arXiv subjects

Euijoon Kwon

Publications and source records attributed to Euijoon Kwon.

11 recordsLinked to original sources

arXiv2026

Quantifying information flow along a stochastic trajectory

Stochastic information flow (SIF) quantifies information flow at the trajectory level, overcoming the limitations of conventional symmetric, ensemble-averaged measures. However, computational difficulties have hindered the empirical application of the SIF. In this work, we propose a scalable deep-learning method for estimating the SIF from general time-series data. Its applications to an exactly solvable two-particle model, Kuramoto oscillators, and empirical trajectories of interacting motile cells demonstrate the utility of SIF as a data-driven indicator of cooperative structures.

cond-mat.stat-mech↗

arXiv2026

Nonequilibrium Fluctuation-Response Theory in the Frequency Domain

We establish a unified fluctuation-response theory in the frequency domain for nonequilibrium steady states governed by overdamped Langevin dynamics and Markov jump processes. The central identity is an exact fluctuation-response relation that reconstructs the power spectrum of general observables from local responses measured at the same frequency. This relation applies to state-dependent observables, current-like observables, and their combinations, and reveals how local dynamics shape global fluctuations. From this identity, we derive a hierarchy of relations in the frequency domain, including response uncertainty relations, kinetic and thermodynamic uncertainty relations, the equilibrium fluctuation-dissipation theorem, and Harada-Sasa relations. This establishes an overarching framework for fluctuation-response theory in the frequency domain. Applications to stochastic networks and driven diffusive systems demonstrate how the theory decomposes fluctuation spectra into local contributions and reveals frequency-dependent tradeoffs between fluctuations, response, and dissipation.

cond-mat.stat-mech↗

arXiv2026

Fluctuation-Response Theory for Nonequilibrium Langevin Dynamics

We establish a unified fluctuation-response relation for Langevin dynamics. By exploiting the common mathematical structures underlying fluctuations and responses of empirical density and current, we derive a unified identity that generalizes the fluctuation-dissipation theorem from equilibrium to nonequilibrium settings. This relation connects global fluctuations of observables with their local responses to perturbations in force, mobility, and temperature. We further derive finite-time fluctuation-response inequalities, leading to response uncertainty relations that complement the identity by providing more practical bounds. These derivations establish a unified theoretical framework linking the fluctuation-dissipation theorem and thermodynamic uncertainty relations. Using the $F_1$-ATPase molecular motor model, we illustrate how these response-based bounds constrain the long-time diffusion coefficient.

cond-mat.stat-mech↗

arXiv2025

Violation of kinetic uncertainty relation in maser heat engines: Role of spontaneous emission

We investigate the kinetic uncertainty relation (KUR)-a fundamental trade-off between dynamical activity and current fluctuations-in two configurations of a maser heat engine. We find that KUR violations arise only in one model. This asymmetry originates from spontaneous emission, which breaks the structural symmetry between the configurations and modifies their coherence dynamics. While we analyze several contributing factors-including statistical signatures such as the Fano factor and the ratio of dynamical activity to current-our results show that the decisive mechanism is the slower decoherence in one configuration, which enables quantum violations of the classical steady-state KUR bound. By contrast, the faster coherence decay in the other configuration suppresses such violations, driving it closer to classical behavior. These findings highlight the critical role of decoherence mechanisms in determining fundamental thermodynamic bounds and provide insights for the design of quantum heat engines in which the control of decoherence is central to suppressing fluctuations and enhancing reliable performance.

quant-ph↗

arXiv2025

Thermodynamically consistent lattice Monte Carlo method for active particles

Recent years have seen a growing interest in the thermodynamic cost of dissipative structures formed by active particles. Given the strong finite-size effects of such systems, it is essential to develop efficient numerical approaches that discretize both space and time while preserving the original dynamics and thermodynamics of active particles in the continuum limit. To address this challenge, we propose two thermodynamically consistent kinetic Monte Carlo methods for active lattice gases, both of which correctly reproduce the continuum dynamics. One method follows the conventional Kawasaki dynamics, while the other incorporates an extra state-dependent prefactor in the transition rate to more accurately capture the self-propulsion velocity. We find that the error scales linearly with time step size and that the state-dependent prefactor improves accuracy at high P\'{e}clet numbers by a factor of $\mathrm{Pe}^2$. Our results are supported by rigorous proof of convergence as well as extensive simulations.

cond-mat.soft↗

arXiv2025

Quantum Thermodynamics on a limit cycle

We consider a periodic quantum clock based on cooperative resonance fluorescence at zero temperature. In the quantum case, this system has an exact steady state and the limit cycle appears in conditional quantum dynamics under homodyne detection. We show that the intrinsic quantum phase diffusion on the limit cycle leads to fluctuations in the period. By simulating the stochastic master equation for homodyne detection, we extract the statistical properties of the clock period. We show that the precision of the clock satisfies the quantum-thermodynamic kinetic uncertainty relations. As energy dissipation increases, the clock quality improves, fully validating, in a quantum stochastic system, the link between energy dissipation and clock precision.

quant-ph↗

arXiv2024

A unified framework for classical and quantum uncertainty relations using stochastic representations

Thermodynamic uncertainty relations (TURs) and kinetic uncertainty relations (KURs) provide tradeoff relations between measurement precision and thermodynamic cost such as entropy production and activity. Conventionally, these relations are derived using the Cram\'er-Rao inequality, which involves an auxiliary perturbation in deterministic differential equations governing the time evolution of the system's probability distribution. In this study, without relying on the previous formulation based on deterministic evolving equation, we demonstrate that all previously discovered uncertainty relations can be derived solely through the stochastic representation of the same dynamics. For this purpose, we propose a unified method based on stochastic representations for general Markovian dynamics. Extending beyond classical systems, we apply this method to Markovian open quantum systems by unraveling their dynamics, deriving quantum uncertainty relations that are physically more accessible and tighter in regimes where quantum effects play a significant role. This fully establishes uncertainty relations for both classical and quantum systems as intrinsic properties of their stochastic nature.

cond-mat.stat-mech↗

arXiv2024

Fluctuation-response inequalities for kinetic and entropic perturbations

We derive fluctuation-response inequalities for Markov jump processes that link the fluctuations of general observables to the response to perturbations in the transition rates within a unified framework. These inequalities are derived using the Cram\'er-Rao bound, enabling broader applicability compared to existing fluctuation-response relations formulated for static responses of current-like observables. The fluctuation-response inequalities are valid for a wider class of observables and are applicable to finite observation times through dynamic responses. Furthermore, we extend these inequalities to open quantum systems governed by the Lindblad quantum master equation and find the quantum fluctuation-response inequality, where dynamical activity plays a central role.

cond-mat.stat-mech↗

arXiv2024

Phase separation of chemokinetic active particles

Motility-induced phase separation (MIPS) is a well-studied nonequilibrium collective phenomenon observed in active particles. Recently, there has been growing interest in how coupling the self-propulsion of active particles to chemical degrees of freedom affects MIPS. Although the effects of chemotaxis on MIPS have been extensively studied, little is known about how chemokinesis affects MIPS. In this study, we demonstrate that various patterns can be induced when active particles consume chemicals and exhibit chemokinesis, where higher chemical concentrations enhance self-propulsion without causing alignment with the chemical gradient. We discover that MIPS is intensified if chemical consumption is proportional to particle density (as in the basal metabolic regime), but it is suppressed if chemical consumption is closely tied to particle motion (as in the active metabolic regime). While the former produces large-scale phase separation via coarsening, the latter suppresses the coarsening process, leading to microphase separation and oscillating patterns. We also derive a hydrodynamic theory that describes these findings.

cond-mat.soft↗

arXiv2023

Unified Hierarchical Relationship Between Thermodynamic Tradeoff Relations

Recent years have witnessed a surge of discoveries in the studies of thermodynamic inequalities: the thermodynamic uncertainty relation (TUR) and the entropic bound (EB) provide a lower bound on the entropy production (EP) in terms of nonequilibrium currents; the classical speed limit (CSL) expresses the lower bound on the EP using the geometry of probability distributions; the power-efficiency (PE) tradeoff dictates the maximum power achievable for a heat engine given the level of its thermal efficiency. In this study, we show that there exists a unified hierarchical structure encompassing all of these bounds, with the fundamental inequality given by a novel extension of the TUR (XTUR) that incorporates the most general range of current-like and state-dependent observables. By selecting more specific observables, the TUR and the EB follow from the XTUR, and the CSL and the PE tradeoff follow from the EB. Our derivations cover both Langevin and Markov jump systems, with the first proof of the EB for the Markov jump systems and a more generalized form of the CSL. We also present concrete examples of the EB for the Markov jump systems and the generalized CSL.

cond-mat.stat-mech↗

arXiv2023

$\alpha$-divergence Improves the Entropy Production Estimation via Machine Learning

Recent years have seen a surge of interest in the algorithmic estimation of stochastic entropy production (EP) from trajectory data via machine learning. A crucial element of such algorithms is the identification of a loss function whose minimization guarantees the accurate EP estimation. In this study, we show that there exists a host of loss functions, namely those implementing a variational representation of the $\alpha$-divergence, which can be used for the EP estimation. By fixing $\alpha$ to a value between $-1$ and $0$, the $\alpha$-NEEP (Neural Estimator for Entropy Production) exhibits a much more robust performance against strong nonequilibrium driving or slow dynamics, which adversely affects the existing method based on the Kullback-Leibler divergence ($\alpha = 0$). In particular, the choice of $\alpha = -0.5$ tends to yield the optimal results. To corroborate our findings, we present an exactly solvable simplification of the EP estimation problem, whose loss function landscape and stochastic properties give deeper intuition into the robustness of the $\alpha$-NEEP.

cond-mat.stat-mech↗