Macroscopic Systems Research Articles

Quantum Control of a Single Magnon in a Macroscopic Spin System.

Nonclassical quantum states are the pivotal features of a quantum system that differs from its classical counterpart. However, the generation and coherent control of quantum states in a macroscopic spin system remain an outstanding challenge. Here we experimentally demonstrate the quantum control of a single magnon in a macroscopic spin system (i.e., 1mm-diameter yttrium-iron-garnet sphere) coupled to a superconducting qubit via a microwave cavity. By tuning the qubit frequency insitu via the Autler-Townes effect, we manipulate this single magnon to generate its nonclassical quantum states, including the single-magnon state and the superposition of single-magnon state and vacuum (zero magnon) state. Moreover, we confirm the deterministic generation of these nonclassical states by Wigner tomography. Our experiment offers the first reported deterministic generation of the nonclassical quantum states in a macroscopic spin system and paves a way to explore its promising applications in quantum engineering.

Tunable magnetic field effects on the near-field radiative heat transfer in planar three-body systems

Recently, the application of an external magnetic field to actively control the near-field radiative heat transfer (NFRHT) has emerged as an appealing and promising technique. Most prior studies have shown that the externally static magnetic field can only reduce the near-field flux transferred between two planar magneto-optical (MO) structures, yet so far the thermomagnetic effect in many-body systems consisting of such structures has not been revealed. Here, we investigate how the presence of an external static magnetic field modifies the nanoscale heat transfer in a MO many-body configuration comprising three noncontact slabs made of $n$-doped InSb. To this end, we first generalize a general Green's function approach for the calculation of the radiative energy transfer in many-body planar geometries composed of materials with complex optical anisotropy. Based on this approach, we show that the presence of a third MO body allows for either the reduction or even the enhancement of the NFRHT between MO slabs by applying external magnetic fields, which depends on the interplay between the zero-field surface waves and propagating hyperbolic modes induced by fields. Our study not only deepens the understanding of the active control of the nanoscale heat transfer via the applied magnetic field, but also paves the way for the magnetic regulation of energy fluxes in complex macroscopic many-body systems.

Interpretation of Josephson junction fluctuations at very low temperatures by superfluid flow equations

The effect of fluctuations on the stability of the zero-voltage state in the Josephson junction has been extensively investigated in the last four decades, due to the fundamental interest in this macroscopic quantum system and in view of possible application as a detector and, more recently, as base for quantum logic. Thermal induced escape from the zero-voltage state is well explained by consolidated theories based on the standard junction electrical model. However, at very low temperatures, significant deviations have been experimentally observed, which have triggered additional theories based on quantization of the Josephson junction effective potential and on macroscopic quantum tunneling. By looking at experiments carried out in the last forty years, we show here that the reported experimental data can be well described by standard theories down to zero temperature, provided that the Josephson potential is shifted by a constant amount, related to the junction plasma frequency. An explanation of this shift is given in terms of Anderson equations, relating chemical potential to phases, energies, and particle numbers in a superfluid flow.

Bidirectional wave-propelled capillary spinners

When a solid body floats at the interface of a vibrating liquid bath, the motion of the object generates outwardly propagating surface waves. We here demonstrate that chiral objects on a vibrating fluid interface are set into steady rotation, with the angular speed and direction of rotation controlled by the interplay between object geometry and driving parameters. Scaling laws and a simplified model of the wavefield reveal the underlying physical mechanism of rotation, while collapsing measurements of the angular velocity across parameters. Leveraging the control over the chiral object’s direction of rotation, we demonstrate that a body with an asymmetric mass distribution and chirality can be remotely steered along two-dimensional trajectories via modulation of the driving frequency. This accessible and tunable macroscopic system serves as a potential platform for explorations of chiral active and driven matter, and demonstrates a mechanism by which wave-mediated forces can be manipulated for directed propulsion.

Accelerating nonequilibrium Green function simulations with embedding self-energies

Real-time nonequilibrium Green functions (NEGFs) have been very successfully used to simulate the dynamics of correlated many-particle systems far from equilibrium. However, NEGF simulations are computationally expensive since the effort scales cubically with the simulation duration. Recently, we introduced the G1--G2 scheme that allows for a dramatic reduction to time-linear scaling [N. Schl\"unzen et al., Phys. Rev. Lett. 124, 076601 (2020); J.-P. Joost et al., Phys. Rev. B 101, 245101 (2020)]. Here we tackle another problem: the rapid growth of the computational effort with the system size. In many situations where the system of interest is coupled to a bath, to electric contacts, or to similar macroscopic systems for which a microscopic resolution of the electronic properties is not necessary, efficient simplifications are possible. This is achieved by the introduction of an embedding self-energy---a concept that has been successful in standard NEGF simulations. Here, we demonstrate how the embedding concept can be introduced into the G1--G2 scheme, allowing us to drastically accelerate NEGF embedding simulations. The approach is compatible with all advanced self-energies that can be represented by the G1--G2 scheme [as described in J.-P. Joost et al., Phys. Rev. B 105, 165155 (2022)] and retains the memoryless structure of the equations and their time-linear scaling. As a numerical illustration, we investigate the charge transfer between a Hubbard nanocluster and an additional site which is of relevance for the neutralization of ions in matter.

Higher order diffusion imaging as a putative index of human sleep-related microstructural changes and glymphatic clearance

The brain has a unique macroscopic waste clearance system, termed the glymphatic system which utilises perivascular tunnels surrounded by astroglia to promote cerebrospinal-interstitial fluid exchange. Rodent studies have demonstrated a marked increase in glymphatic clearance during sleep which has been linked to a sleep-induced expansion of the extracellular space and concomitant reduction in intracellular volume. However, despite being implicated in the pathophysiology of multiple human neurodegenerative disorders, non-invasive techniques for imaging glymphatic clearance in humans are currently limited.Here we acquired multi-shell diffusion weighted MRI (dwMRI) in twenty-one healthy young participants (6 female, 22.3 ± 3.2 years) each scanned twice, once during wakefulness and once during sleep induced by a combination of one night of sleep deprivation and 10 mg of the hypnotic zolpidem 30 min before scanning. To capture hypothesised sleep-associated changes in intra/extracellular space, dwMRI were analysed using higher order diffusion modelling with the prediction that sleep-associated increases in interstitial (extracellular) fluid volume would result in a decrease in diffusion kurtosis, particularly in areas associated with slow wave generation at the onset of sleep.In line with our hypothesis, we observed a global reduction in diffusion kurtosis (t15=2.82, p = 0.006) during sleep as well as regional reductions in brain areas associated with slow wave generation during early sleep and default mode network areas that are highly metabolically active during wakefulness. Analysis with a higher-order representation of diffusion (MAP-MRI) further indicated that changes within the intra/extracellular domain rather than membrane permeability likely underpin the observed sleep-associated decrease in kurtosis. These findings identify higher-order modelling of dwMRI as a potential new non-invasive method for imaging glymphatic clearance and extend rodent findings to suggest that sleep is also associated with an increase in interstitial fluid volume in humans.

On the balance of hydrophilicity and electrostatic interactions during particle filtration and backwash at microfiltration pores

Colloidal fouling is a notoriously limiting factor in many membrane processes. Although intensely studied in mostly macroscopic filtration systems, the underlying phenomena leading to generally decreased fouling remain unknown. This study systematically investigates fouling and backwashing of anionic polystyrene particles in microfluidic structures with engineered surface charge and hydrophilicity. We interpret the observed phenomena with the extended Derjaguin–Landau–Verwey–Overbeek (xDLVO) theory for interaction potentials. Polyelectrolyte coatings of the membrane-mimicking structure allow us to investigate particle–membrane interactions in (1) hydrophobic structures with negative zeta potential, (2) hydrophilic structures with positive zeta potential, and (3) hydrophilic structures with negative zeta potential. We assess the qualitative changes in membrane–particle interaction potentials regarding their hydrophilic and electrostatic double-layer interactions on the basis of the extensive study of particle deposition and resuspension during filtration and backwash. The results generally confirm the known trend of decreased fouling with increased hydrophilicity. However, we also show that hydrophilicity alone is an insufficient measure to estimate a membrane’s particle deposition and removal behavior. More important to achieve a low-fouling membrane that can be cleaned with pure flow reversal during regular backwashing is the design of a hydrophilic membrane with an engineered surface charge.

Enhanced weak force sensing based on atom-based coherent quantum noise cancellation in a hybrid cavity optomechanical system

The weak force sensing based on a coherent quantum noise cancellation (CQNC) scheme is presented in a hybrid cavity optomechanical system containing a trapped ensemble of ultracold atoms and an optical parametric amplifier (OPA). In the proposed system, the back-action noise can be completely eliminated at all frequencies and through the proper choice of the OPA parameters, and the noise spectral density can also be reduced at lower frequencies. This leads to a significant enhancement in the sensitivity of the cavity optomechanical weak force sensor, and the noise spectral density also surpasses the standard quantum limit (SQL) even for the small input power at the lower detection frequency. Furthermore, the experimental feasibility of this scheme is also briefly discussed. This study can be used for the realization of a force sensor based on hybrid cavity optomechanical systems and for the coherent quantum control in macroscopic systems.

Requirements on quantum superpositions of macro-objects for sensing neutrinos

We examine a macroscopic system in a quantum superposition of two spatially separated localized states as a detector for a stream of weakly interacting relativistic particles. We do this using the explicit example of neutrinos with $\mathrm{MeV}$-scale energy scattering from a solid object via neutral-current neutrino-nucleus scattering. Presuming the (anti)neutrino source to be a nuclear fission reactor, we utilize the estimated flux and coherent elastic neutrino-nucleus cross section to constrain the spatial separation $\mathrm{\ensuremath{\Delta}}x$ and describe the temporal evolution of the sensing system. Particularly, we find that a potentially measurable relative phase between quantum superposed components is obtained for a single gram scale mass placed in a superposition of spatial components separated by ${10}^{\ensuremath{-}14} \mathrm{m}$ under sufficient cooling and background suppression.

Three-Dimensionally Complex Phase Behavior and Collective Phenomena in Mixtures of Acoustically Powered Chiral Microspinners.

The process of dynamic self-organization of small building blocks is fundamental to the emergent function of living systems and is characteristic of their out-of-equilibrium homeostasis. The ability to control the interactions of synthetic particles in large groups could lead to the realization of analogous macroscopic robotic systems with microscopic complexity. Rotationally induced self-organization has been observed in biological systems and modeled theoretically, but studies of fast, autonomously moving synthetic rotors remain rare. Here, we report switchable, out-of-equilibrium hydrodynamic assembly and phase separation in suspensions of acoustically powered chiral microspinners. Semiquantitative modeling suggests that three-dimensionally (3D) complex spinners interact through viscous and weakly inertial (streaming) flows. The interactions between spinners were studied over a range of densities to construct a phase diagram, which included gaseous dimer pairing at low density, collective rotation and multiphase separation at intermediate densities, and ultimately jamming at high density. The 3D chirality of the spinners leads to self-organization in parallel planes, forming a three-dimensionally hierarchical system that goes beyond the 2D systems that have so far been modeled computationally. Dense mixtures of spinners and passive tracer particles also show active-passive phase separation. These observations are consistent with recent theoretical predictions of the hydrodynamic coupling between rotlets generated by autonomous spinners and provide an exciting experimental window to the study of colloidal active matter and microrobotic systems.

Molecular Weight Distribution of Branched Polymers: Comparison between Monte Carlo Simulation and Flory-Stockmayer Theory.

It is challenging to predict the molecular weight distribution (MWD) for a polymer with a branched architecture, though such information will significantly benefit the design and development of branched polymers with desired properties and functions. A Monte Carlo (MC) simulation method based on the Gillespie algorithm is developed to quickly compute the MWD of branched polymers formed through step-growth polymerization, with a branched polyetherimide from two backbone monomers (4,4'-bisphenol A dianhydride and m-phenylenediamine), a chain terminator (phthalic anhydride), and a branching agent (tris[4-(4-aminophenoxy)phenyl] ethane) as an example. This polymerization involves four reactions that can be all reduced to a condensation reaction between an amine group and a carboxylic anhydride group. A comparison between the MC simulation results and the predictions of the Flory-Stockmayer theory on MWD shows that the rates of the reactions are determined by the concentrations of the functional groups on the monomers involved in each reaction. It further shows that the Flory-Stockmayer theory predicts MWD well for systems below the gel point but starts to fail for systems around or above the gel point. However, for all the systems, the MC method can be used to reliably predict MWD no matter if they are below or above the gel point. Even for a macroscopic system, a converging distribution can be quickly obtained through MC simulations on a system of only a few hundred to a few thousand monomers that have the same molar ratios as in the macroscopic system.

Theoretical Study of Supercavitation Bubble Formation Based on Gillespie’s Algorithm

Understanding the creation and development of a supercavitation bubble is essential for the design of supercavitational underwater vehicles and applications. The pressure field of the supercavitation bubble is one of the most significant factors in these processes, and it should be taken into account in the analysis. The underwater vessel is surrounded by a supercavitation bubble which is, in fact, an inhomogeneous fluid containing cavities (also described as microbubbles). The distribution of the cavities in the supercavitation volume dictates the pressure field and thus determines the stresses and forces that act on the vessel and affect its motion and stability. In this research, we suggest a new approach to studying the bubbles’ formation and learning about the cavities’ distribution in the low-pressure volume that envelops the underwater vehicle. We used Logvinovich’s principle to describe a two-dimensional ring of fluid that is created at the front edge of the supercavitation body and moves downstream along the vessel. To describe the distribution of the cavities we used Gillespie’s algorithm, which is usually used to describe biological and chemical systems. The algorithm succeeded in describing the random movement of the cavities in the cross-section under various conditions and also in describing their distribution and effects on the macroscopic system. A few factors of the physical characteristics of the fluid and the flow conditions were examined (the initial bubble supply, and the rate coefficients of creation and collapse). The results led to the conclusion that with an examination of those factors and using Gillespie’s algorithm, predictions of the distribution and thus the development of supercavitation could be achieved. The main finding of the analysis was that asymmetric development of the bubbles took place, in spite of the symmetry of the physical problem, as observed in high-resolution experiments.

Vortex Counting and Velocimetry for Slitted Superconducting Thin Strips

The maximal speed $v^\ast$ for magnetic flux quanta is determined by the energy relaxation of unpaired electrons and is thus essential for superconducting microstrip single-photon detectors (SMSPDs). However, the deduction of $v^\ast$ from the current-voltage ($I$-$V$) curves at zero magnetic field is hindered by the unknown number of vortices, $n_\mathrm{v}$, as a small number of fast-moving vortices can induce the same voltage as a large number of slow-moving ones. Here, we introduce an approach for the quantitative determination of $n_\mathrm{v}$ and $v^\ast$. The idea is based on the Aslamazov and Larkin prediction of kinks in the $I$-$V$ curves of wide and short superconducting constrictions when the number of fluxons crossing the constriction is increased by one. We realize such conditions in wide MoSi thin strips with slits milled by a focused ion beam and reveal quantum effects in a macroscopic system. By observing kinks in the $I$-$V$ curves with increase of the transport current, we evidence a crossover from a single- to multi-fluxon dynamics and deduce $v^\ast\simeq12\,$km/s. Our experimental observations are augmented with numerical modeling results which reveal a transition from a vortex chain over a vortex jet to a vortex river with increase of $n_\mathrm{v}$ and the vortex velocity. Our findings are essential for the development of 1D and 2D few-fluxon devices and provide a demanded approach for the deduction of $v^\ast$ at the SMSPD operation conditions.

Polaron level spectrum in double-potential quantum wells

The description of macroscopic systems of quasi-particles is a bright spot in quantum physics, providing an accurate description of the physics development. The current study presents a theoretical study of the energy level spectrum of weakly coupled polarons in polar crystals (GaAs, PbSe, PbTe) in double potential quantum wells (parabolic and asymmetric Gaussian confinement potentials) using the linear combination operator and two simple unitary transformations. Rigorous electron-phonon coupling interaction results are obtained under the weak coupling limitation. The results indicate that the polaron energy is relatively lower than that of the electron and the coupling strength affects the energy difference between polaron and electron. The influence of quantum wells’ confinement potential on energy is also discussed. These findings provide the possibility for potential application of polaron.

Supramolecular glasses with color-tunable circularly polarized afterglow through evaporation-induced self-assembly of chiral metal–organic complexes

The fabrication of chiral molecules into macroscopic systems has many valuable applications, especially in the fields of optical displays, data encryption, information storage, and so on. Here, we design and prepare a serious of supramolecular glasses (SGs) based on Zn-L-Histidine complexes, via an evaporation-induced self-assembly (EISA) strategy. Metal-ligand interactions between the zinc(II) ion and chiral L-Histidine endow the SGs with interesting circularly polarized afterglow (CPA). Multicolored CPA emissions from blue to red with dissymmetry factor as high as 9.5 × 10−3 and excited-state lifetime up to 356.7 ms are achieved under ambient conditions. Therefore, this work not only communicates the bulk SGs with wide-tunable afterglow and large circular polarization, but also provides an EISA method for the macroscopic self-assembly of chiral metal–organic hybrids toward photonic applications.

Revision of the edge channel picture for the integer quantum Hall effect

State of the art computing opens now a new window to the integer quantum Hall effect (IQHE) regime, which enforces a major revision of the common knowledge accumulated so far. In our record-breaking application of the Hartree–Fock method we use up to 3000 electrons distributed over up to 5000 states for almost macroscopic system size of 1000 × 1000 nm. In particular, the formation of compressible and in-compressible edge stripes turns out to develop essentially different from the common picture used so far. Oppositely to the theory of Chklovskii, Shklovskii and Glazman (CSG), the narrow channels, as assumed by the early models of the IQHE, do not widen up into wide compressible stripes. Instead, the wide compressible stripes of CSG transform into a mixture of clusters of full and empty spin-split LLs, while the cluster boundaries create a network of still narrow quantum channels sitting on top of the wide compressible stripes. On this background the early models based on narrow edge channels do not suffer from neglecting electron–electron interaction as falsely stated in the past. Quite oppositely, in contrast to the common believe, our modeling demonstrates that also the IQHE regime carries the hallmark of many-body physics which stabilizes narrow edge channels also in the presence of electron–electron interaction.

Joint-embeddings reveal functional differences in default-mode network architecture between marmosets and humans

The default-mode network (DMN) is a distributed functional brain system integral for social and higher-order cognition in humans with implications in a myriad of neuropsychological disorders. In this study, we compared the functional architecture of the DMN between humans and marmosets to assess their similarities and differences using joint gradients. This approach permits simultaneous large-scale mapping of functional systems across the cortex of humans and marmosets, revealing evidence of putative homologies between them. In doing so, we find that the DMN architecture of the marmoset exhibits differences along its anterolateral-posterior axis. Specifically, the anterolateral node of the DMN (dorsolateral prefrontal cortex) displayed weak connections and inconsistent connection topographies as compared to its posterior DMN-nodes (posterior cingulate and posterior parietal cortices). We also present evidence that the marmoset medial prefrontal cortex and temporal lobe areas correspond to other macroscopical distributed functional systems that are not part of the DMN. Given the importance of the marmoset as a pre-clinical primate model for higher-order cognitive functioning and the DMN's relevance to cognition, our results suggest that the marmoset may lack the capacity to integrate neural information to subserve cortical dynamics that are necessary for supporting diverse cognitive demands.

Equilibria (equilibrium states) problem statement for city transport systems

Introduction. The article formulates the problems statement about equilibrium states in the model of a generalized transport system of the city, consisting of a street and road network, centers of mass gravity, places of residence of people, vehicles, as well as road users themselves, including passengers. The solution of these problems makes it possible to identify the equilibrium distributions of the system elements over various subsets of states (a subset of the elements of the road network; a subset of the center of mass gravity; a subset of trips of a certain type, etc.), depending on the type of vehicle, individual preferences, knowledge about the state of the transport system and other factors.At the same time, the transport system is considered as an object of research within the framework of the theory of macrosystems. A set of problems statements on the search for equilibrium states of the transport system for various modelling objects has been compiled for various structural levels (scales) of the objects under consideration.Materials and methods. In this paper, the theory of transport macrosystems is applied, which follows from a wellknown scientific discipline - the theory of macrosystems. Among its tasks there are statements about the distribution of elements over subsets of states and problems about the equilibrium of the system as a whole. In macroscopic systems, by definition, the stochastic behavior of a large number of elements transforms the deterministic behavior of the system as a whole. A macro system is thus a dynamic converter of the chaotic behavior of elements into a certain set of behavior parameters (phase variables) forming a space of small dimension. Therefore, within the framework of the theory of macrosystems, the basic concepts of entropy maximization at equilibrium states of the system are used. In this case, the distribution function of macrostates is selected depending on the method of filling some states with elements from the corresponding subsets; the necessary values of a priori probabilities and proofs of parametric properties of models of macrosystems with various statistics (Fermi-, Einstein- and Boltzmann-distributions). On the basis of the theory of macrosystems, for example, problems are solved to find equilibrium in such systems as: 1) megapolis with its functional and spatial structures (probabilistic states of hierarchical systems), 2) transport networks of cities formed by the movement of vehicles and residents of the city between different areas (distribution of trips along routes in the network); 3) logistics systems in the interregional exchange of products (problems of economic equilibrium in the exchange of resources).Results. The paper presents the results of research concerning the uniform description of the elements of the road network and the centers of mass gravity as components of the general transport system of the city (agglomeration) in the framework of the theory of transport macrosystems. At the same time, the study identifies various structural levels of description that can be used to solve particular problems, for example, finding equilibrium in individual subsets of the transport system, such as groups of centers of mass gravity of a certain type, or traffic flows on routes, stretches, network sections, etc.Discussion and conclusions. Within the framework of the work, the following tasks were solved: a description of the structural levels of the objects of the road network and the centers of mass gravity as the main components of the model of transport systems was developed; the formulation of problems about the equilibrium states of transport systems at the corresponding structural levels was developed; the analysis of the obtained methodology was performed; a methodological analogy is established between different subsets of states at the same structural level, for example, between the centers of mass gravity and elements of the road network as objects of modeling by methods of the theory of macrosystems (this analogy can be extended to other subsets of states in transport systems, for example, types of transport systems, travel purposes, parking spaces, subsystems of the intelligent transport system and much more).

Lévy models for collapse of the wave function

Recently there has been much progress in the development of stochastic models for state reduction in quantum mechanics. In such models, the collapse of the wave function is a physical process, governed by a nonlinear stochastic differential equation that generalizes the Schrödinger equation. The present paper considers energy-based stochastic extensions of the Schrödinger equation. Most of the work carried out hitherto in this area has been concerned with models where the process driving the stochastic dynamics of the quantum state is Brownian motion. Here, the Brownian framework is broadened to a wider class of models where the noise process is of the Lévy type, admitting stationary and independent increments. The properties of such models are different from those of Brownian reduction models. In particular, for Lévy models the decoherence rate depends on the overall scale of the energy. Thus, in Lévy reduction models, a macroscopic quantum system will spontaneously collapse to an eigenstate even if the energy level gaps are small.

More on the demons of thermodynamics

In her article “The demons haunting thermodynamics” (Physics Today, November 2021, page 44), Katie Robertson concludes the introductory historical summary by saying that modern developments in quantum foundations have banished the demons “once and for all.” Unfortunately, no explanation or reference is given for that optimistic but controversial conclusion.Robertson presents Erwin Hahn’s 1950 spin-echo experiments11. E. L. Hahn, Phys. Rev. 80, 580 (1950). https://doi.org/10.1103/PhysRev.80.580 as the realization of Josef Loschmidt’s vision of reversing momentum. But Hahn clearly described his spin-echo experiments as the effect of traditional spin dynamics for noninteracting spins in a spatially inhomogeneous magnetic field. Although the detailed explanation involves many particular subtleties of NMR dynamics in liquids, Hahn’s interpretation does not imply any violation of the “second law”; it uses only the mild assumption that the spin observables are at thermal equilibrium before each start signal. Robertson’s misunderstanding clearly appears when she writes that “atomic spins that have dephased and become disordered are taken back to their earlier state by an RF pulse” and, a few lines later, “it turns out that the spin-echo experiment is a special case; most systems approach equilibrium instead of retracing their steps back to nonequilibrium states.” The spins have not become disordered: The phase of each spin remains directly related to the magnetic field at the spin’s location, and that relationship explains the echo.Two illuminating articles by Won-Kyu Rhim, Alexander Pines, and John Waugh describe spin-echo experiments in which the irreversible time evolution of a coupled nuclear spin system in solids is apparently “reversed” for a limited duration.22. W.-K. Rhim, A. Pines, J. S. Waugh, Phys. Rev. Lett. 25, 218 (1970); https://doi.org/10.1103/PhysRevLett.25.218W.-K. Rhim, A. Pines, J. S. Waugh, Phys. Rev. B 3, 684 (1971). https://doi.org/10.1103/PhysRevB.3.684 As the authors explain, the results arise from uniform spin manipulation and are still consistent with the laws of thermodynamics.Another aspect of Robertson’s article that disturbed me is the lack of discussion of the relations between the actual experiments performed on large (macroscopic) systems and the microscopic-scale models used in attempts to make predictions about those results.Consider a mixture of oxygen and hydrogen at atmospheric pressure and room temperature. Standard thermodynamics and statistical mechanics will lead to excellent predictions of the equation of state, specific heat, and the like. But the standard models ignore the possibility of the chemical reaction producing water. Improved models are needed if one is to allow for, say, a slow, isothermal catalytic reaction or—much more complicated—an explosion in an isolated system at constant volume. Spin systems offer a rich variety of experimental possibilities for external manipulation and observation, but the corresponding models are related to the real experimental situations by complicated transformations and approximations that must be chosen according to the situation under study.Robertson invokes two models in the context of Maxwell’s demon. One is a biological machine using a ratchet-style mechanism. But in the cited article, the abstract carefully indicates that the evolution does not violate the second law because the microscopic mechanism is coupled to the exterior of the system.33. V. Serreli et al., Nature 445, 523 (2007). https://doi.org/10.1038/nature05452 In the demon-style experiment of Robertson’s figure 3, a complete realistic discussion of an actual implementation of the experiment leads to the similar conclusion that the second law is not violated.My conclusion is that demons and the related controversies are features of models and that the interpretation of actual experiments should be subjected to critical examination, preferably by those who performed the experiment and have a complete knowledge of all details.ReferencesSection:ChooseTop of pageReferences <<CITING ARTICLES1. E. L. Hahn, Phys. Rev. 80, 580 (1950). https://doi.org/10.1103/PhysRev.80.580, Google ScholarCrossref, ISI2. W.-K. Rhim, A. Pines, J. S. Waugh, Phys. Rev. Lett. 25, 218 (1970); https://doi.org/10.1103/PhysRevLett.25.218, Google ScholarCrossref, ISIW.-K. Rhim, A. Pines, J. S. Waugh, Phys. Rev. B 3, 684 (1971). https://doi.org/10.1103/PhysRevB.3.684, , Google ScholarCrossref, ISI3. V. Serreli et al., Nature 445, 523 (2007). https://doi.org/10.1038/nature05452, Google ScholarCrossref, ISI© 2023 American Institute of Physics.