Localized surface plasmon and Klein tunneling resonances are two phenomena that were previously thought to be unrelated, where the former plays an important role in subwavelength optics while the latter is fundamental to relativistic quantum mechanics and physics of Dirac materials. We develop a rigorous theory for spin-1 Dirac-Weyl particles, which establishes a striking analogy between the two phenomena and unveils a deep connection between the distinct physical contexts, paving the way for gate-controlled surface plasmon mimetic electronics as well as for realizing localized spoof plasmons in a scope wider than previously thought. A possible experimental scheme is articulated.

We describe and benchmark a method to accurately calculate the quantum droplet states that can be produced from a dipolar Bose-Einstein condensate. Our approach also allows us to consider vortex states, where the atoms circulate around the long-axis of the filament-shaped droplet. We apply our approach to determine a phase diagram showing where self-bound droplets are stable against evaporation, and to quantify the energetics related to the fission of a vortex droplet into two nonvortex droplets.

One-particle reduced density matrix functional theory would potentially be the ideal approach for describing Bose-Einstein condensates. It namely replaces the macroscopically complex wave function by the simple one-particle reduced density matrix, and therefore provides direct access to the degree of condensation and still recovers quantum correlations in an exact manner. We initiate and establish this theory by deriving the respective universal functional $\mathcal{F}$ for homogeneous Bose-Einstein condensates with arbitrary pair interaction. Most importantly, the successful derivation necessitates a particle-number conserving modification of Bogoliubov theory and a solution of the common phase dilemma of functional theories. We then illustrate this approach in several bosonic systems such as homogeneous Bose gases and the Bose-Hubbard model. Remarkably, the general form of $\mathcal{F}$ reveals the existence of a universal Bose-Einstein condensation force which provides an alternative and more fundamental explanation for quantum depletion.

Driving an inertial many-body system out of equilibrium generates complex dynamics due to memory effects and the intricate relationships between the external driving force, internal forces, and transport effects. Understanding the underlying physics is challenging and often requires carrying out case-by-case analysis. To systematically study the interplay between all types of forces that contribute to the dynamics, a method to generate prescribed flow patterns could be of great help. We develop a custom flow method to numerically construct the external force field required to obtain the desired time evolution of an inertial many-body system, as prescribed by its one-body current and density profiles. We validate the custom flow method in a Newtonian system of purely repulsive particles by creating a slow-motion dynamics of an out-of-equilibrium process and by prescribing the full time evolution between two distinct equilibrium states. The method can also be used with thermostat algorithms to control the temperature.

The onset of polar flocking in active matter is discontinuous, akin to gas-liquid phase transitions, except that the steady state exhibits microphase separation into polar clusters. While these features have been observed in theoretical models and experiments, little is known about the underlying mesoscopic processes at the cluster level. Here we show that emergence and maintenance of polar order are governed by the interplay between the assembly and disassembly dynamics of clusters with varying size and degree of polar order. Using agent-based simulations of propelled filaments in a parameter regime relevant for actomyosin motility assays, we monitor the temporal evolution of cluster statistics and the transport processes of filaments between clusters. We find that, over a broad parameter range, the emergence of order is determined by nucleation and growth of polar clusters, where the nucleation threshold depends not only on the cluster size but also on its polar moment. Growth involves cluster self-replication, and polar order is established by cluster growth and fragmentation. Maintenance of the microphase-separated, polar-ordered state results from a cyclic dynamics in cluster size and order, driven by an interplay between cluster nucleation, coagulation, fragmentation, and evaporation of single filaments. These findings are corroborated by a kinetic model for the cluster dynamics that includes these elementary cluster-level processes. It consistently reproduces the cluster statistics as well as the cyclic turnover from disordered to ordered clusters and back. Such cyclic kinetic processes could represent a general mechanism for the maintenance of order in active matter systems.

The study of free-space quantum communications requires tools from quantum information theory, optics, and turbulence theory. Here we combine these tools to bound the ultimate rates for key and entanglement distribution through a free-space link, where the propagation of quantum systems is generally affected by diffraction, atmospheric extinction, turbulence, pointing errors, and background noise. Besides establishing ultimate limits, we also show that the composable secret-key rate achievable by a suitable (pilot-guided and postselected) coherent-state protocol is sufficiently close to these limits, therefore showing the suitability of free-space channels for high-rate quantum key distribution. Our paper provides analytical tools for assessing the composable finite-size security of coherent-state protocols in general conditions from the standard assumption of a stable communication channel (as is typical in fiber-based connections) to the more challenging scenario of a fading channel (as is typical in free-space links).

By means of first-principles calculations and crystal structure searching techniques, we predict that a different NaCaBi family crystallized into the ZrBeSi-type structure (i.e., $P{6}_3/mmc$) are strong topological insulators (STIs). Taking $P{6}_3/mmc$ NaCaBi as an example, the calculated band structure indicates that there is a band inversion between two opposite-parity bands at the $\mathrm{\Gamma }$ point. In contrast to the well-known ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ family, the band inversion in the NaCaBi family has already occurred even without spin-orbit coupling (SOC), giving rise to a nodal ring surrounding $\mathrm{\Gamma }$ in the $k_z=0$ plane (protected by $M_z$ symmetry). With time-reversal symmetry $\left(\mathcal{T}\right)$ and inversion symmetry $\left(\mathcal{I}\right)$, the spinless nodal-line metallic phase protected by ${\left[\mathcal{TI}\right]}^2=1$ is the weak-SOC limit of the spinful topological insulating phase. Upon including SOC, the nodal ring is gapped, driving the system into a STI. Besides inversion symmetry, the nontrivial topology of NaCaBi can also be indicated by $\overline{6}$ symmetry. More surprisingly, the SOC-induced band gap in NaCaBi is about 0.34 eV, which is larger than the energy scale of room temperature. Four other compounds (KBaBi, KSrBi, RbBaBi, and RbSrBi) in the family are stable at ambient pressure, both in thermodynamics and lattice dynamics, even though their gaps are smaller than that of NaCaBi. Thus, they provide good platforms to study topological states both in theory and experiments.

We put forward the concept of an optomagnonic crystal: a periodically patterned structure at the microscale based on a magnetic dielectric, which can co-localize magnon and photon modes. The co-localization in small volumes can result in large values of the photon-magnon coupling at the single quanta level, which opens perspectives for quantum information processing and quantum conversion schemes with these systems. We study theoretically a simple geometry consisting of a one-dimensional array of holes with an abrupt defect, considering the ferrimagnet yttrium iron garnet (YIG) as the basis material. We show that both magnon and photon modes can be localized at the defect, and use symmetry arguments to select an optimal pair of modes in order to maximize the coupling. We show that an optomagnonic coupling in the kHz range is achievable in this geometry, and discuss possible optimization routes in order to improve both coupling strengths and optical losses.

The Edelstein effect produces a homogeneous magnetization in nonmagnetic materials with broken inversion symmetry which is generated and tuned exclusively electrically. Often the spin Edelstein effect—that is, a spin density in response to an applied electric field—is considered. In this paper we report on the electrically induced magnetization that comprises contributions from the spin and the orbital moments. Our theory for these spin and orbital Edelstein effects is applied to the topologically nontrivial two-dimensional electron gas at ${\mathrm{SrTiO}}_3$ interfaces. In this particular system the orbital Edelstein effect exceeds the spin Edelstein effect by more than one order of magnitude. This finding is explained mainly by orbital moments of different magnitude in the Rashba-like split band pairs, while the spin moments are of almost equal magnitude.

Finite key analysis of quantum key distribution (QKD) is an important tool for any QKD implementation. While much work has been done on the framework of finite key analysis, the application to individual protocols often relies on the specific protocol being simple or highly symmetric as well as represented in small finite-dimensional Hilbert spaces. In this work, we extend our pre-existing reliable, efficient, tight, and generic numerical method for calculating the asymptotic key rate of device-dependent QKD protocols in finite-dimensional Hilbert spaces to the finite key regime using the security analysis framework of Renner. We explain how this extension preserves the reliability, efficiency, and tightness of the asymptotic method. We then explore examples which illustrate both the generality of our method as well as the importance of parameter estimation and data processing within the framework.

We derive a thermodynamic uncertainty relation (TUR) for systems with unidirectional transitions. The uncertainty relation involves a mixture of thermodynamic and dynamic terms, namely the entropy production from bidirectional transitions, and the net flux of unidirectional transitions. The derivation does not assume a steady state, and the results apply equally well to transient processes with arbitrary initial conditions. As every bidirectional transition can also be seen as a pair of separate unidirectional ones, our approach is equipped with an inherent degree of freedom. Thus, for any given system, an ensemble of valid TURs can be derived. However, we find that choosing a representation that best matches the system's dynamics over the observation time will yield a TUR with a tighter bound on fluctuations. More precisely, we show that a bidirectional representation should be replaced by a unidirectional one when the entropy production associated with the transitions between two states is larger than the sum of the net fluxes between them. Thus, in addition to offering TURs for systems in which such relations were previously unavailable, the results presented herein also provide a systematic method to improve TUR bounds via physically motivated replacement of bidirectional transitions with pairs of unidirectional transitions. The power of our approach and its implementation are demonstrated on a model for a random walk with stochastic resetting and on the Michaelis-Menten model of enzymatic catalysis.

Recent progress in optically trapped ultracold atomic gases is now making it possible to access microscopic observables in doped Mott insulators, which are the parent states of high-temperature superconductors. This makes it possible to address longstanding questions about the temperature scales at which attraction between charge carriers are present, and their mechanism. Controllable theoretical results for this problem are not available at low temperature due to the sign problem. In this work, we make important progress with this problem by employing worm-algorithm Monte Carlo, which allows us to obtain completely unbiased results for two charge carriers in a Mott insulator. Our method gives access to lower temperatures than what is currently possible in experiments, and provides evidence for attraction between dopants at a temperature scale that is now feasible in ultracold atomic systems. We also report on spin correlations in the presence of charge carriers, which are directly comparable to experiments.

${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$, a quasi-one-dimensional (1D) crystal, shows a characteristic temperature-driven metal-insulator phase transition. Above the charge density wave (CDW) temperature $T_c$, ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$ has been predicted to harbor a Weyl semimetal phase. Below $T_c$, it becomes an axion insulator. Here, we performed angle-resolved photoemission spectroscopy measurements on the (110) surface of ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$ and observed two sets of Dirac-like energy bands in the first Brillouin zone, which agree well with our first-principles calculations. Moreover, we found that each Dirac band exhibits an energy splitting of hundreds of millielectron volts under certain circumstances. In combination with core level measurements, our theoretical analysis showed that this Dirac band splitting is a result of surface charge polarization due to the loss of surface iodine atoms. Our findings here shed light on the interplay between band topology and CDW order in Peierls compounds and will motivate more studies on topological properties of strongly correlated quasi-1D materials.

Ab initio simulations of a range of interferometric experiments are used to identify a strong dependence on multiphoton phase shifts in above-threshold ionization (ATI). A simple rule of thumb for interaction phase shifts is derived to explain both the conservation of photoelectron yield and its absolute CEP dependence. For instance, it is found that interferometric (ATI) experiments are shifted by $\pi /4$ relative to RABBIT experiments, and that there is no RABBIT term in a laser-assisted photoionization experiment with odd and even harmonics. Thus, our work helps to resolve the issues of interpretation of quantum dynamics in attosecond and free-electron laser sciences.

Due to the lack of full rotational symmetry in condensed matter physics, solids exhibit new excitations beyond Dirac and Weyl fermions, of which the sixfold excitations have attracted considerable interest owing to the presence of maximum degeneracy in bosonic systems. Here, we propose that a single linear dispersive sixfold excitation can be found in the electride ${\mathrm{Li}}_{12}{\mathrm{Mg}}_3{\mathrm{Si}}_4$ and its derivatives. The sixfold excitation is formed by the floating bands of elementary band representation $A@12a$ originating from the excess electrons centered at the vacancies (i.e., the $12a$ Wyckoff sites). There exists a unique topological bulk-surface-edge correspondence for the spinless sixfold excitation, resulting in trivial surface “Fermi arcs” but topological hinge arcs. All gapped $k_z$ slices belong to a two-dimensional higher-order topological insulating phase, which is protected by a combined symmetry $\mathcal{T}{\tilde{S}}_{4z}$ and characterized by a quantized fractional corner charge $Q_{\text{corner}}=\frac{3\left|e\right|}{4}$. Consequently, the hinge arcs are obtained in the hinge spectra of the ${\tilde{S}}_{4z}$-symmetric rod structure. The state with a single sixfold excitation, stabilized by both nonsymmorphic crystalline symmetries and time-reversal symmetry, is located at the phase boundary and can be driven into various topologically distinct phases by explicit breaking of symmetries, making these electrides promising platforms for the systematic studies of different topological phases.

The planar Hall effect (PHE), wherein a rotating magnetic field in the plane of a sample induces oscillating transverse voltage, has recently garnered attention in a wide range of topological metals and insulators. The observed twofold oscillations in ${\rho }_{yx}$ as the magnetic field completes one rotation are the result of chiral, orbital, and/or spin effects. The antiperovskites $A_{3}B\mathrm{O}$ ($A=\mathrm{Ca}$, Sr, Ba; $B=\mathrm{Sn}$, Pb) are topological crystalline insulators whose low-energy excitations are described by a generalized Dirac equation for fermions with total angular momentum $J=\frac{3}{2}$. We report unusual sixfold oscillations in the PHE of ${\mathrm{Sr}}_3\mathrm{Sn}\mathrm{O}$, which persisted nearly up to room temperature. Multiple harmonics (twofold, fourfold, and sixfold), which exhibited distinct field and temperature dependencies, were detected in ${\rho }_{xx}$ and ${\rho }_{yx}$. These observations are more diverse than those in other Dirac and Weyl semimetals and point to a richer interplay of microscopic processes underlying the PHE in the antiperovskites.

During the last decades, the study of cities has been transformed by new approaches combining engineering and complexity sciences. Network theory is playing a central role, facilitating the quantitative analysis of crucial urban dynamics, such as mobility, city growth, or urban planning. In this work we focus on the spatial aspects of congestion. Analyzing a large amount of real city networks, we show that the location of the onset of congestion changes according to the considered urban area, defining, in turn, a set of congestion regimes separated by abrupt transitions. To help unveiling this spatial dependencies of congestion (in terms of network betweenness analysis), we introduce a family of planar road network models composed by a dense urban center connected to an arboreal periphery. These models, coined as GT and DT-MST models, allow us to analytically, numerically, and experimentally describe how and why congestion emerges in particular geographical areas of monocentric cities and, subsequently, to describe the congestion regimes and the factors that promote the appearance of their abrupt transitions. We show that the fundamental ingredient behind the observed abrupt transitions is the spatial separation between the urban center and the periphery, and the number of separated areas that form the periphery. Elaborating on the implications of our results, we show that they may have influence in the design and optimization of road networks regarding urban growth and the management of daily traffic dynamics.

Hair cells of the auditory and vestibular systems display astonishing sensitivity, frequency selectivity, and temporal resolution to external signals. These specialized cells utilize an internal active amplifier to achieve highly sensitive mechanical detection. One of the manifestations of this active process is the occurrence of spontaneous limit-cycle motion of the hair-cell bundle. As hair bundles under in vivo conditions are typically coupled to each other by overlying structures, we explore the role of this coupling on the dynamics of the system, using a combination of theoretical and experimental approaches. Our numerical model suggests that the presence of chaotic dynamics in the response of individual bundles enhances their ability to synchronize when coupled, resulting in significant improvement in the system's ability to detect weak signals. This synchronization persists even for a large frequency dispersion and when tens of oscillators comprise the system. Further, the amplitude and coherence of the active motion are not reduced upon increasing the number of oscillators. Using artificial membranes, we impose mechanical coupling on groups of live and functional hair bundles, selected from in vitro preparations of the sensory epithelium, allowing us to explore the role of coupling experimentally. Consistent with the numerical simulations of the chaotic system, synchronization occurs even for large frequency dispersion and a large number of hair cells. Further, the amplitude and coherence of the spontaneous oscillations are independent of the number of hair cells in the network. We therefore propose that hair cells utilize their chaotic dynamics to stabilize the synchronized state and avoid the amplitude death regime, resulting in collective coherent motion that could play a role in generating spontaneous otoacoustic emissions and an enhanced ability to detect weak signals.

We study the stability of a quantum droplet and its associated phase transitions in ultracold Bose-Bose mixtures uniformly confined in quasi-two-dimensions with a periodic boundary condition. We show that the confinement-induced boundary effect can be significant when increasing the atom number or reducing the confinement length, which destabilizes the quantum droplet towards the formation of a soliton bound state that has no density modulation along the confined direction. In particular, as increasing the atom number we find the soliton reentrance, while the droplet is stabilized only within a finite number window that sensitively depends on the confinement length. Near the droplet-soliton transitions, they can coexist with each other as two local minima in the energy landscape. Finally, we map out the phase diagram for the droplet-soliton transition and coexistence in the parameter plane of the atom number and confinement length for ${}^{39}\mathrm{K}$ boson mixtures.