Physical Review E

In the field of quantum thermodynamics, scientists are trying to reconcile how the laws of classical thermodynamics apply to systems with only a small number of particles or atoms that are linked via quantum entanglement.

Polo-Gómez showed how the second law of thermodynamics — which states that heat flows from hot to cold, increasing entropy — impacts quantum state discrimination, the ability to distinguish quantum states from one another. By modifying a model for how an ideal gas could be split in two, Polo-Gómez found an upper limit on how easy it is to tell the difference between two pure quantum states. Along with providing insights on the ability to retrieve information in quantum computing, these results could help resolve disagreements between thermodynamic and von Neumann entropy.

Metamaterials have unique, tunable properties thanks to their precisely engineered internal structures. Some metamaterials can amplify electrical or mechanical signals through parametric resonance — the injection of energy into an oscillator, akin to pumping one’s legs on a swing — but it’s not well understood how this manifests in large, interconnected groups of oscillators.

Melkani and Paulose analyzed parametric resonance in a model of oscillators connected by a long chain made of springs. They identified a new operator that provides more information about the metamaterial’s dynamics than Floquet theory, which describes systems with periodic forces. Using this operator, they were able to predict the mechanical responses of dynamic mechanical metamaterials. The researchers say this new theory could bolster advances in metamaterial design.

What do a pile of sand, an engineered tissue scaffold, and a pane of glass all have in common? They are all solidlike systems, behaving like something in between a solid and a liquid. They also exhibit stress relaxation — a reduction in forces that cause a system to deform when subjected to constant strain. While stress relaxation is crucial for understanding how semisolid materials behave under sustained loads, developing a theory that can describe this phenomenon across a diverse set of systems, ranging from particles to tissues, has been a challenge.

Building on their previous work, where they predicted some unusual properties of glass and granular materials, Livné and colleagues have derived a theory of stress relaxation for two-dimensional semisolids. Taking inspiration from classical theories in electric-field screening — the energy loss caused by particles that carry an electric charge — their new theory predicts states of matter that are akin to the solid-liquid phases seen in two-dimensional crystals. Their theory could inform ongoing research across a range of fields, including material sciences, mechanical engineering, and more.

Complex systems — such as economic markets or ecosystems — can be represented as networks of interacting components that become unstable under certain conditions. In prior studies, researchers have modeled ecosystems as networks of “species” whose stability is modulated by random, well-connected food web interactions. Such techniques could offer insights into ecosystem resilience under environmental change or other potential disturbances.

Expanding on this approach, Mambuca and colleagues investigated the stability requirements for large, complex ecosystems by representing them as linear dynamical systems with random, sparse distributions of predator-prey, mutualistic, and competitive interactions. The system generally became unstable when it was large and complex enough, implying a tug-of-war between an ecosystem’s stability and its diversity. However, networks that featured solely predator-prey interactions remained stable regardless of size, and displayed oscillations when perturbed in infinitely large and sparse network structures. Such oscillatory behavior could shed light on oscillations in species populations documented in real-world ecosystems.

To understand swimming at microscopic levels, biologists and physicists have long investigated Chlamydomonas reinhardtii, a common, single-celled green alga that measures about as long as a human red blood cell and uses two flagella to breaststroke in fluids. Previous experimental and theoretical studies of this model microorganism have focused on its movements and models of the flow fields it produces in low Reynolds number environments.

Here, Jones et al. took a different approach. They placed a C. reinhardtii in an optical trap, sculpted using a near-infrared laser, and let the organism flail around with its flagella. By tracking the disruption of the photon momentum of the trapping beam, the researchers could directly measure the stochastic forces produced by the swimmer. After analyzing the force dynamics and energetics, the group discovered that their findings were in agreement with previous studies.

“Critical points,” in statistical mechanics, refer to points where a system changes from one phase to another, as from a liquid to a solid. Studies of biological systems suggest that collective behavior among living things—like cells that respond to each other through sensing—may also emerge at a kind of critical point. Whether or not biological systems operating near critical points offer a functional benefit remains an open question.

Vennettilli, Erez, and Mugler here use a simple biochemical model that incorporates nonlinear feedback and multicellular communication to investigate whether operating near a critical point confers an advantage for gaining information from a chemical signal. Their investigation turned up costs and benefits. For slow signal fluctuations, for example, they found that the mutual information between the signal and readout was maximized at criticality. However, they also found that the information flow rate—as opposed to the information itself—is minimized at criticality, due to critical slowing down.

Self-replicating objects manufacture copies of themselves using nearby building blocks from the environment. Researchers study such systems because an understanding of the rules behind this process could inform new insights into prebiotic chemistry—the study of how life originated from chemical compounds—and lead to innovative methods for generating high yields of particular products.

Sarkar and England investigated the physical conditions that promote the spontaneous emergence of self-replicating structures. They designed a toy model centered on objects that draw from collections of chemical building blocks in all possible stoichiometric combinations (and without triggering unwanted side reactions) and produced a system of rate equations that describe the process. Using the model, they described general conditions that lead to exponential growth in systems without explicit catalysis.

Amorphous solids like supercooled fluids, glass, foam, and plastic are materials with internal arrangements of atoms and molecules that don’t line up neatly in crystalline structures. The microscopic details of these states differ by material, but some researchers suspect that universal macroscopic properties may exist. Previous computer simulations and experimental work have probed both long-range stress correlations and plasticity (leading to yielding) in many of these materials.

Here, DeGiuli described the new “Edwards field theory” that provides a minimal description of the internal states of amorphous solids. (The name honors physicist Sam Edwards, who sought a theory of granular matter.) In two and three dimensions, the theory predicted long-range stress correlations that agree with data from simulations of supercooled liquids.

The general principles of macroscopic systems in equilibrium are well-described using the tools of statistical mechanics and thermodynamics. But small systems like individual particles, molecular motors, or chemical reactions can fluctuate in response to their surroundings and end up in steady states far from equilibrium, and researchers have been investigating the basic principles of this behavior.

The thermodynamic uncertainty relation offers an energetic constraint on the relative magnitude of current fluctuations in nonequilibrium steady states, but the relation had only been derived for long observation times. In this paper, Horowitz and Gingrich presented a proof of a recent conjecture on a finite-time uncertainty relation for steady-state current fluctuations. The derivation of this relation, they found, almost exactly followed a modified derivation of the long-time uncertainty relation, and they expect the proof to hold for diffusion processes as well.