAM review 9, 1 (1967)
work page 1967
-
[74]
L. Erd˝ os, B. Schlein, and H.-T. Yau, Local semicircle law and complete delocalization for wigner random matrices, Communications in Mathematical Physics287, 641–655 (2008)
work page 2008
-
[75]
J. L. Rubio, D. Viscor, J. Mompart, and V. Ahufinger, Atomic-frequency-comb quantum memory via piecewise adiabatic passage, Phys. Rev. A98, 043834 (2018)
work page 2018
-
[76]
K. Debnath, A. H. Kiilerich, A. Benseny, and K. Mølmer, Coherent spectral hole burning and qubit isolation by stimulated raman adiabatic passage, Phys. Rev. A100, 023813 (2019)
work page 2019
-
[77]
C. Laplane, P. Jobez, J. Etesse, N. Timoney, N. Gisin, and M. Afzelius, Multiplexed on-demand storage of po- larization qubits in a crystal, New J. Phys.18, 013006 (2015)
work page 2015
-
[78]
H. S. Dhar, M. Zens, D. O. Krimer, and S. Rotter, Vari- ational renormalization group for dissipative spin-cavity systems: Periodic pulses of nonclassical photons from mesoscopic spin ensembles, Phys. Rev. Lett.121, 133601 (2018)
work page 2018
-
[79]
M. Zens, D. O. Krimer, H. S. Dhar, and S. Rotter, Peri- odic cavity state revivals from atomic frequency combs, Phys. Rev. Lett.127, 180402 (2021)
work page 2021
- [80]
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.