arXiv subjects

Marius Lemm

Publications and source records attributed to Marius Lemm.

At least 19 recordsLinked to original sources

arXiv2026

Lieb-Robinson bounds for Bose-Hubbard Hamiltonians via path expansions

We study the transport properties of Bose-Hubbard-type Hamiltonians. Kuwahara, Vu, and Saito proved sharp bounds on (i) particle transport and (ii) the Lieb-Robinson velocity for initial states with controlled boson density. We provide simpler and shorter proofs for both results using a new set of methods. For (i), boson transport, we use a class of weighted extended particle number operators that directly enables closing the associated differential inequalities. For (ii), the Lieb-Robinson bounds, we develop a bosonic version of the Chen-Lucas path expansion technique and recover the sharp velocity bound $t^{d-1}$ up to logarithmic corrections, where $d$ denotes the lattice dimension. We also extend the transport bounds (i) to long-range hopping with a power-law exponent $α>d$. The range $α\in (d,d+1)$ corresponds to an algebraic light cone for boson transport and has not been previously covered.

math-ph↗

arXiv2026

A First-Order Phase Transition in Spin-Imbalanced BCS Theory of Superconductivity

We study a translation-invariant BCS functional of superconductivity for a system of spin-$1/2$ fermions with spin-dependent chemical potentials, $μ_+\neqμ_-$. Numerical work of Caldas and Mota and analytical work of Freiji, Hainzl, and Seiringer has suggested an unusual first-order phase transition in spin-imbalanced BCS models, in which the normal state becomes globally unstable while remaining locally stable. Here, we prove that a first-order phase transition occurs in such spin-imbalanced BCS functionals. For each sufficiently small fixed spin imbalance and sufficiently low temperature, there is a coexistence coupling $λ_{\mathrm{fo}}$ strictly below the linear-stability threshold $λ_{\mathrm{lin}}$ of the normal state at which the Cooper pair density exhibits a nonzero jump and the minimized energy has a kink. The methodological insight is a coercivity estimate obtained by decomposing the BCS energy minimization into two parts, where the ``inner'' problem minimizes over the one-particle density matrices at fixed pairing and can be exactly solved by a Legendre--Fenchel transform.

math-ph↗

arXiv2026

Dissipation-Enabled Operator-Norm Locality Bounds for Bose-Hubbard Hamiltonians

Closed bosonic lattice systems do not generally admit operator norm Lieb-Robinson bounds uniformly in the initial state. The reason is that the information propagation velocity can grow with the local boson occupancy, which can be macroscopically large. Here, we show that local dissipation restores an operator-norm Lieb-Robinson bound. We consider the dissipative Bose-Hubbard model described by a Lindbladian operator with on-site $\ell$-photon loss, $\ell>2$. For our main result, we establish an almost-ballistic Lieb-Robinson bound. Dissipation rapidly depletes highly occupied sites, thus regularizing the state on the Sobolev-type scale of local particle moments. The resulting moment bounds diverge as time approaches zero but remain integrable near zero at sufficiently low orders, which is crucial for our argument. We extend these ideas to treat cat-code dissipation and, for initial states in the code space, we prove a local adiabatic approximation uniform in the total volume. Further consequences include local channel approximation, a thermodynamic limit, and efficient hybrid bosonic-qubit simulation. The point is that these applications are now available uniformly in the input state, as for quantum spin systems, but in contrast to closed Bose-Hubbard systems.

quant-ph↗

arXiv2026

Response theory at low temperature for weakly interacting quantum spin systems

Recent work by Greenblatt, Lange, Marcelli, and Porta (Commun. Math. Phys. 2024) provides the first rigorous justification of adiabatic theory for interacting quantum systems at positive temperature. Their analysis treats weakly interacting lattice fermions and combines a rigorous Wick rotation with fermionic cluster expansion techniques. Here, we study a class of weakly interacting quantum spin systems and derive response theory at positive temperature, thereby covering non-Fermi systems for the first time. Our proof identifies and exploits the locality property of local indistinguishability as a key ingredient in carrying out low-temperature adiabatic expansions. The fact that these systems satisfy local indistinguishability was recently established jointly with Adhikari (arXiv:2508.15907) using cluster expansion methods together with a probabilistic swapping trick.

math-ph↗

arXiv2026

Dynamics and equilibrium states of infinite systems of lattice bosons

We consider the dynamics of systems of lattice bosons with infinitely many degrees of freedom. We show that their dynamics defines a group of automorphisms on a $C^*$--algebra introduced by Buchholz, which extends the resolvent algebra of local field operators. For states that admit uniform bounds on moments of the local particle number, we derive propagation bounds of Lieb--Robinson type. Using these bounds, we show that the dynamics of local observables gives rise to a strongly continuous unitary group in the GNS representation. Moreover, accumulation points of finite-volume Gibbs states satisfy the KMS condition with respect to this group. This, in particular, proves the existence of KMS states.

math-ph↗

arXiv2026

Diffusion bounds for non-autonomous degenerate parabolic equations

We consider possibly degenerate non-autonomous parabolic equations in $L^p$ spaces for all $1\le p\le\infty$, and prove Gaussian space-time upper bounds of the Davies-Gaffney, or integrated Nash-Aronson, type on their solutions. Our bounds cover both linear and nonlinear equations.

math.AP↗

arXiv2026

Lieb-Robinson bounds for Bose-Hubbard Hamiltonians: A review with a simplified proof

We review recent progress on state-dependent Lieb--Robinson bounds for Bose--Hubbard Hamiltonians. In particular, Kuwahara, Vu, and Saito established that, for general bounded-density initial states, the Lieb--Robinson velocity is bounded by $t^{d-1}$ for large times, where $d$ denotes the lattice dimension. We present a shorter proof of the weaker, but still polynomial velocity bound $t^{d+ε}$.

math-ph↗

arXiv2026

The spin-one Motzkin chain is gapped for any area weight $t<1$

We consider the spin-one Motzkin chain with area weight $t>0$. We resolve three questions from the literature about this model. We prove (i) existence of a uniform spectral gap for all $t<1$ as conjectured by Zhang--Ahmadein--Klich \cite{zhang2017novel} (ii) an explicit formula for the long-distance limit of the string order parameter, which implies it is non-vanishing at small $t$, confirming a conjecture by Barbiero et al. \cite{barbiero2017haldane}, and (iii) that gaplessness for $t>1$ is robust and extends to hard boundary conditions, answering a question of Zhang--Klich \cite{zhang2017entropy}. These conclusions rest on an effective approximate description of the ground states of finite open Motzkin chains in terms of height-controlled imbalanced Motzkin walks.

quant-ph↗

arXiv2026

Learning Coulomb Potentials and Beyond with Free Fermions in Continuous Space

The first-principles formulation of quantum mechanics relevant for quantum chemistry and trapped quantum gases involves particles in the continuous space $\mathbb R^d$. We present a unified framework and modular algorithm for learning external potentials $V$ with free-fermion models in the continuum. Compared to the lattice-based approaches, the continuum presents new mathematical challenges: the state space is infinite-dimensional and the Hamiltonian contains the Laplacian, which is unbounded in the continuum and produces an unbounded speed of information propagation. We address these through novel optimization methods and information-propagation bounds in combination with a priori regularity assumptions on the external potential. The resulting algorithm provides a unified and robust approach to learn parametric interactions (e.g., Coulomb potentials or periodic potentials) and general smooth functions. Our results lay the foundation for a scalable and generalizable toolkit to learn Hamiltonians in continuous space.

quant-ph↗

arXiv2026

Uniform-in-temperature locality estimates for weakly interacting quantum systems

The locality of thermal quantum states has emerged as a key input for applications to thermalization, response theory, and efficient simulability. Locality is either captured by the decay of correlations or by local indistinguishability, which allows to approximate local expectation values by those of local thermal states. Most techniques for deriving locality bounds deteriorate at small temperature, a physically highly relevant regime and so it is of interest to identify conditions for uniform-in-temperature bounds. Here we prove that a class of weakly interacting quantum Hamiltonians satisfies exponential decay of correlations and local indistinguishability uniformly in the temperature. The proof uses a low-temperature cluster expansion and a quantum version of a probabilistic swapping trick developed by the first author and Cao (Ann. Probab. 53, 2025) in the context of lattice gauge theories.

math-ph↗

arXiv2025

On Bose-Einstein condensates in disordered media

We consider the quantum dynamics of interacting bosons in the mean-field regime when they are subjected to a disordered potential, which is either random or quasi-periodic. Starting from a spatially localized Bose-Einstein condensate, we prove that fluctuations around the condensate propagate with a small velocity due to the disorder. This provides an example of a disordered many-body system with provably slow transport behavior in any spatial dimension. The main technical novelty is an interaction picture analysis relative to the Anderson-localized one-body dynamics.

math-ph↗

arXiv2025

A ballistic upper bound on the accumulation of bosonic on-site energies

In this note, we study transport properties of the dynamics generated by translation-invariant and possibly long-ranged Hamiltonians of Bose-Hubbard type. For translation-invariant initial states with controlled boson density, we improve the known bound on the local repulsive energy at time $t$ from $\langle n^2_x\rangle_t\lesssim t^{2d}$ to $\langle n^2_x\rangle_t\lesssim t^d$. This shows that bosonic on-site energies accumulate at most ballistically. Extending the result to higher moments would have powerful implications for bosonic Lieb-Robinson bounds. While previous approaches focused on controlling particle transport, our proof develops novel ASTLOs (adiabatic space-time localization observables) that are able to track the growth of local boson-boson correlations.

math-ph↗

arXiv2025

Macroscopic suppression of supersonic quantum transport

We consider a broad class of strongly interacting quantum lattice gases, including the Fermi-Hubbard and Bose-Hubbard models. We focus on macroscopic particle clusters of size $θN$, with $θ\in(0,1)$ and $N$ the total particle number, and we study the quantum probability that such a cluster is transported across a distance $r$ within time $t$. Conventional effective light cone arguments yield a bound of the form $\exp(v t-r)$. We report a substantially stronger bound $\exp(θN(vt-r))$, which provides exponential suppression that scales with system size. Our result establishes a universal dynamical large deviation principle: macroscopic suppression of supersonic macroscopic transport (MASSMAT).

math-ph↗

arXiv2025

Non-perturbatively slow spread of quantum correlations in non-resonant systems

Strong disorder often has drastic consequences for quantum dynamics. This is best illustrated by the phenomenon of Anderson localization in non-interacting systems, where destructive quantum wave interference leads to the complete absence of particle and information transport over macroscopic distances. In this work, we investigate the extent to which strong disorder leads to provably slow dynamics in many-body quantum lattice models. We show that in any spatial dimension, strong disorder leads to a non-perturbatively small velocity for ballistic information transport under unitary quantum dynamics, almost surely in the thermodynamic limit, in every many-body state. In these models, we also prove the existence of a "prethermal many-body localized regime", where entanglement spreads logarithmically slowly, up to non-perturbatively long time scales. More generally, these conclusions hold for all models corresponding to quantum perturbations to a classical Hamiltonian obeying a simple non-resonant condition. Deterministic non-resonant models are found, including spin systems in strong incommensurate lattice potentials. Consequently, quantum dynamics in non-resonant potentials is asymptotically easier to simulate on both classical or quantum computers, compared to a generic many-body system.

cond-mat.dis-nn↗

arXiv2025

Two classes of quantum spin systems that are gapped on any bounded-degree graph

We study translation-invariant quantum spin Hamiltonians on general graphs with non-commuting interactions either given by (i) a random rank-$1$ projection or (ii) Haar projectors. For (i), we prove that the Hamiltonian is gapped on any bounded-degree graph with high probability at large local dimension. For (ii), we obtain a gap for sufficiently large local dimension. Our results provide examples where the folklore belief that typical translation-invariant Hamiltonians are gapped can be proved, which extends a result by Bravyi and Gosset from 1D qubit chains with rank-$1$ interactions to general bounded-degree graphs. We derive the gaps by analytically verifying generalized Knabe-type finite-size criteria that apply to any bounded-degree graph.

quant-ph↗

arXiv2025

Enhanced Lieb-Robinson bounds for commuting long-range interactions

Recent works have revealed the intricate effect of long-range interactions on information transport in quantum many-body systems: In $D$ spatial dimensions, interactions decaying as a power-law $r^{-α}$ with $α> 2 D+1$ exhibit a Lieb-Robinson bound (LRB) with a linear light cone and the threshold $2D +1$ is sharp in general. Here, we observe that mutually commuting, long-range interactions satisfy an enhanced LRB of the form $t \, r^{-α}$ for any $α>0$, and this scaling is sharp. In particular, the linear light cone occurs at $α= 1$ in any dimension. Part of our motivation stems from quantum error-correcting codes. As applications, we derive enhanced bounds on ground state correlations and an enhanced local perturbations perturb locally (LPPL) principle for which we adapt a recent subharmonicity argument of Wang-Hazzard. Similar enhancements hold for commuting interactions with stretched exponential decay.

math-ph↗

arXiv2025

The charge gap is greater than the neutral gap in fractional quantum Hall systems

Past studies of fractional quantum Hall systems have found that the charge gap dominates the neutral gap for all relevant parameter choices. We report a wide-ranging proof that this domination is in fact a universal property of any Hamiltonian that satisfies a few simple structural properties: translation-invariance, charge conservation, dipole conservation, and a fractionally filled ground state. The result applies to both fermions and bosons. Our main tool is a new mathematical scheme, the gap comparison method, which provides a sequence of inequalities that relate the spectral gaps in successive particle number sectors. Our finding sheds new light on dipole conservation's profound effects on many-body physics.

cond-mat.str-el↗

arXiv2025

Recursive spectral relations and the charge versus neutral gap in fractional quantum Hall systems

We consider quantum lattice Hamiltonians and derive recursive spectral relations bridging successive particle number sectors. One relation gives conditions under which the charge gap dominates the neutral gap. We verify these conditions under a triad of symmetries (translation-invariance, charge and dipole conservation) that are present, e.g., in periodic fractional quantum Hall systems. Thus, this gap domination, previously observed numerically, is a universal feature imposed by symmetry. A second relation yields a new induction-on-particle-number method for deriving spectral gaps. The results cover both bosons and fermions.

math-ph↗