Statistical Mechanics · Cross-listed from quant-ph
Approximation theorems for fermionic Gaussian states
Amir-Reza Negari, Farzin Salek, Zoltán Zimborás et al. (+3 more)
A recurring principle in many-body physics is that sufficiently distributed interactions or correlations lead to an effectively mean-field description. For fermionic Gaussian states, we establish a quantitative version of this principle directly at the level of Majorana covariance
matrices. The main observation is that the admissibility condition for fermionic covariance
matrices imposes a quantitative monogamy constraint on two-point correlations: a fixed region has only a bounded covariance budget to distribute among many disjoint regions. We use this principle in three directions. First, for quadratic fermionic
Hamiltonians on finite simple graphs of high degree, we obtain product-state approximations to both the ground-state energy and the finite-temperature free energy, with error of order \(D^{-1/2}\) for \(D\)-regular graphs. Second, we prove a finite fermionic Gaussian de Finetti
theorem: a \(k\)-copy Gaussian state that is \(n\)-exchangeable under graded fermionic copy permutations is \(O(k/n)\)-close in trace norm to the product of its one-copy marginal. In particular, infinite exchangeability within the Gaussian class implies exact product structure. Third, for spatial fermionic Gaussian states with exponentially clustering mutual information, we prove that the conditional mutual information across a separating buffer decays as \(O(r^{-1})\), and hence, by recoverability, the state admits an \(O(r^{-1/2})\) trace-norm approximation by a recovered state. These results show that product and de Finetti approximations and recoverability estimates for fermionic Gaussian systems can all be derived directly from covariance-level structure.
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All authors Amir-Reza Negari, Farzin Salek, Zoltán Zimborás, Aram Harrow, Patrick Hayden, Jens Eisert
Statistical Mechanics · Cross-listed from hep-ph
Generalized fluctuation-dissipation theorem and Einstein relation in rotating equilibrium
Shuo Fang and Shi Pu
We derive a generalized fluctuation-dissipation
theorem (FDT) for vector fields in rotating thermal
equilibrium from an exact phase-space Kubo-Martin-Schwinger (KMS) relation. The resulting FDT contains rotation-induced tensorial contributions beyond a scalar thermal factor. In the local transport limit, we obtain a model-independent generalized Einstein relation with rotation in which, through second order in thermal vorticity, symmetric momentum
diffusion depends on low-frequency spectral information beyond that encoded in the rotation-dependent dissipative drag. Remarkably, we find a new Einstein-type relation linking zero modes in the spectral function to the nonrotating drag coefficient. These zero modes underlie a new mechanism for the orbital polarization of heavy quarkonium. The same FDT constraint yields a modified detailed-balance relation whose first-order vortical correction is governed by transition polarization, without explicit dependence on the bath model. Our findings establish microscopic
equilibrium constraints for studying rotating
quantum matter in various fields.
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Statistical Mechanics · Cross-listed from hep-th
DSSYK, open ASEP/TASEP and 2d dilaton gravity at strong coupling
Alexander Gorsky, Elizaveta Kovalenko and Sergei Nechaev
We discuss the triality between
stochastic growth models in the KPZ universality class, the DSSYK model, and low dimensional dilaton
gravity. The TASEP model, which will be our primary focus, is identified with the strong-coupling limit of two-dimensional
quantum gravity. The
Hamiltonian of DSSYK and the transfer
matrix of ASEP coincide up to an additive constant, and we derive the exact expression for the resolvent of the corresponding transfer
matrix in ASEP on a finite interval. We also mention the relation of cumulants with the Tutte
polynomial $T(1,q)$ of the crossing graph dual to the chord diagram, which implies the relationship of the models considered with the $Q=0$ Potts model coupled with
gravity. We further exploit the realization of all these models in terms of different versions of discrete and continuous random walks on two-dimensional
manifolds, which parameterize the
Hilbert spaces of the respective theories. The representation of TASEP as the growth model of heaps provides new insight into strong-coupling
gravity, suggesting its interpretation as an emerging phenomenon. The new dualities between ASEP and q-TASEP models and heap models providing a useful growth interpretation have been developed.
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Statistical Mechanics · Cross-listed from quant-ph
Exact first-detection probability in a locally monitored solvable quantum circuit
Cecilia De Fazio, Igor Lesanovsky and Gabriele Perfetto
We study the interplay between local stroboscopic measurements and the
unitary quantum circuit dynamics of the deterministic Floquet-
quantum East model. We derive exact analytical results for the probability of first detecting a subset of
qubits back in their initial state. This is achieved via a renewal equation linking the first-detection probability to the subsystem Loschmidt echo, which encodes the effective open dynamics of the monitored
qubits. The resulting statistics exhibit two regimes set by the competition between the measurement probing time and the relaxation timescale of the
unitary circuit dynamics: while infrequent monitoring leads to
geometric first-detection statistics, frequent monitoring induces correlations that manifest as deviations from this
geometric behavior. These results provide a rare exact characterization of how local measurement backaction reshapes detection statistics in an interacting
quantum many-body system. In our local protocol, return probabilities are controlled by the size of the monitored subregion and can therefore be accessed on digital
quantum simulators via local mid-
circuit measurements.
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Statistical Mechanics
Scalar field theory for (chiral) active Brownian particles: bottom-up derivation revisited
Yuta Kuroda and Thomas Speck
The active Brownian particle (ABP) model is one of the most widely used particle-based models in studies of scalar active fluids. A collection of interacting ABPs is known to exhibit intriguing phenomena that are absent in
equilibrium counterparts. A prominent example is spontaneous phase separation, known as motility-induced phase separation (MIPS), which occurs even in the absence of explicit attractive interactions. In parallel, the large-scale behavior of scalar active fluids is often modeled using continuum descriptions, namely, scalar field theories. Such scalar active field theories are usually formulated in a top-down manner, and establishing their connections to particle-based models remains challenging, with these connections not yet fully understood. To advance the microscopic derivation of scalar active field theories, we revisit the connection between ABPs interacting via a two-body potential in two dimensions and scalar field theories that include all possible terms up to fourth order in spatial gradients. Our approach is based on a pressure expansion and the renormalization group (RG) method in the context of singular perturbation theory. The RG method provides a systematic way to eliminate fast variables and identify the dynamics on the slow invariant
manifold. Within this framework, scalar field theories can be obtained as RG flow equations. We also apply this method to a chiral variant of the ABP model, the chiral ABP (cABP) model, in which a constant torque biases particle rotation to the left or right. We show that the scalar field theory corresponding to the cABP model contains two “odd” terms in addition to those present in the scalar field theory for ABPs. The method developed here may also be useful for deriving hydrodynamic descriptions of other types of active matter systems.
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Statistical Mechanics
Optimal sampling strategies in event-chain Monte Carlo
James Gulliford, Gareth O. Roberts and Michael F. Faulkner
Event-chain Monte Carlo (ECMC) has revolutionised computational sampling over recent years, providing a powerful alternative to the molecular-dynamics (MD) and
Hamiltonian Monte Carlo (HMC)
algorithms. Each method outperforms the ubiquitous random-walk Metropolis
algorithm by advancing particles along deterministic trajectories, but ECMC achieves this without being constrained by Newtonian dynamics. Recent advances exploited this dynamical freedom to induce a collective particle dynamics that relax local density variations on fast timescales. In a foundational model of $N$ pairwise-interacting particles on the 1D
torus, this led to an $O(N^{3/4})$ improvement on the industry-leading computational efficiency of MD and HMC - but with an impractically small prefactor at low/high mean particle density for general attractive/repulsive interactions. Here we present a universal framework that generalises this high-efficiency sampling strategy to all translationally symmetric pairwise models on the
torus - creating the potential to surpass MD and HMC as the state of the art. We also numerically elucidate the collective particle dynamics and discuss broad impact across the physical sciences and computational statistics.
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Statistical Mechanics
Gate Control Improves Routing Efficiency for Periodic Traffic
Hyunwook Song, Hoyun Choi and B. Kahng
Traffic movement on highways and data packet transmission across the Internet serve as prototypical examples of collective dynamical behavior in complex systems. Although enhancing traffic flow in complex networks has been extensively investigated, most existing studies focus on single-queue models. In this paper, we introduce a simple yet effective two-queue scheme with gate control that regulates vehicle entry and integrates it with the routing protocol. When vehicles arrive at a on-ramp gate, they first join a waiting queue and are permitted to enter the main flowing queue once it has emptied. This two-queue strategy is especially advantageous when highway inflow varies periodically with large fluctuations, as is commonly observed in real traffic. Moreover, it modifies the system's failure characteristics, replacing sudden, discontinuous jamming transitions with gradual, continuous ones. The approach is based on a simple rule using only local queue information, making it easy to implement and widely applicable. Beyond the specific setup studied here, it can be applied to other transportation networks under different conditions or extended to other queuing systems, such as Internet data packet routing.
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Statistical Mechanics · Cross-listed from nlin.CD
Evidence for an Obstruction to Perturbative Renormalization Group in Stochastic Turbulence
L. Ts. Adzhemyan, D. A. Davletbaeva, D. A. Evdokimov et al. (+1 more)
We examine whether the perturbative renormalization group (RG) regime of
stochastic turbulence can be continued from small $\varepsilon$ to the physical value $\varepsilon_{\mathrm{phys}}=2$, corresponding to large-scale forcing. Exploiting the simplifications of the large-$d$ limit, we calculate the correction exponent $\omega$ to five-loop order and find a simple structure suggesting a closed analytic form. This form implies that the fixed point stable near $\varepsilon=0$ ceases to be simultaneously real and infrared stable before reaching $\varepsilon_{\mathrm{phys}}=2$, pointing to a possible change of infrared regime rather than a controlled perturbative RG extrapolation.
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All authors L. Ts. Adzhemyan, D. A. Davletbaeva, D. A. Evdokimov, M. V. Kompaniets
Statistical Mechanics
Learning ab initio phase-field models
Mengyi Chen, Peichen Zhong, Zihan Zhang et al. (+1 more)
Simulating microstructure
evolution requires
quantum-mechanical accuracy and mesoscopic reach in length and time scales, a combination that no current method achieves. Classical phase-field models provide this reach, but their accuracy is limited by phenomenological free energies and mobilities. Here we develop a framework for learning ab initio phase-field models, where the mesoscopic equation is not postulated but derived from a Mori-Zwanzig projection of molecular dynamics onto species-density fields under explicit assumptions. The nonlocal free energy and mobility left unspecified by this equation are parametrized by neural networks and learned from short molecular dynamics trajectories generated with machine-learning interatomic potentials of ab initio accuracy. We demonstrate the framework on an iron-boron melt and on hydrogen-helium mixtures under planetary conditions. For iron-boron, the model shows that the melt at the FeB$_4$ composition is spinodally unstable at ambient pressure but stabilized at 10 GPa, offering a thermodynamic rationale for why FeB$_4$ has been synthesized only under high pressure. For hydrogen-helium, the model predicts the immiscibility boundary and captures droplet nucleation and growth in helium-rain simulations of a column corresponding to 2.2 million atoms, far beyond the scale of atomistic modeling at comparable accuracy. Trained across compositions and conditions, such models could provide a mesoscopic counterpart to ab initio molecular dynamics.
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All authors Mengyi Chen, Peichen Zhong, Zihan Zhang, Qianxiao Li
Statistical Mechanics
On thermalization in random free fermions: Statistical origins and energy-dependent ETH structure
Xiao-Kan Guo and Zhiqiang Huang
We study the thermalization in the random free
fermion model by a detailed random-
matrix analysis. By computing the ensemble average and fluctuations of $\operatorname{Tr}(A\rho(t))$ for a single-particle observable $A$, we derive the thermalization function $g^2(t/\tau_\lambda)$ with $\tau_\lambda = \hbar/(2\eta\sqrt{N})$, prove its $|t|^{-3}$ asymptotic decay, and show that the variance of the fluctuations vanishes as $O(1/N)$ in the thermodynamic limit.Particle number conservation is also incorporated into the model, by which we further study the energy-shell eigenstate statistics and prove a factorization
theorem of the eigenstate statistics. We further show that under a controlled eigenvector-
eigenvalue correlation deformation, the diagonal energy-resolved slope and the off-diagonal eigenstate thermalization hypothesis spectral function acquire an energy dependence.Finally, we compute the fluctuations of correlation functions and compare the fluctuation scales with those of a fully chaotic system, revealing quantitative differences rooted in the Gaussian nature of the eigenstates.Our work establishes the random free
fermion model as an analytically solvable realization of weak eigenstate thermalization hypothesis type self-averaging, and as a controllable setting in which eigenbasis chaos can be separated from spectral chaos.
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Statistical Mechanics · Cross-listed from cond-mat.soft
Ideal gas of active particles in a box
Siran Li, Ralf Eichhorn and Lennart Dabelow
Extending a fundamental concept of thermodynamics to the nonequilibrium realm of active matter, we derive an ideal gas law for a suspension of noninteracting active particles confined to a finite rectangular box. The law is exact for run-and-tumble particles (RTPs) in one dimension and established by a systematic analytical expansion of the steady-state probability density for active Ornstein-Uhlenbeck particles in any dimension. It also agrees well with numerical results for active Brownian particles and RTPs in higher dimensions and recovers previously known relations for the limit of large system sizes. Together, these results demonstrate the applicability of the ideal active gas law to the three most widely studied theoretical models of colloidal active matter. As an application, we analyze thermodynamic processes involving active matter on a macroscopic level, without recourse to microscopic particle trajectories or self-propulsion fluctuations.
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Statistical Mechanics
Work fluctuation speed limit in boundary conformal field theories
Shihao Xia, Ahsan Nazir and Harry J. D. Miller
We explore the fundamental limits on finite-time driving in
quantum critical systems described by boundary conformal field theory. We show that
stochastic work fluctuations arising from external driving are a resource for speedy control, and derive an exact, saturable fluctuation-based speed limit in weakly driven boundary conformal field theories at finite temperature. The bound and saturating protocol can be expressed entirely in terms of the universal scaling dimension, and the result interpolates between the Kibble–Zurek regime,where temporal correlations are strongly nonlocal, and an adiabatic regime where linear driving becomes optimal. For small scaling dimension, the enhanced temporal correlations produce pronounced departures from linear protocols and a larger optimization advantage. These results establish a universal work precision–time tradeoff for boundary-critical control, applicable to
quantum impurity, fractional
quantum Hall, and superconducting-
circuit platforms.
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Statistical Mechanics · Cross-listed from quant-ph
Fermionic Gaussian Scrooge Ensembles in Deep Thermalization
Ning Sun and Pengfei Zhang
Measuring part of a many-body wave function generates a projected ensemble of pure
quantum states on the unmeasured subsystem. Recent advances in deep thermalization have shown that, in chaotic systems, this ensemble universally converges to the maximally random ensemble compatible with its average density
matrix, known as the Scrooge ensemble. By contrast, free-
fermion systems are nonchaotic, and their
quantum states are constrained to remain Gaussian. Motivated by this distinction, we introduce the fermionic Gaussian Scrooge ensemble to describe deep thermalization in generic free-
fermion systems. This ensemble is defined as a distortion of the Gaussian Haar ensemble by the average density
matrix, and its moments reveal an enlarged
symmetry of Gaussian states in the replicated
Hilbert space. We demonstrate the emergence of the fermionic Gaussian Scrooge ensemble in two complementary settings: (1) we analytically prove that the projected ensemble of the SYK$_2$ model follows the fermionic Gaussian Scrooge ensemble at arbitrary
evolution times; (2) we provide numerical evidence that it emerges at sufficiently long times in random Gaussian
circuits with charge conservation. Our results establish the fermionic Gaussian Scrooge ensemble as a universal description of deep thermalization in free-
fermion systems.
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Statistical Mechanics · Cross-listed from quant-ph
Rapid mixing of quantum spin chains at any finite temperature
Leeseok Kim
We prove that a quasi-local
quantum Gibbs sampler rapidly mixes to the Gibbs state of any 1D finite-range
Hamiltonian at all finite temperatures in $\mathcal{O}\left(\log (n/\varepsilon)\right)$ time. Each update traces out a contiguous block of constant length and reconstructs it using the Petz recovery map, following early proposals for
quantum heat-bath Gibbs samplers by Kastoryano and Brandão~\cite{kastoryano2016
quantum}. Our proof directly bounds the distance between the updated state and the target Gibbs state using
quantum Wasserstein distance of order $1$. We further provide $\mathcal{O}(\text{polylog}(n/\varepsilon))$ depth of
quantum circuits to prepare these Gibbs states. We hope these techniques will facilitate the analysis and design of other Gibbs samplers.
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Statistical Mechanics
Dynamical Critical Properties of the Random-Bond three-state Potts Model
Zeynep Demir Vatansever, Ulvi Kanbur, Mohammed Sadra Najafi et al. (+2 more)
We study the dynamic critical behavior of the two-dimensional random-bond three-state Potts model using large-scale Monte Carlo simulations with the Wolff and Swendsen–Wang cluster
algorithms. At the disorder-dependent critical temperature, we compute integrated and exponential autocorrelation times and extract the dynamic critical exponent $z$ via finite-size scaling. Critical slowing down is significantly weakened by bond randomness, with the dynamic exponent decreasing from $z \approx 0.54$ in the pure system to $z \approx 0.24$ at strong disorder. From the finite-size scaling of the Wolff cluster size, we obtain $\gamma/\nu \approx 1.73$, where $\gamma$ and $\nu$ are the susceptibility and correlation-length exponents, respectively, consistent with the universality class of the two-dimensional three-state Potts model. These results indicate that bond randomness strongly affects the critical dynamics but leaves the underlying static universality class unchanged. Furthermore, both static and dynamic observables, including the specific heat and the autocorrelation times, show a lack of self-averaging.
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All authors Zeynep Demir Vatansever, Ulvi Kanbur, Mohammed Sadra Najafi, Muktish Acharyya, Erol Vatansever
Statistical Mechanics · Cross-listed from quant-ph
Dissipation-Sensitivity Trade-Off in Dissipative Bosonic Systems
Kohei Yoshimura and Ryusuke Hamazaki
We discover a universal trade-off between the dissipation required to maintain a
quantum state more sensitive than a thermal one and the sensitivity gain for a driven single-mode bosonic system under a thermal environment. Specifically, for Gaussian states, we rigorously derive a lower bound on the
entropy production rate in the steady state in terms of the
quantum Fisher information gain relative to a thermal reference state. Numerical investigations of Kerr nonlinearities further support that these bounds can hold beyond the Gaussian regime. Our results establish a new connection between steady-state thermodynamics and
quantum metrological performance.
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Statistical Mechanics
Effects of on-site Gaussian disorder in the ferromagnetic Blume Capel model
Kimberly J. Silverman, Daria Lhommedieu, Alykhan Rajan et al. (+3 more)
We conduct a numerical study of a generalization of the Blume-Capel model in which the
crystal field $D_i$, which controls the local vacancy density, is site dependent rather than uniform. We employ Markov Chain Monte Carlo to compute the shifted phase transition line $T_c$ as a function of the width $\sigma$ of a gaussian distribution $P(D_i) \sim \exp( -(D_i - \bar{D})^2/2 \sigma^2)$ at fixed $\bar{D}$. We characterize how the presence of this on-site Gaussian disorder reduces $T_c$ across the second order transition line, and discuss how our results suggest that $T_c$ remains finite even at large values of $\sigma$. We also determine the effects of disorder on the tricritical point of the conventional, uniform, Blume-Capel model, finding that it significantly suppresses the first-order phase transition, eliminating it entirely at large values of $\sigma$.
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All authors Kimberly J. Silverman, Daria Lhommedieu, Alykhan Rajan, Kathryn A. Cooper, Richard T. Scalettar, Eduardo Ibarra-García-Padilla
Statistical Mechanics · Cross-listed from cond-mat.mtrl-sci
Landau-like formalism for the thermal-runaway and filamentation instabilities in switching Mott devices
Aniket Bajaj, Anudeep Tullibilli, Arup Basak et al. (+2 more)
Correlated oxides can switch to a low-resistance state when exposed to a voltage, making these
insulator-to-metal transition materials promising candidates for memristor technology. Switching in materials such as VO2 occurs because of a thermal-runaway instability that leads to the abrupt formation of a hot current-carrying metallic filament within the
insulator. We study, theoretically and experimentally, this bias-induced switching as a non-linear dynamics stability problem. We construct Landau free energy-like Lyapunov functions to show that both the on-state ("set") and the off-state ("reset") transitions can be described as separate saddle-node bifurcations. Starting from the reaction-
diffusion equation for the heat transport, the abrupt snap-back (reset) to the insulating state is systematically formulated as a mean field Landau-like dynamics with the filament width playing the role of the order parameter. The formulation predicts an abrupt first-order phase transition with a break-off of a finite width filament across the current-controlled reset transition. The formulation is quantitatively compared against our electrical and optical microscopy experiments on VO2 thin film devices. Multi-cycle switching dynamics experiments further reveal the emergence of a stable
stochastic regime following an initial electro-spatial forming phase. Beyond establishing a cross-disciplinary dynamical framework that may be readily compared to instabilities in various reaction-
diffusion scenarios, the work also provides a predictive foundation for the rational design and optimization of Mott-transition-based technologies.
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All authors Aniket Bajaj, Anudeep Tullibilli, Arup Basak, Pavan Nukala, Bhavtosh Bansal
Statistical Mechanics · Cross-listed from math.PR
Convergence of stationary distributions for a class of zero-range processes and its application to a wealth distribution model with debt
Hironobu Sakagawa
Understanding how macroscopic wealth distributions emerge from microscopic transaction rules among
agents is a central problem in econophysics. While many traditional models restrict
agents' wealth to non-negative values, debt is an essential feature of realistic economic systems. In this paper, we introduce a
stochastic transaction mechanism into the
agent-based model with a central-bank-mediated collective debt limit, as studied in [14]. In our model, each
agent probabilistically determines whether to transfer a coin, depending on their current asset or debt level. From a probabilistic perspective, the resulting system can be formulated as a zero-range process with $\mathbb{Z}$-valued site occupation numbers, where negative values represent debt. We prove that, under an appropriate scaling, the wealth distribution in the stationary state converges to either a two-sided Gamma distribution or a Gamma distribution, depending on the asymptotic behavior of the function governing coin transfers. Our results rigorously identify these previously unconsidered limiting distributions and provide a mathematical framework for understanding how
stochastic monetary exchanges give rise to macroscopic wealth distributions.
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