Microcavity exciton-polaritons, strongly mixed light-matter states, are promising candidates for novel solid-state photonic devices such as classical and quantum simulators, qubits, and topologically protected photonic on-chip circuitry. Although different schemes for creation of confined polariton states and lattices have been proposed, a unified approach for the advanced preparation and dynamic manipulation of individual confined states, as well as for the control of interactions between remote states, is still missing. Here, we introduce a solid-state platform for dynamic polariton control based on the combination of intracavity potential traps, defined by a spatial modulation of the microcavity thickness, with dynamic control via nonlinear polariton-polariton interactions, as well as time and spatially dependent strain fields. We first demonstrate important functionalities of the platform, such as dynamic control of the energy and internal degrees of freedom, as well as the energy matching of neighboring traps. We then show the coherent modulation and symmetry control of a lattice of coupled traps by strain fields, which enables the creation of dynamic states within the band gap of the lattice. Important characteristics of the acoustic modulation are that it maintains the coherence of macroscopic polariton states and does not depend on polariton density, thus being applicable down to the single-polariton level. These functionalities of the strongly nonlinear, tunable, and spatially compact polariton platform are well described by a theoretical framework and represent important steps towards tunable polaritonic devices based on lattices of interacting polariton traps.

Understanding collective electronic states such as superconductivity and charge density waves is pivotal for fundamental science and applications. The layered transition metal dichalcogenide $1T-\mathrm{TiS}{\mathrm{e}}_2$ hosts a unique charge density wave (CDW) phase transition whose origins are still not fully understood. Here, we present ultrafast time- and angle-resolved photoemission spectroscopy (TR-ARPES) measurements complemented by time-resolved reflectivity (TRR) which allows us to establish the contribution of excitonic and electron-phonon interactions to the CDW. We monitor the energy shift of the valence band (VB) and coupling to coherent phonons as a function of laser fluence. The VB shift, directly related to the CDW gap closure, exhibits a markedly slower recovery dynamics at fluences above $F_{\mathrm{th}}=60\mu \mathrm{J}\mathrm{c}{\mathrm{m}}^{-2}$. This observation coincides with a shift in the relative weight of coherently coupled phonons to higher-frequency modes in TRR, suggesting a phonon bottleneck. Using a rate equation model, the emergence of a high-fluence bottleneck is attributed to an abrupt reduction in coupled phonon damping and an increase in exciton dissociation rate linked to the loss of CDW superlattice phonons. Thus, our work establishes the important role of both excitonic and phononic interactions in the CDW phase transition and the advantage of combining complementary femtosecond techniques to understand the complex interactions in quantum materials.

Understanding the ultrafast coupling and relaxation mechanisms between valleys in transition metal dichalcogenide semiconductors is of crucial interest for future valleytronic devices. Recent ultrafast pump-probe experiments showed an unintuitive significant bleaching at the excitonic $B$ transition after optical excitation of the energetically lower excitonic $A$ transition. Here, we present a possible microscopic explanation for this surprising effect. It is based on the joint action of exchange coupling and phonon-mediated thermalization into dark exciton states and does not involve a population of the $B$ exciton. Our work demonstrates how intra- and intervalley coupling on a femtosecond timescale governs the optical valley response of 2D semiconductors.

The effect of an external perpendicular magnetic field on the magnetization and crystallization of magnetic particles trapped in multilayered arrays is investigated theoretically. General ground-state symmetry considerations corroborated by numerical results allow us to develop an exact analytical theory. When dealing with monolayers, the continuous degeneracy of the in-plane dipole orientation is broken and accompanied by a rhombic crystal structure at finite applied magnetic field. On the other hand, bilayer systems are more robust against applied magnetic fields and preserve the in-plane hexagonal lattice symmetry. The onset of disaggregation is predicted as well in both scenarios. Our findings can be experimentally verified in colloidal suspensions (typically ferrofluids or magnetorheological fluids) and provide enlightening hints to further understanding the phase behavior of magnetic multilayers in more conventional molecular and atomic materials such as thin metal films obtained by epitaxy in the presence of an external magnetic field.

Confining light to scales beyond the diffraction limit, quantum plasmonics supplies an ideal platform to explore strong light-matter couplings. The light-induced localized surface plasmons (LSPs) on the metal-dielectric interface acting as a quantum bus have wide potential in quantum information processing; however, the loss nature of light in the metal hinders their application. Here we propose a mechanism to make the reversible energy exchange and the multipartite quantum correlation of a collective of quantum emitters (QEs) mediated by the LSPs persistent. Via investigating the quantized interaction between the QEs and the LSPs supported by a spherical metal nanoparticle, we find that the diverse signatures of the quantized QE-LSP coupling in the steady state, including the complete decay, population trapping, and persistent oscillation, are essentially determined by the different number of bound states formed in the energy spectrum of the QE-LSP system. Enriching our understanding on the light-matter interactions in a lossy medium, our result is instructive in the design of quantum devices using plasmonic nanostructures.

Synchronization, the temporal coordination of coupled oscillators, allows fireflies to flash in unison, neurons to fire collectively, and human crowds to fall in step on the London millenium bridge. Here, we interpret active (or self-propelled) chiral microswimmers with a distribution of intrinsic frequencies as motile oscillators and show that they can synchronize over very large distances, even for local coupling in two dimensions (2D). This opposes canonical nonactive oscillators on static or time-dependent networks, leading to synchronized domains only. A consequence of this activity-induced synchronization is the emergence of a “mutual flocking phase,” where particles of opposite chirality cooperate to form superimposed flocks moving at a relative angle to each other, providing a chiral active matter analogue to the celebrated Toner-Tu phase. The underlying mechanism employs a positive feedback loop involving the two-way coupling between oscillators' phase and self-propulsion and could be exploited as a design principle for synthetic active materials and chiral self-sorting techniques.

We propose a qubit efficient scheme to study ground-state properties of quantum many-body systems on near-term noisy intermediate-scale quantum computers. One can obtain a tensor network representation of the ground state using a number of qubits smaller than the physical degrees of freedom. By increasing the number of qubits, one can exponentially increase the bond dimension of the tensor network variational ansatz on a quantum computer. Moreover, we construct circuits blocks which respect $\mathrm{U}\left(1\right)$ and $\mathrm{SU}\left(2\right)$ symmetries of the physical system and show that they can significantly speed up the training process and alleviate the gradient vanishing problem. To demonstrate the feasibility of the qubit efficient variational quantum eigensolver in a practical setting, we perform first-principles classical simulation of differentiable programming of the circuits. Using only six qubits, one can obtain the ground state of a $4\times 4$ square lattice frustrated Heisenberg model with fidelity over 97%. Arbitrarily-long-range correlations can also be measured on the same circuit after variational optimization.

Multidimensional stochastic integrate-and-fire (IF) models are a standard spike-generator model in studies of firing variability, neural information transmission, and neural network dynamics. Most popular is a version with Gaussian noise and adaptation currents that can be described via Markovian embedding by a set of $d+1$ stochastic differential equations corresponding to a Fokker-Planck equation (FPE) for one voltage and $d$ auxiliary variables. For the specific case $d=1$, we find a set of partial differential equations that govern the stationary probability density, the stationary firing rate, and, central to our study, the spike-train power spectrum. We numerically solve the corresponding equations for various examples by a finite-difference method and compare the resulting spike-train power spectra to those obtained by numerical simulations of the IF models. Examples include leaky IF models driven by either high-pass-filtered (green) or low-pass-filtered (red) noise (surprisingly, already in this case, the Markovian embedding is not unique), white-noise-driven IF models with spike-frequency adaptation (deterministic or stochastic) and models with a bursting mechanism. We extend the framework to general $d$ and study as an example an IF neuron driven by a narrow-band noise (leading to a three-dimensional FPE). The many examples illustrate the validity of our theory but also clearly demonstrate that different forms of colored noise or adaptation entail a rich repertoire of spectral shapes. The framework developed so far provides the theory of the spike statistics of neurons with known sources of noise and adaptation. In the final part, we use our results to develop a theory of spike-train correlations when noise sources are not known but emerge from the nonlinear interactions among neurons in sparse recurrent networks such as found in cortex. In this theory, network input to a single cell is described by a multidimensional Ornstein-Uhlenbeck process with coefficients that are related to the output spike-train power spectrum. This leads to a system of equations which determine the self-consistent spike-train and noise statistics. For a simple example, we find a low-dimensional numerical solution of these equations and verify our framework by simulation results of a large sparse recurrent network of integrate-and-fire neurons.

We investigate the quantum transition to a correlated state of coupled oscillators in the regime where they display period tripling in response to a drive at triple the eigenfrequency. Correlations are formed between the discrete oscillation phases of individual oscillators. The evolution toward the ordered state is accompanied by the transient breaking of the symmetry between seemingly equivalent configurations. We attribute this to the nontrivial geometric phase that characterizes period tripling. We also show that the Wigner distribution of a single damped quantum oscillator can display a minimum at the classically stable zero-amplitude state.

In this paper, we take a fresh look at the long-standing issue of the nature of macroscopic density fluctuations in the grand canonical treatment of the Bose-Einstein condensation (BEC). Exploiting the close analogy between the spherical and mean-spherical models of magnetism with the canonical and grand canonical treatments of the ideal Bose gas, we show that BEC stands for different phenomena in the two ensembles: an ordering transition of the type familiar from ferromagnetism in the canonical ensemble and condensation of fluctuations, i.e., growth of macroscopic fluctuations in a single degree of freedom, without ordering, in the grand canonical case. We further clarify that this is a manifestation of nonequivalence of the ensembles, due to the existence of long-range correlations in the grand canonical one. Our results shed new light on the recent experimental realization of BEC in a photon gas, suggesting that the observed BEC when prepared under grand canonical conditions is an instance of condensation of fluctuations.

We propose a scheme to entangle two magnon modes via the Kerr nonlinear effect when driving the systems far from equilibrium. We consider two macroscopic yttrium iron garnets interacting with a single-mode microcavity through the magnetic dipole coupling. The Kittel mode describing the collective excitations of a large number of spins is excited through the driving cavity with a strong microwave field. We demonstrate how Kerr nonlinearity creates the entangled quantum states between the two macroscopic ferromagnetic samples, when the microcavity is strongly driven by a blue-detuned microwave field. Such quantum entanglement survives at the steady state. Our work offers insights and guidance in designing experiments for observing entanglement in massive ferromagnetic materials. It can also find broad applications in macroscopic quantum effects and magnetic spintronics.

Non-Markovianity may significantly speed up quantum dynamics when the system interacts strongly with an infinite large reservoir, of which the coupling spectrum should be fine-tuned. The potential benefits are evident in many dynamics schemes, especially the continuous-time quantum walk. Difficulty exists, however, in producing closed-form solutions with controllable accuracy against the complexity of memory kernels. Here, we introduce a new multiple-scale perturbation method that works on integro-differential equations for general study of memory effects in dynamical systems. We propose an open-system model in which a continuous-time quantum walk is enclosed in a non-Markovian reservoir, that naturally corresponds to an error correction algorithm scheme. By applying the multiple-scale method we show how emergence of different timescales is related to transition of system dynamics into the non-Markovian regime. We find that up to two long-term modes and two short-term modes exist in regular networks, limited by their intrinsic symmetries. In addition to the effective approximation by our perturbation method on general forms of reservoirs, the speed-up of quantum walks assisted by non-Markovianity is also confirmed, revealing the advantage of reservoir engineering in designing time-sensitive quantum algorithms.

Intracellular bidirectional transport of cargo on microtubule filaments is achieved by the collective action of oppositely directed dynein and kinesin motors. Experiments have found that in certain cases, inhibiting the activity of one type of motor results in an overall decline in the motility of the cellular cargo in both directions. This counterintuitive observation, referred to as the paradox of codependence, is inconsistent with the existing paradigm of a mechanistic tug of war between oppositely directed motors. Unlike kinesin motors, dynein motors exhibit catch bonding, wherein the unbinding rates of these motors decrease with increasing force on them. Incorporating this catch-bonding behavior of dyneins in a theoretical model, we show that the functional divergence of the two motor species manifests itself as an internal regulatory mechanism, and leads to codependent-transport behavior in biologically relevant regimes. Using analytical methods and stochastic simulations, we analyze the processivity characteristics and probability distribution of run times and pause times of transported cellular cargoes. We show that catch bonding can drastically alter the transport characteristics and also provide a plausible resolution of the paradox of codependence.

We investigate how the ionization rate of molecules consisting of a pair of phenyl rings depends on the torsional dihedral angle between these two rings. We consider different orientations of the molecules with respect to the external field direction and find differences in the rates in biphenyl and substituted biphenyl. The evaluation of the rates is made possible by application of the integral representation of the weak-field asymptotic theory for tunneling ionization. We report a strong variation of the ionization rates with dihedral angle with contributions from several orbitals and a large difference between biphenyl and substituted biphenyl related to the permanent dipole of the latter system.

We study the time-evolving currents flowing in an interacting ring-shaped nanostructure after a bias voltage has been switched on. The source-to-drain current exhibits the expected relaxation towards its quasistatic equilibrium value at a rate ${\mathrm{\Gamma }}_0$ reflecting the lead-induced broadening of the ring states. In contrast, the current circulating within the ring decays with a different rate $\mathrm{\Gamma }$, which is a rapidly decaying function of the interaction strength and thus can take values orders of magnitude below ${\mathrm{\Gamma }}_0$. This implies the existence of a regime in which the nanostructure is far from equilibrium even though the transmitted current is already stationary. We discuss experimental setups to observe the long-lived ring transients.

Cooled oil emulsion droplets in aqueous surfactant solution have been observed to flatten into a remarkable host of polygonal shapes with straight edges and sharp corners, but different driving mechanisms—(i) a partial phase transition of the liquid bulk oil into a plastic rotator phase near the droplet interface and (ii) buckling of the interfacially frozen surfactant monolayer enabled by a drastic lowering of surface tension—have been proposed. Here, combining experiment and theory, we analyze the initial stages of the evolution of these “shape-shifting” droplets, during which a polyhedral droplet flattens into a polygonal platelet under cooling and gravity. Using reflected-light microscopy, we reveal how icosahedral droplets evolve through an intermediate octahedral stage to flatten into hexagonal platelets. This behavior is reproduced by a theoretical model of the phase transition mechanism, but the buckling mechanism can only reproduce the flattening if the deformations are driven by buoyancy. This requires surface tension to decrease by several orders of magnitude during cooling and yields bending modulus estimates orders of magnitude below experimental values. The analysis thus shows that the phase transition mechanism underlies the observed “shape-shifting” phenomena.

From a semiclassical perspective, the Bohr-Sommerfeld quantization of the closed cyclotron orbits for charged particles such as electrons in an external magnetic field gives rise to discrete Landau levels and a series of fascinating quantum Hall phenomena. Here, we consider topologically nontrivial physics from a distinct origin, where the cyclotron orbits take nontrivial knotting structures such as a trefoil knot. We present a scenario of a Weyl semimetal with a slab geometry, where the Fermi arcs on the opposing surfaces can cross without interfering with each other and form a knot together with the bulk Weyl nodes, and in an external magnetic field, the resulting chiral Landau levels. We provide a microscopic lattice model to realize a cyclotron orbit with an unconventional geometry of a trefoil knot and study the corresponding quantum oscillations. Interestingly, unlike the conventional ring-shaped cyclotron orbit, a trefoil knot is self-threading, allowing the magnetic field line along the cyclotron orbit to contribute to the overall Berry phase and therefore altering the external magnetic field for each quantization level. The cyclotron orbit knot offers an arena of the nontrivial knot theory in three spatial dimensions and its subsequent physical consequences.

Recent progress in nanoscale optics is driven by the physics of electric and magnetic resonances supported by high-index dielectric nanoparticles. Here, we exploit optical bound states in the continuum in a subwavelength particle enhanced by an engineered substrate undergoing an epsilon-near-zero transition from an insulator to a conductor, and uncover how to boost dramatically high-order parametric nonlinear effects. Our strategy makes feasible an observation of a variety of multistep cascaded and multifrequency nonlinear effects in an individual subwavelength resonator. This would expand substantially the range of applications of Mie-resonant dielectric metaphotonics for highly efficient subwavelength optical circuitry, nonlinear metadevices, ultrasensitive hyperspectral sensing, and quantum nanophotonics.

The first indication of a pseudogap in cuprates came from a sudden decrease of the NMR Knight shift at a doping-dependent temperature $T^{*}\left(\delta \right)$. Since then, experiments have found phase transitions at a lower $T_{\text{phase}}^{*}\left(\delta \right)$. Using plaquette cellular dynamical mean field for the square-lattice Hubbard model at high temperature, where the results are reliable, we show that $T^{*}\left(\delta \right)$ shares many features of $T_{\text{phase}}^{*}\left(\delta \right)$. The remarkable agreement with several experiments, including quantum critical behavior of the electronic specific heat, supports the view that the pseudogap is controlled by a finite-doping extension of the Mott transition. We propose further experimental tests.

Interaction of electromagnetic radiation with time-variant objects is a fundamental problem whose study involves foundational principles of classical electrodynamics. Such study is a necessary preliminary step for delineating the novel research field of linear time-varying metamaterials and metasurfaces. A closer look to the literature, however, reveals that this crucial step has not been addressed and important simplifying assumptions have been made. Before proceeding to studies of linear time-varying metamaterials and metasurfaces with their effective parameters, we need to rigorously describe the electric and magnetic responses of a temporally modulated meta-atom. Here, we introduce a theoretical model which describes a time-variant meta-atom and its interaction with incident electromagnetic waves in time domain. The developed general approach is specialized for a dipole emitter/scatterer loaded with a time-varying reactive element. We confirm the validity of the theoretical model with full-wave simulations. Our study is of major significance also in the area of nanophotonics and nano-optics because the optical properties of all-dielectric and plasmonic nanoparticles can be varied in time in order to achieve intriguing scattering phenomena.