arXiv subjects

Robert L. Jack

Publications and source records attributed to Robert L. Jack.

At least 19 recordsLinked to original sources

arXiv2026

When does nonreciprocity matter? Scale-dependence and nonequilibrium signatures

Nonreciprocity is increasingly recognized as a unifying concept linking diverse nonequilibrium phenomena found across physics, chemistry, and biology. It gives rise to distinctive behavior including run-and-chase dynamics and spatio-temporal patterns, often associated with a breaking of time-reversal symmetry. However, nonreciprocity and its nonequilibrium signatures are fundamentally scale-dependent, and may emerge or disappear under coarse-graining. A central challenge is therefore to understand when and how nonreciprocity manifests itself on different scales, for example via irreversible fluctuations or macroscopic currents. In this Perspective, we discuss the physical origin and fate of effective nonreciprocal interactions and the characteristic irreversible dynamics they give rise to across scales.

cond-mat.stat-mech↗

arXiv2026

Supermartingales in Quantum Resources Theories: Where do quantum resources go when you're watching?

We establish a connection between quantum resource theory and probability theory, under repeated application of free operations. We show that strong monotonicity of a resource measure implies that the resource is a supermartingale. We then use the optional stopping and martingale convergence theorems to derive bounds on the efficiency of post-selection, and other free adaptive strategies. We also describe the asymptotic dynamics of the conditional state, where resource fluctuations are necessarily absent, showing one of two distinct phenomena occurs: resource vanishing or resource freezing.

quant-ph↗

arXiv2026

Non-equilibrium pathways between cluster morphologies in active phase separation: necking, rupture and cavitation

We investigate the dynamical pathways of a morphological transition in a two-dimensional active lattice gas undergoing motility-induced phase separation. The transition is between two locally stable morphologies of the liquid cluster: a system-spanning "slab" and a compact "droplet". We generate trajectories of this transition in both directions using forward flux sampling. We find that the droplet-to-slab transition always follows a similar mechanism to its equilibrium counterpart, but the reverse (slab-to-droplet) transition depends on rare non-equilibrium fluctuations. At low Péclet numbers the equilibrium and non-equilibrium pathways compete, while at high Péclet numbers the equilibrium pathway is entirely suppressed, and the only allowed mechanism involves a large vapour bubble. We discuss the implications of these findings for active matter systems more generally.

cond-mat.soft↗

arXiv2026

Nonreciprocal Disorder Prevents Zero-Temperature Freezing in a Ferromagnet

Nonreciprocal interactions underpin diverse nonequilibrium phenomena, yet the effects of quenched nonreciprocity in extended systems remain largely unexplored. We study a $2d$ Ising model with randomly distributed nonreciprocal bonds at density $p$, finding a continuous nonequilibrium transition down to $T=0$ with finite $p_c$. A gauge-invariance argument yields $p_c(T)\leq1/2$, and mean-field theory predicts a qualitatively correct phase diagram. Unlike equilibrium disordered models, the zero-temperature dynamics remains active, with athermal rare-region reversals and logarithmic "activated" coarsening.

cond-mat.stat-mech↗

arXiv2026

Hydrodynamic theory of wetting by active particles

The accumulation of self-propelled particles on repulsive barriers is a widely observed feature in active matter. Despite being implicated in a broad range of biological processes, from biofilm formation to cytoskeletal movement, wetting of surfaces by active particles remains poorly understood. In this work, we study this active wetting by considering a model comprising an active lattice gas, interacting with a permeable barrier under periodic boundary conditions, for which an exact hydrodynamic description is possible. We consider a hydrodynamic scaling limit that eliminates dynamical noise while retaining microscopic interpretability, enabling a precise characterisation of steady-states and their transitions. We demonstrate that the accumulation of active particles has remarkable similarities to equilibrium wetting, and that active wetting transitions display all the salient characteristics of the equilibrium critical wetting transition -- despite fundamental differences in underlying microscopic dynamics. However, our framework also enables the investigation of subtle but important nonequilibrium effects in active wetting, including a spontaneous ratchet effect which leads to a global steady-state current, departure of the bulk densities from their binodal values, and a novel dynamical transition pathway. Our results provide an intrinsically nonequilibrium framework in which to study active wetting, precisely demonstrating the connection to passive wetting while clarifying the nonequilibrium consequences of activity.

cond-mat.stat-mech↗

arXiv2026

Spontaneous Ratchet Currents and Transition Dynamics in Active Wetting

Self-propelled particles accumulate on repulsive barriers in so-called active wetting, whose relationship with equilibrium wetting remains unclear. Using an exact (noiseless) hydrodynamic framework for an active lattice gas, we show, using a slit geometry with periodic boundary conditions, that active matter exhibits both fully-wet and partially-wet states, with a critical wetting transition between them. Furthermore, we demonstrate the existence of a spontaneous-symmetry-breaking ratchet current in the partially-wet state, leading to departure of the bulk densities from their binodal values and the emergence of a novel dynamical pathway for the full-to-partial wetting transition. We elucidate this modified dynamical pathway using a minimal model. The results, while establishing a direct connection between active and equilibrium wetting, also identify the nonequilibrium consequences of activity.

cond-mat.stat-mech↗

arXiv2026

Odd pathways speed up self-assembly

Active self-assembly can bypass equilibrium bottlenecks through external energy injection. However, generic driving typically distorts target structures and requires sustained energy input even after assembly is complete. Here, we investigate a class of non-reciprocal interactions that accelerates assembly while preserving the equilibrium Boltzmann distribution. The probability currents induced by these odd interactions reshape fundamental processes, including activated barrier crossing, soft-mode relaxation, and transitions between metastable states. In particular, these currents enhance Arrhenius rates by driving particles across otherwise inaccessible free-energy barriers. We show that this acceleration arises from an effective increase in the mobility of the reaction coordinate, mediated by non-reciprocal coupling between mechanical modes. In turn, we discover a trade-off between kinetic acceleration and power dissipation when active forces are engaged. Our results suggest a route to energy-efficient, high-fidelity self-assembly via active catalysts that transiently accelerate relaxation toward equilibrium targets and deactivate upon reaching the desired state.

cond-mat.soft↗

arXiv2025

Nonreciprocal dynamics with weak noise: aperiodic "Escher cycles" and their quasipotential landscape

We present an explicit construction of the Freidlin-Wentzell quasipotential of a stochastic system with two degrees of freedom and nonreciprocal interactions. This model undergoes noise-induced transitions between four metastable attractors, forming recurrent but aperiodic ``Escher cycles,'' similar to the cyclic nucleation dynamics observed in the nonreciprocal Ising model. We calculate the quasipotential analytically to first order in nonreciprocality. We characterise it along a one-dimensional reaction coordinate that connects the attractors, and we also obtain the full two-dimensional landscape, at leading order in perturbation theory. The resulting landscapes feature flat regions and extended plateaus, together with non-differentiable switching lines. These singular structures arise from two geometric mechanisms: the handover of dominance between competing transition paths, and the competition between basins of attraction. The system provides a rare case where the geometry of nonequilibrium rare events can be fully resolved, and a simple analytically tractable example of a quasipotential in more than one coordinate that captures a rich set of nonequilibrium features.

cond-mat.stat-mech↗

arXiv2025

Avalanche-like lithium intercalation and intraparticle correlations in graphite

Graphite is the most widely used anode material in lithium-ion batteries with over 98% market share. However, despite its first application over 30 years ago, the lithium insertion processes and associated dynamics in graphite remain poorly understood, especially for the dilute stages. A fundamental understanding of how the symmetry-breaking phase transitions occur pseudo-continuously under operating conditions is still lacking. Here, we provide a unified picture of ion intercalation dynamics during the dilute stages of graphite intercalation, using operando optical microscopy combined with random field Ising modelling. We show that during the dilute stages, single graphite particle undergoes rapid, localised avalanche-like (de)intercalation, leading to micron-sized regions (de)intercalating within seconds. These avalanches are reminiscent of phase transition behaviour seen in disordered materials such as martensitic transformations, Barkhausen noise and ferroelectric/elastic materials - associated with step changes in the order parameter, where the system changes from one phase to another under an applied driving force by jumping from one metastable state to another. Here, using a modified random field Ising model, we relate these avalanches to static disorder in graphite, which disrupts ion filling dynamics, leading to pseudo-continuous transitions between stages, accounting for the experimental electrochemistry profile as well as the temperature dependent avalanche dynamics. Finally, we develop a methodology to spatio-temporally analyse avalanches between intraparticle regions, revealing spatially heterogeneous connectivity and temporal patterns between regions during the dilute stages. Our work highlights the role of local and static disorder in eliciting unexpected phase transition behaviour, and provides new tools and concepts for studying layered battery materials.

cond-mat.mtrl-sci↗

arXiv2025

Exact large deviations and emergent long-range correlations in sequential quantum East circuits

Exploiting quantum measurements is a promising route for preparation of correlated quantum states. We use methods from large deviation theory to solve this problem exactly for a specific system: the deterministic quantum East circuit with boundary measurements. We show that conditioning on measurement outcomes generates a long-range correlated state, despite typical trajectories being trivial. We derive the channel that optimally realizes the rare measurement trajectories, and establish a formal connection with the Petz recovery (time-reversal) map. We compute one- and two-point correlation functions in the conditioned state, revealing finite two-body correlations at arbitrarily large separations, and an underlying fractal structure, related to the Sierpiński triangle. These results demonstrate explicitly how boundary measurements can be used to control bulk properties of a quantum system.

cond-mat.stat-mech↗

arXiv2025

Dynamical patterns and nonreciprocal effective interactions in an active-passive mixture through exact hydrodynamic analysis

The formation of dynamical patterns is one of the most striking features of nonequilibrium physical systems. Recent work has shown that such patterns arise generically from forces that violate Newton's third law, known as nonreciprocal interactions. These nonequilibrium phenomena are challenging for modern theories. Here, we introduce a model mixture of active (self-propelled) and passive (diffusive) particles amenable to exact mathematical analysis. We exploit state-of-the-art methods to derive exact hydrodynamic equations for the particle densities, which reveal effective nonreciprocal couplings between the active and passive species. We study the resulting collective behavior, including the linear stability of homogeneous states and phase coexistence in large systems. This reveals a novel phase diagram with the spinodal associated with active phase separation protruding through the associated binodal, heralding the emergence of dynamical steady states. We analyze these states in the thermodynamic limit of large system size, showing, for example, that sharp interfaces may travel at finite velocities, but traveling phase-separated states are forbidden. The model's mathematical tractability enables precise new conclusions beyond those available by numerical simulation of particle models or field theories.

cond-mat.stat-mech↗

arXiv2025

Gauge freedoms in unravelled quantum dynamics: When do different continuous measurements yield identical quantum trajectories?

Quantum trajectories of a Markovian open quantum system arise from the back-action of measurements performed in the environment with which the system interacts. In this work, we consider counting measurements of quantum jumps, corresponding to different representations of the same quantum master equation. We derive necessary and sufficient conditions under which these different measurements give rise to the same unravelled quantum master equation, which governs the dynamics of the probability distribution over pure conditional states of the system. Since that equation uniquely determines the stochastic dynamics of a conditional state, we also obtain necessary and sufficient conditions under which different measurements result in identical quantum trajectories. We then consider the joint stochastic dynamics for the conditional state and the measurement record. We formulate this in terms of labelled quantum trajectories, and derive necessary and sufficient conditions under which different representations lead to equivalent labelled quantum trajectories, up to permutations of labels. As those conditions are generally stricter, we finish by constructing coarse-grained measurement records, such that equivalence of the corresponding partially-labelled trajectories is guaranteed by equivalence of the trajectories alone. These general results are illustrated by two examples that demonstrate permutation of labels, and equivalence of different quantum trajectories.

quant-ph↗

arXiv2025

Weak unitary symmetries of open quantum dynamics: beyond quantum master equations

We consider Markovian open quantum dynamics with weak unitary symmetries. Starting from the quantum master equation for the system alone, it is known that the joint dynamics of the system and its environment can be obtained by dilation, leading to a closed dynamics for a continuous matrix product state. Performing counting measurements on the environment gives rise to stochastic dynamics of quantum trajectories for the system, which when averaged yield back the quantum master equation. In this work, we identify necessary and sufficient conditions under which the dynamics of these different descriptions retain the weak symmetry of the quantum master equation and we characterise the resulting symmetries of the different descriptions in terms of their generators. We find that the joint dynamics always features a separable symmetry directly related to that of the quantum master equation, but for quantum trajectories the corresponding symmetry is present only if the counting measurement satisfies certain conditions.

quant-ph↗

arXiv2025

Cyclically sheared colloidal gels: structural change and delayed failure time

We present experiments and simulations on cyclically sheared colloidal gels, and probe their behaviour on several different length scales. The shearing induces structural changes in the experimental gel, changing particles' neighborhoods and reorganizing the mesoscopic pores. These results are mirrored in computer simulations of a model gel-former, which show how the material evolves down the energy landscape under shearing, for small strains. By systematic variation of simulation parameters, we characterise the structural and mechanical changes that take place under shear, including both yielding and strain-hardening. We simulate creeping flow under constant shear stress, for gels that were previously subject to cyclic shear, showing that strain-hardening also increases gel stability. This response depends on the orientation of the applied shear stress, revealing that the cyclic shear imprints anisotropic structural features into the gel.

cond-mat.soft↗

arXiv2025

Entropy production rate in thermodynamically consistent flocks

We study the entropy production rate (EPR) of aligning self-propelled particles which undergo a flocking transition towards a polarized collective motion. In our thermodynamically consistent lattice model, individual self-propulsion is the exclusive source of irreversibility. We derive the fluctuating hydrodynamics for large system sizes using a controlled coarse-graining: our procedure entails an exact correspondence between the EPR evaluated at the hydrodynamic and particle-based levels. We reveal that EPR is maximal when the system adopts a homogeneous configuration, either apolar or polar, and reduced in the non-homogeneous state where a polar band travels in a apolar background due to strong spatial EPR modulations. By analyzing the latter we also show that asymmetric energetic exchanges occur at the trailing and leading edges, which we map into a thermodynamic cycle in density-polarization space. Finally, we demonstrate that the regime of weak self-propulsion features a singular scaling of EPR, and a non-analyticity of the travelling band profiles.

cond-mat.stat-mech↗

arXiv2025

Learning strategies for optimised fitness in a model of cyclic dominance

A major problem in evolutionary biology is how species learn and adapt under the constraint of environmental conditions and competition of other species. Models of cyclic dominance provide simplified settings in which such questions can be addressed using methods from theoretical physics. We investigate how a privileged ("smart") species optimises its population by adopting advantageous strategies in one such model. We use a reinforcement learning algorithm, which successfully identifies optimal strategies based on a survival-of-the-weakest effect, including directional incentives to avoid predators. We also characterise the steady-state behaviour of the system in the presence of the smart species and compare with the symmetric case where all species are equivalent.

cond-mat.stat-mech↗

arXiv2025

Efficient post-selection in light-cone correlations of monitored quantum circuits

We consider how to target evolution conditioned on atypical measurement outcomes in monitored quantum circuits, i.e., the post-selection problem. We show that for a simple class of measurement schemes, post-selected light-cone dynamical correlation functions can be obtained efficiently from the averaged correlations of a different unitary circuit. This connects rare measurement outcomes in one circuit to typical outcomes in another one. We derive conditions for the existence of this rare-to-typical mapping in brickwork quantum circuits made of XYZ gates. We illustrate these general results with a model system that exhibits a dynamical crossover (a smoothed dynamical transition) in event statistics, and discuss extensions to more general dynamical correlations.

cond-mat.stat-mech↗

arXiv2024

Interfacial and density fluctuations in a lattice model of motility-induced phase separation

We analyze motility-induced phase separation and bubbly phase separation in a two-dimensional lattice model of self-propelled particles. We compare systems where the dense (liquid) phase has slab and droplet geometries. We find that interfacial fluctuations of the slab are well-described by capillary wave theory, despite the existence of bubbles in the dense phase. We attribute this to a separation of time scales between bubble expulsion and interfacial relaxation. We also characterize dependence of liquid and vapor densities on the curvature of the liquid droplet, as well as the density fluctuations inside the phases. The vapor phase behaves similarly to an equilibrium system, displaying a Laplace pressure effect that shifts its density, and Gaussian density fluctuations. The liquid phase has large non-Gaussian fluctuations, but this is not accompanied by a large density shift, contrary to the equilibrium case. Nevertheless, the shift of the vapor density can be used to infer an effective surface tension that appears to also quantify capillary wave fluctuations.

cond-mat.soft↗