Characterizing the Many Body Localization Crossover as a Metal-Insulator Transition: Localization length from Polarization and Quantum Metric
Pith reviewed 2026-05-24 06:00 UTC · model grok-4.3
The pith
The many-body quantum metric supplies a localization length that marks the MBL regime in interacting disordered chains.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
In the MBL regime of the disordered one-dimensional Bose-Hubbard model the many-body quantum metric and the localization parameter remain related, so that a localization length characterizing the real-space spread of the wave function can be read directly from the quantum metric.
What carries the argument
Many-body quantum metric (MBQM) defined on the space of twist boundary conditions, compared with the localization parameter of the modern theory of polarization.
If this is right
- The MBL phase can be treated as an insulator whose localization length is defined consistently with other insulating phases.
- The quantum metric offers an experimentally accessible route to that length without direct wave-function imaging.
- The ergodic-MBL crossover acquires a geometric signature based on the mismatch or agreement of the two quantities.
Where Pith is reading between the lines
- The same geometric diagnostic could be tested in other interacting localized phases, such as many-body mobility edges or Floquet MBL.
- Cold-atom experiments that already measure polarization or Berry curvature could extract the proposed localization length.
- If the relation survives in higher dimensions, it would supply a dimension-independent way to quantify MBL insulation.
Load-bearing premise
The relation between many-body quantum metric and localization parameter that holds for non-interacting Anderson insulators continues to diagnose localization once interactions are added.
What would settle it
In the MBL regime of an interacting disordered chain, the localization length inferred from the quantum metric differs systematically from the actual real-space spread of the many-body eigenstates.
Figures
read the original abstract
Many body localization (MBL) represents a unique physical phenomenon, providing a testing ground for exploring thermalization, or more precisely its failure. Here we characterize the MBL regime geometrically by the many-body quantum metric (MBQM), defined in the parameter space of twist boundary, and the localization parameter as defined in the modern theory of polarization and insulators. First, we demonstrate that the quantum metric can be used to characterize disordered insulating states by applying this theoretical framework to excited states of the 1D Anderson insulator. There we observe that the MBQM and localization parameter are related in finite realizations despite the states being gapless in the thermodynamic limit. Then, we consider a disordered 1D Bose-Hubbard model and find that we can characterize the ergodic-MBL crossover by comparing the MBQM and localization parameter. We find that we can extract a natural localization length in the MBL regime that characterizes the real space spread of the wave function and can be measured by extracting the quantum metric. Our analysis provides complementary insight into the MBL regime focusing on its insulating properties and providing a localization length whose definition is consistent across a range of insulating phases.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript claims that the many-body quantum metric (MBQM), defined via twist-boundary conditions, is related to the localization parameter from modern polarization theory in finite realizations of the 1D Anderson insulator, and that comparing these quantities in the disordered 1D Bose-Hubbard model characterizes the ergodic-MBL crossover while allowing extraction of a natural localization length from the MBQM alone in the MBL regime. This provides a geometric characterization of MBL as an insulator with a localization length consistent across phases.
Significance. If the reported numerical relation between MBQM and the localization parameter extends to interacting systems, the work supplies a complementary geometric diagnostic for the MBL crossover and a definition of localization length that is uniform across insulating phases and potentially extractable from quantum metric measurements. The approach is grounded in established polarization theory and offers insight into the insulating character of MBL states.
major comments (1)
- [abstract and Bose-Hubbard section] The abstract and the paragraph on the disordered 1D Bose-Hubbard model: the functional relation between MBQM and localization parameter is established numerically only for non-interacting Anderson-insulator excited states; no analytic argument or additional numerical test is supplied demonstrating that the same quantitative relation remains valid once on-site interactions are added. This extrapolation is load-bearing for the claim that the comparison marks the ergodic-MBL crossover and yields a localization length in the interacting model.
minor comments (2)
- [abstract] The abstract states that the quantities are related in finite Anderson realizations and that the comparison marks the crossover in the Bose-Hubbard model, but provides no error bars, system sizes, or details on how the crossover point is identified.
- [MBL regime discussion] The localization length is extracted from the same quantities used to define the crossover; explicit fitting procedures or cross-validation against independent measures (e.g., participation ratio) would clarify the diagnostic power.
Simulated Author's Rebuttal
We thank the referee for the careful reading and for highlighting this important point regarding the scope of our numerical evidence. We address the comment below.
read point-by-point responses
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Referee: [abstract and Bose-Hubbard section] The abstract and the paragraph on the disordered 1D Bose-Hubbard model: the functional relation between MBQM and localization parameter is established numerically only for non-interacting Anderson-insulator excited states; no analytic argument or additional numerical test is supplied demonstrating that the same quantitative relation remains valid once on-site interactions are added. This extrapolation is load-bearing for the claim that the comparison marks the ergodic-MBL crossover and yields a localization length in the interacting model.
Authors: We acknowledge that the quantitative functional relation between the many-body quantum metric and the localization parameter is established numerically only for excited states of the non-interacting 1D Anderson insulator. Both quantities are defined in a manner that does not presuppose the absence of interactions: the many-body quantum metric is obtained from the fidelity susceptibility under twist boundary conditions, and the localization parameter follows from the modern theory of polarization. In the disordered Bose-Hubbard model we therefore employ the comparison as a diagnostic that becomes meaningful once the system enters the MBL regime, where both quantities are expected to reflect strong real-space localization. We agree that an explicit numerical check of the same quantitative relation inside the interacting model, or an analytic derivation, would strengthen the presentation. We will revise the abstract and the Bose-Hubbard section to state explicitly that the relation is demonstrated in the non-interacting case and is used as a benchmark diagnostic when applied to the interacting model. revision: partial
Circularity Check
No significant circularity; numerical relation observed independently in each model without reduction to definition or fit.
full rationale
The paper first numerically observes a relation between MBQM and the localization parameter on finite Anderson insulator realizations, then compares the same two quantities in the interacting Bose-Hubbard model to characterize the ergodic-MBL crossover. The claim that MBQM alone yields a localization length in the MBL regime follows directly from this cross-model numerical correspondence rather than from any self-definitional equation, fitted parameter renamed as prediction, or self-citation chain. No load-bearing step reduces by construction to its own inputs; the derivation is an empirical characterization that remains self-contained against external benchmarks.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption The many-body quantum metric defined in twist-boundary parameter space and the localization parameter from polarization theory remain meaningfully related for excited states of the 1D Anderson model even though the spectrum is gapless in the thermodynamic limit.
Reference graph
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