Papers

Stabilizer codes provide exact lattice realizations of bosonic topological orders. In contrast, systematic stabilizer descriptions of intrinsically fermionic topological phases remain much less developed. In this work, we introduce Majorana-Pauli stabilizer codes, a class of exactly solvable fermionic lattice models whose stabilizers are built from both generalized Pauli operators and Majorana operators. As a main example, we construct an exactly solvable stabilizer realization of the fermionic toric code: an intrinsically fermionic $\mathbb Z_2$ topological order in $(2{+}1)$ dimensions, using $\mathbb Z_8$ Pauli operators coupled to Majorana modes. Within this stabilizer framework, the anyons, string operators, fusion rules, and braiding statistics all follow naturally from the stabilizer algebra. More broadly, we show that the fermionic toric code belongs to a duality web generated by anyon condensation and by gauging or ungauging bosonic or fermionic symmetries. This web connects bosonic topological orders, symmetry-enriched topological (SET) phases, and both bosonic and fermionic symmetry-protected topological (SPT) phases, all within a common stabilizer description. We extend this construction to all Abelian fermionic topological orders with gapped boundaries and to fermionic SPT phases whose fully gauged theories are Abelian. Going beyond Majorana operators, we introduce fermionic versions of the clock and shift operators and use them to construct an exact bosonization map for $\mathbb Z_D^F$ symmetries for $D$ even. Using this, we realize a stabilizer model for a nontrivial $\mathbb Z_8^F$ fermionic SPT phase with no free-fermion analog. Altogether, these results extend the stabilizer-code paradigm to a broad class of intrinsically fermionic phases bridging fermionic quantum many-body physics to quantum error correction.

Pair-density wave (PDW) superconductors are exotic phases in which Cooper pairs carry finite center-of-mass momentum. Despite a variety of theoretical and experimental reports on PDW states, exact PDW ground states in topological bands have remained elusive. Here we construct exactly solvable models with PDW ground states in topological flat bands using a generalized version of the recently proposed quantum geometric nesting (QGN) framework. Our construction broadly applies to systems with non-commuting magnetic translation symmetries (MTS) and time-reversal symmetry, exemplified by the time-reversal invariant version of the Kapit-Mueller model with two ideal flat Chern bands with opposite Chern number. Our construction thus provides a platform for further studies of band topology and quantum geometry in PDW superconductors.

Quantum Hall systems host emergent quasiparticles with unusual charge, spin, and statistics, such as fractionally charged anyons. Although transport measurements have revealed many of their collective properties, identifying and visualizing individual quasiparticles remain elusive. Here we use scanning tunneling spectroscopy (STS) to image quantum Hall quasiparticles in graphene. Within incompressible quantum Hall states, we observe spatial variation of Landau level energies originating from electrostatic potentials created by charged defects in graphene and the underlying hexagonal boron nitride (hBN). For surface and near-surface defects, the Coulomb potential lifts the degeneracy of Landau orbitals, producing discrete energy splittings that reveal Landau orbital wavefunctions. In quantum Hall ferromagnetic states, quasiparticles bound to defect potentials produce distinct spatial and spectroscopic signatures that serve as hallmarks of the presence and number of localized excitations. In the fractional quantum Hall regime at one-third filling, our theoretical calculations predict discrete spectroscopic changes associated with the sequential addition of localized anyons, with a three-anyon bound state quantitatively reproducing our experimental data at $\nu = 5/3$. These observations establish spectroscopic fingerprints of quantum Hall quasiparticles and provide a pathway toward imaging and manipulating individual anyons in real space.

Fractional quantum Hall states host anyons, emergent quasiparticles with fractional charge and nontrivial exchange statistics. Controlling, trapping, and braiding anyons are central goals for both fundamental physics and topological quantum computation. A key step toward such control is understanding how anyons behave when confined in local potentials, where their internal structure can become relevant. Here, we use the scanning tunneling microscopy/spectroscopy (STM/STS) to study the excitation spectrum in integer and fractional quantum Hall states of monolayer graphene near individual charged impurities. In the integer quantum Hall states, the STS spectra show lifting of orbital degeneracy near defects, appearing as a band of discrete energy levels. In fractional states, (v=1/3 and 2/5), however, we observe an additional energy splitting of the lowest-energy spectral feature that occurs only when the chemical potential lies within a fractional gap and is absent in compressible or integer regimes. We attribute this to many-body configurations of anyons trapped by an impurity potential. Strikingly, numerical calculations show that the splitting requires an anisotropic confining potential, vanishing for a rotationally symmetric trap. The competing multi-anyon states carry nearly identical charge within the core of the potential but differ in how that charge is redistributed at larger radius. Our results establish local tunneling spectroscopy as a direct probe of anyon bound states, providing a key step toward understanding and controlling their behavior in confined geometries relevant for braiding and fusion.

We study the strict locked-orientation limit of hard spherocylinders on a sphere, in which the rod axes are rigidly locked to a prescribed tangential director field and cannot reorient. Because the bulk hard-rod phase diagram contains no smectic-C phase, any coherent tilt isolates a geometric curvature mechanism rather than a finite-stiffness equilibrium effect. A ratio-symmetric recognition cost fixes the layer spacing at the bulk close-contact value and yields a hierarchy of geometric statements: the lower edge of the smectic-area window at $45^\circ$ follows from reciprocal symmetry; the upper edge at $58.3^\circ$ is a falsifiable channel-saturation hypothesis; the smectic-A to smectic-C boundary is a closed-form prediction; and the rod tilt angle is set by the rod-to-radius ratio, modulated by a chirality envelope peaking near $24^\circ$. Locked-orientation Monte Carlo across fifteen geometries confirms these predictions with no fitted elastic constants: the smectic area peaks at $55^\circ$, and a coherent smectic-C window is detected.

Blackbody infrared radiative dissociation (BIRD) activates molecules through successive absorption of ambient thermal photons until the internal energy reaches a dissociation threshold. Because these radiative transition rates depend on the electromagnetic density of states (DOS), structured infrared environments provide a route to control thermal unimolecular dissociation. Here we develop a state-resolved master-equation framework for polyatomic BIRD in a planar Au/MgO multilayer cavity, where the reactive cluster $\mathrm{(H_2O)_2Cl^-}$ is studied. The cavity modifies the kinetics through the DOS sampled by anharmonic fundamental, overtone, and combination transitions. We show that MgO surface phonon polaritons produce strong near-field enhancements in the central vacuum reaction region of a microcavity. We find that short cavities with thick polar crystal layers yield the largest BIRD enhancements due to enhanced evanescent surface phonon polariton contributions. We further include collisions with a methane bath gas and show that cavity DOS engineering shifts the crossover between BIRD and collisional activation. These results establish Reststrahlen-band DOS engineering as a practical strategy for controlling polyatomic BIRD in infrared microcavities.

We study the universal spatial statistics of point-like topological defects formed during the nonequilibrium condensation of a coherently coupled Bose gas using the stochastic projected Gross-Pitaevskii equation. The symmetry-breaking transition is driven by a linear quench of the chemical potential, leading to stochastic vortex nucleation in the individual condensate components. When the two components are considered together, these elementary defects may combine across components to emerge as composite topological defects known as full quantum vortices. Beyond the mean defect density predicted by the Kibble-Zurek mechanism (KZM), we investigate the spatial organization of both the elementary and composite defects and show that their positions are well described by a Poisson point process, revealing a universal stochastic geometry. This universality is further described through Voronoi tessellation, whose cell-area statistics follow Poisson-Voronoi predictions. We also introduce the spatial form factor for characterizing the vortex configurations and demonstrate the emergence of a characteristic dip-ramp-plateau structure. Our results establish universal stochastic geometry of topological defects beyond conventional Kibble-Zurek scaling and identify it as a fundamental feature of nonequilibrium condensation in coherently coupled Bose gases.

What kind of physical object is an artificial-intelligence system: a machine in the sense a refrigerator is – dissipating free energy while holding an order imposed from outside – or a dissipative structure in the sense a convection cell is – an ordered state that exists only under throughput and loses stability past a critical drive? We argue the answer is split, as it is for the living cell and the star – the trained artifact is a machine, quenched and storable; the operating loop is a dissipative structure in the informational sense – and develop the framework in which the loop's classification becomes decidable. Modeling systems that couple inference to irreversible action as finite-capacity decoders under sustained informational driving, we characterize recoverable operation by a feasibility margin, local invertibility, and a stability diagnostic that diverges as capacity saturates. Making the feedback of uncertified output onto load explicit converts this continuous transition into a first-order one at mean-field level, sharpening in the fleet limit: lucid and collapsed states coexist in a cusp-organized bistable region with closed-form spinodals, collapse pre-empts the divergence, recovery is hysteretic, and for ungatedness alpha >= 1 load reduction alone cannot restore operation; reset restores only what is archived, making certification the operative stability lever. Cascades are subcritical branching with mean-field exponent 3/2 and a cutoff set by the grounded fraction of input. An instrumented real-workload pipeline experiment exhibits the collapse just below the spinodal computed from the measured service law, the backlog-delayed hysteretic recovery, and the cascade statistics. This supplies a statistical-mechanics account of the "metastable failures" documented in large-scale distributed systems, identifying recoverable dissipation as the stability criterion.

Band gaps are commonly attributed to Bragg scattering or local resonance, yet it remains unclear whether these mechanisms govern band gap formation in beam networks. In this work, we explain band gap formation in beam networks in terms of a new mechanism, geometry-induced coupling between deformation modes. Specifically, band gap onset arises from axial-bending coupling at lattice nodes and scales with the axial cutoff frequency of a one-dimensional periodic beam, whereas band gap termination is primarily governed by high-frequency rotational branches associated with beam geometry. This mechanism holds for both periodic and disordered beam networks. In periodic lattices, it manifests through beam orientations at lattice nodes, whereas in disordered networks it manifests through short-beam statistics arising from variations in beam length. Together, these results establish a unified mechanism for band gap formation across both periodic and disordered beam networks, providing new insight into the physical origin of band gaps in beam-network materials.

Quasiparticles in fractional quantum Hall systems are anyons, carrying a fraction of the electron charge. Exchanging two of them gives rise to a fractional exchange phase. While the fractional charge and the braiding phase – twice the exchange phase – have been measured, the exchange phase itself has remained inaccessible. We study a quantum antidot embedded in a Fabry-Perot interferometer. Within a systematic non-equilibrium Keldysh treatment that consistently includes the occupation and level broadening of the antidot, we find that the transmission phase evolves non-monotonically when a gate voltage tunes the antidot through a resonance, in contrast to the monotonic evolution for electrons. The bare exchange phase can be extracted from the difference between the phase plateaus.

We present an exact analytical reformulation of a growing social group model – a Hamiltonian-free nonequilibrium process in which a group grows by noisy, consensus-driven admission. Cast as a gradient flow on logarithmic time, the fixed-point structure collapses to a single self-consistent equation: $\arctanh(\phi^*) = m \cdot \arctanh(\alpha\phi^*)$, where $\phi$ is the polarization, $\alpha=1-2\eta$ the evaluation reliability, and $m$ the number of evaluators. The equation has a direct log-odds interpretation: each verdict contributes log-likelihood ratio $2\arctanh(\alpha\phi)$; unanimity accumulates $m$ independent evidence pieces. The dynamics thus constitutes an exact mean-field theory of self-consistent inference, ordering when the collective gain $m\alpha$ overcomes the dilution of growth. We develop a systematic three-layer framework: core theory (Landau-like effective potential, comparison with the mean-field Ising model, and features without equilibrium counterpart), mathematical foundations (criticality from correlated verdicts, Pólya-urn martingale convergence, and an RG-like flow with group size as scale), and complementary perspectives on irreversibility and information geometry. A frozen-$N$ Freidlin–Wentzell quasipotential yields Kramers-type escape estimates for metastable states, while Monte Carlo simulations collapse onto a parameter-free deterministic master curve on logarithmic time. Systematic comparison with the mean-field Ising model reveals shared critical exponents but a nested arctanh structure unique to growth. These results provide a detailed analytical characterization of a minimal model of growth-driven collective behavior and map which elements of the equilibrium critical toolbox – suitably reinterpreted – survive without a Hamiltonian.

In confined space, deviations from bulk structure and properties are expected due to additional thermodynamic variables. In particular, composition variations arising from surface interactions may lead to additional phases and altered phase transitions. Here, we introduce a non-invasive method for nanoscale composition mapping in confined liquids using the surface force balance (SFB). The method extends conventional SFB analysis from apex measurements to spatially resolved reconstruction of refractive index profiles within confined fluids. When multiple phases are present, the refractive index profiles provide direct access to the position and geometry of the nanoconfined fluid interfaces. We describe the interferogram analysis in detail and establish its range of validity through two model scenarios. First, measurements in air demonstrate the precision of the method and allow detection of a nanometric wetting capillary. Second, we analyse dynamic evaporation of a confined heptane droplet down to 0.1 pL volume. The method provides a time-resolved reconstruction of the meniscus geometry throughout the evaporation process. Although evaporation continuously drives the system out of equilibrium, the meniscus remains well described by a catenoidal geometry down to heights of approximately 80 nm. At smaller separations, systematic deviations from the catenoidal profile emerge, indicating a crossover from a surface tension-dominated regime to a confinement-dominated regime. Overall, we demonstrate composition profiling as a framework to analyse confinement-induced composition variations and to quantify interfacial thermodynamic effects at the nanoscale.

Manipulating phonons through symmetry is a fundamental approach to alter the dynamic responses of materials. Most often new phases are sought through control of the unit-cell (e.g. via strain, doping, light, etc.). By comparison, there is vast potential to look beyond the unit-cell into higher-order architectures to control wave scattering and interference effects that remains less explored. We describe the THz-induced dynamics of an SrTiO$_{3}$ thin film with a nanoscale ordered interfacial dislocation network probed with 2- and 3-dimensional time-resolved x-ray diffraction and classical molecular dynamics simulations. We find that symmetry breaking at all scales is an effective approach to create a dynamical electric polarization and to control phonon mixing that generates previously unreported collective modes in the THz regime with circular vortex-like displacements. This work opens a new pathway to explore dynamical functional properties that can be extended to magnetic, electric, and ferroelectric systems by controlling the real-space topology via epitaxy.

These notes provide an introduction to tensor-network methods in quantum many-body physics, with an emphasis on matrix-product states (MPS). They develop the basic tensor-network language, including graphical notation, virtual indices, bond dimensions, gauge freedom, canonical forms, QR and singular-value decompositions, and the role of entanglement in controlling the efficiency of the representation. The main MPS algorithms are then introduced, including contractions, correlation functions, matrix-product operators, DMRG, and time-evolution methods. The notes also briefly discuss projected entangled-pair states (PEPS) as a higher-dimensional generalization of MPS, together with the basic ideas behind approximate PEPS contraction. Finally, tensor-network representations of mixed states, quantum channels, and Lindblad dynamics are presented, with applications to thermal states and open quantum systems. The presentation is accompanied by short Julia code examples based on ITensor, ITensorMPS, and TensorMixedStates. These notes were written for the 9th Les Houches Summer School on Computational Physics: Open Quantum Systems, held in June 2026.

Charge order driven by electron-phonon coupling is well understood at equilibrium but pump-probe experiments raise a new question: how does this order melt and recover after strong photoexcitation? A pump pulse promotes carriers across the charge-order gap and creates a nonequilibrium high-energy electronic population. In a closed system the subsequent dynamics is constrained by energy conservation. In an `open system' - where the system is coupled to a thermal bath at some temperature $T_{\rm bath}$ - there are new fluctuation and dissipation processes at play. One can attempt a computational scheme that incorporates coupling of electrons to a laser pump, the coupling of system phonons to a thermal bath, and the Holstein interaction that couples electrons and phonons. We attempt an approximation where the pump induced electronic excitations are modeled by a slowly time varying `electron temperature', $T_{\rm el}(t)$, indicative of a quasi-equilibrium electronic state. We solve the problem for different combinations of $T_{\rm el}$ and $T_{\rm bath}$, probing the order parameter dynamics, the static properties and excitations in the long time `quasi steady state', and establish a `phase diagram' in terms of bath temperature and electron temperature.

Strongly correlated electron systems often display competing or even intertwined ordering tendencies, hinting to extremely close or degenerate many-electron energies. While degeneracies are directly rooted in the underlying symmetries of the problem under investigation, their multifaceted effects on different response functions and their mutual relations often remain elusive. Here we put this subject on a rigorous basis by investigating the degeneracies of charge, spin, and pairing susceptibilities for the unfrustrated, bipartite Hubbard model. Exploiting its pseudospin symmetry, we analytically derive the mutual relations between these response functions for generic spatial modulations, highly relevant, e.g., for the competition of stripe and superconducting orders. By means of two-particle numerical simulations we demonstrate the occurrence of a simultaneous $d$-wave pairing/$d$-density wave (loop current) instability in the vicinity of the metal-insulator transition, driven by short-ranged spin fluctuations for the two-dimensional case. We show how this degeneracy is gradually lifted by geometrical frustration, which favors superconductivity. Our study provides a general tool for revealing symmetry relations in correlated electron systems and establishes a unifying perspective on the nature of their intermingled charge/loop current, pairing, and spin orders.

Near-UV optical absorption is increasingly reported in hydrogen-bonded organic and biomolecular materials lacking aromatic or extended pi-conjugated chromophores, yet its microscopic origin remains unresolved and electronic-structure calculations often overestimate experimental absorption onsets. Here, we combine machine-learned interatomic potentials for large-scale classical and quantum nuclear sampling with periodic excited-state calculations to address this discrepancy in L-pyroglutamine ammonium, an experimentally established glutamine-derived crystal containing a well-resolved short hydrogen bond and exhibiting non-aromatic near-UV optical response. Using controlled in silico ion substitutions that vary the surrounding hydrogen-bond environment while preserving this scaffold, we compute optical spectra from configurations sampled along classical and quantum nuclear trajectories using hybrid-functional time-dependent density functional theory. We show that nuclear quantum effects stabilise proton-sharing configurations that are strongly suppressed classically, redshifting the lowest bright excitations by 0.5-0.8 eV and raising the fraction of configurations with bright excitations below 6 eV from approximately 3% to approximately 30%. Explicit Brillouin-zone sampling provides a further, mechanistically distinct redshift of 0.5-1.1 eV, reflecting modest but significant indirect electronic character. Only when both effects are incorporated does the calculated onset recover the experimental 3.8-4.5 eV range. These results establish quantum proton fluctuations and reciprocal-space convergence as cooperative but physically distinct ingredients required for predictive optical spectroscopy of strongly hydrogen-bonded molecular materials.

Despite their potential similarity between the mosaic Aubry-André (MAA) and AA models, the MAA model allows mobility edges (MEs), whereas the AA model does not. Here we develop a new double quasiperiodic MAA (DMAA) model consisting of one primitive MAA with nonzero even-site potentials and the other modified one with both nonzero odd-site potentials and a tunable amplitude factor, to reveal how localization transitions evolve from MAA to AA models. Interplays and competitions among the extended, critical and localized states arising from superpositions of double quasi-periodic MAA potentials enable new twice and multiple localization-delocalization transitions besides the original single localization transition. Our numerical calculations on inverse participation ratio, normalized participation ratio, fractal dimension and real-space wavefunction distribution confirm such localization features. The continuum model simulations on the experimental polariton modes also yield consistent results and hence validate their experimental feasibility. The constructed DMAA model provides a new framework for studying the localization transition processes between two analogous quasiperiodic models and broadens the understanding of Anderson localization.

Symmetric mass generation is the interaction-induced opening of a fermion gap without spontaneous symmetry breaking. The anomaly-free 3-4-5-0 model of Wang and Wen provides a minimal one-dimensional setting for this phenomenon, but a direct lattice realization faces two obstacles: fermion doubling for local chiral discretizations and perturbative irrelevance of the six-fermion gapping interaction. We address both obstacles. First, we formulate the model on a strictly one-dimensional tangent-fermion lattice, where a nonlocal hopping produces a single chiral branch without a mirror partner while retaining an efficient tensor-network representation. Second, we add a Hubbard-type density-density interaction (Luttinger parameter $K$) that reduces the scaling dimension of the 3-4-5-0 interaction from $5$ to $5K$, making it relevant for $K<2/5$. Density-matrix renormalization group calculations show the opening of an excitation gap in this regime without the appearance of a degenerate ground state, the hallmark of symmetric mass generation.

Vapor-deposited organic glasses can show enhanced kinetic stability relative to liquid-cooled glasses. When such stable glasses of model glassformers are annealed above the glass transition temperature Tg, they lose their thermal stability and transform into the supercooled liquid via constant velocity propagating fronts. In this work, we show that vapor-deposited glasses of an organic semiconductor, N,N-bis(3-methylphenyl)-N,N-diphenylbenzidine (TPD), also transform via propagating fronts. Using spectroscopic ellipsometry and a new high-throughput annealing protocol, we measure transformation front velocities for TPD glasses prepared with substrate temperatures (TSubstrate) from 0.63 to 0.96 Tg, at many different annealing temperatures. We observe that the front velocity varies by over an order of magnitude with TSubstrate, while the activation energy remains constant. Using dielectric spectroscopy, we measure the structural relaxation time of supercooled TPD. We find that the mobility of the liquid and the structure of the glass are independent factors in controlling the thermal stability of TPD films. In comparison to model glassformers, the transformation fronts of TPD have similar velocities and a similar dependence on TSubstrate, suggesting universal behavior. These results may aid in designing active layers in organic electronic devices with improved thermal stability.

Magnonic frequency combs (MFCs) are spectacular phenomena in microwave-driven high-quality magnets. Like the equally spaced prongs in a comb, conventional integer MFCs are sharp resonances with an equal and constant frequency difference. Here we report fractional MFCs in a high-quality magnetic sphere that emerges when adding a low-power, precisely detuned microwave to the main drive that compresses the frequency spacings to a rational fraction of the original comb, generating high-density spectral grids with hundreds of lines. The theoretical analysis finds that parametric three-magnon scattering is the dominant non-linear process that reproduces the observation well. This mechanism is unique to magnets: it does not exist in an optomechanical system, where the Kerr and optical nonlinearities govern comb formation at a much higher power input. Since our platform operates as a frequency “vernier caliper" with much higher sensitivity than integer MFCs, it has application potential in precision metrology.

Anderson localization - the suppression of wave propagation by interference in a sufficiently disordered medium - is one of the most remarkable phenomena in condensed matter physics. These lecture notes provide a pedagogical introduction to Anderson localization on the Bethe lattice, an infinite tree with fixed connectivity and no loops, and one of the few settings in which the transition can be studied exactly. The problem is important for two main reasons. First, the Anderson model on the Bethe lattice provides a mean-field, infinite-dimensional limit in which the transition point and critical behavior can be determined exactly. Second, it provides a simplified description of the high-dimensional configuration spaces of interacting quantum systems, and thus a useful toy model for many-body localization. We focus on the Green's function formalism and the cavity method. After recalling standard diagnostics of localization based on transport, spectral statistics, and eigenfunction moments, we show that they are encoded in the statistics of the Green's functions, and that the full distribution of the local density of states is the natural order parameter of the transition. We derive the exact self-consistent cavity equations on the Bethe lattice, use them to characterize the two phases and critical behavior - including an analytic solution at large connectivity - and describe population-dynamics algorithms that solve them with high precision. We explain how an exponentially large correlation volume and the localized nature of the critical point account for the strong finite-size effects on random regular graphs, which led to a long debate over a possible intermediate non-ergodic phase. Finally, we discuss the mapping onto directed polymers in random media, interpreting localization as a freezing transition dominated by a few rare resonant paths.

The Haldane model represents the minimal lattice-based realization of a Chern insulator, exhibiting a quantized Hall conductance in the absence of Landau levels. Despite its conceptual elegance, the implementation in crystalline solids of the requisite pattern of Peierls phases breaking time-reversal symmetry remains experimentally demanding. In this work, we theoretically investigate the possibility to simulate the Haldane model in ultra-clean GaAs/AlGaAs heterostructures. Our proposal relies on recent experiments in which a high-mobility two-dimensional electron gas is subject to a gate-defined honeycomb electrostatic potential and a laterally periodic magnetic field generated by patterned ferromagnetic structures. The combined electrostatic and magnetic superlattices furnish a viable route to emulate the topological properties of the Haldane model.

We studied the ac conductance of an $n$-InSb quantum well structure using acoustic methods in magnetic fields up to 18 T and at temperatures ranging from 20 to 500 mK. We attribute the unusual magnetic field dependences of surface acoustic wave (SAW) attenuation and velocity observed in the experiment to the presence of a conducting layer parallel to the quantum well in the sample. We successfully separated the contributions from both the quantum well and the shunting layer, enabling the identification of their distinct conduction mechanisms. Furthermore, by employing the coincidence technique, we determined the electron g-factor in the quantum well and investigated its dependence on the degree of spin polarization.

Nanostructures can be designed to absorb light efficiently at resonance despite their subwavelength footprint, but causality and passivity fundamentally limit the bandwidth over which strong absorption can be maintained. Here we derive fundamental absorption-bandwidth limits for passive, causal, linear, and temporally dispersive subwavelength objects by rigorously casting electromagnetic scattering as an equivalent impedance-matching problem. This mapping yields ultimate Bode-Fano-type constraints for optical absorption and provides rational synthesis guidelines for the material dispersion of passive nanoparticles that can approach the bounds. Our results clarify the ultimate limits for broadband light harvesting and dissipation, with implications for solar-energy conversion, photothermal hyperthermia, thermal management, and related nanophotonic technologies.

Previous work has shown that vapor-deposition can prepare organic glasses with extremely high kinetic stabilities and other properties that would be expected from liquid-cooled glasses only after aging for thousands of years or more. However, recent reports have shown that some molecules form vapor-deposited glasses with only limited kinetic stability when prepared using conditions expected to yield a stable glass. In this work, we vapor deposit glasses of 2-ethyl-1-hexanol over a wide range of deposition rates and test several hypotheses for why this molecule does not form highly stable glasses under normal deposition conditions. The kinetic stability of 2-ethyl-1-hexanol glasses is found to be highly dependent on the deposition rate. For deposition at Tsubstrate = 0.90 Tg, the kinetic stability increases by 3 orders of magnitude (as measured by isothermal transformation times) when the deposition rate is decreased from 0.2 nm/s to 0.005 nm/s. We also find that, for the same preparation time, a vapor-deposited glass has much more kinetic stability than an aged liquid-cooled glass. Our results support the hypothesis that the formation of highly stable 2-ethyl-1-hexanol glasses is inhibited by limited surface mobility. We compare our deposition rate experiments to similar ones performed with ethylcyclohexane (which readily forms glasses of high kinetic stability); we estimate that the surface mobility of 2-ethyl-1-hexanol is more than 4 orders of magnitude less than that of ethylcyclohexane at 0.85 Tg.

Active matter comprises particles that extract energy from their local environment and convert it into motion. Although active particles have been miniaturized down to the nanoscale, realizing activity at the fundamentally smaller scale of individual atoms remains an open challenge, where quantum effects become increasingly relevant. Here, we experimentally demonstrate that individual Cs-133 atoms confined in an optical dipole trap extract energy from an ultracold bath of Rb-87 atoms via quantum-mechanical spin interactions and convert it into active motion. We quantitatively reproduce the resulting dynamics using a parameter-free active Langevin model derived from kinetic theory and support it with event-driven Monte Carlo collision simulations. The microscopic origin of activity is identified as quantum spin exchange, which transfers discrete internal spin energy into kinetic motion. Our work establishes a quantum-enabled route to active matter at the fundamental size limit of single atoms and opens perspectives for exploring the interplay of activity, quantum physics, and mesoscopic non-equilibrium thermodynamics.

The dynamics of flexible polymers and chains under follower activity is known to produce diverse nonequilibrium states. A prominent feature of such systems is the emergence of periodic motion arising from the coupling between internal activity and chain conformation. Recently, it has been shown that flexible and extensible chains of active particles exhibit rich dynamical patterns in the overdamped limit, where inertia is negligible. Here, we study the complex dynamics of a flexible and extensible chain of active particles under follower activity when inertia is significant. Using numerical simulations, we quantify the chain dynamics as a function of chain length ($N$), segment mass, and activity. To rationalize the numerical results, we develop theoretical descriptions in the limit of short chains ($N=3$) and long chains ($N \gg 1$). In both these limits, we derive approximate expressions for the bond lengths and bond angles along the contour, which show excellent agreement with the numerical results. In addition, for short chains, we derive the stability conditions for a periodic motion as a function of segment mass and activity. For long chains ($N\gg1$) we identify parameter regime in which the circular, periodic solution becomes structurally unstable. Our theoretical and numerical analysis provides insights into the emergence of ordered and periodic behaviour in active chains.

Motivated by recent experiments on light-matter interacting systems, we investigate a dilute Bose gas and self-bound quantum droplets in a one-dimensional optical lattice coupled to a lossy cavity mode. Using a classical-field approach, we determine the stationary states and collective excitations of this non-equilibrium system. Apart from the usual Bogoliubov modes, we identify a polariton-like gapped excitation, the frequency of which softens as a precursor of the density ordering transition. Moreover, its relaxation time diverges as the critical point is approached, signaling the non-equilibrium nature of this transition. Dynamically, this polariton-like mode can be probed by inducing cavity field fluctuations, which in turn generates spatio-temporal oscillations of both the condensate and droplet states. In the droplet regime, we also analyze the bound modes which bear the characteristics of such non-equilibrium self-bound state. In addition, we uncover solitonic non-equilibrium states, including condensate with kink-like configuration and double-droplet state, and investigate their robustness following a sudden quench. Remarkably, although these states become unstable beyond a critical coupling, they continue to manifest in the dynamics, akin to the scarring phenomena. Our results demonstrate that dissipative cavity coupling provides a versatile route for exploring rich non-equilibrium dynamics of condensates and quantum droplets within experimentally accessible settings.

We report on optical spectroscopic measurements of ultracold and partially condensed 87Rb gases, which show distinct spectral features due to the thermal and the Bose condensed components in the frequency domain. These features are detected in-situ by using a dark-field configuration with single-photon sensitivity and a frequency-agile laser system. To interpret the observed spectra, we develop a model for light propagation through an inhomogeneous atomic cloud. This model enables the extraction of temperature and atom number, which we benchmark against conventional time-of-flight absorption imaging. The spectroscopically obtained cloud parameters show enhanced sensitivity to small thermal fractions in nearly pure condensates. We further compare the spectroscopy in dark-field and bright-field configurations, demonstrating the superior performance of the former. Our results reveal previously unexplored spectral structure in optically dense ultracold gases and establish spectroscopy as a tool for characterizing ultracold systems at very low temperatures.