Van der Waals interactions are ubiquitous and they play an important role for the stability of materials. Current understanding of this type of coupling is based on linear response theory, while optical nonlinearities are rarely considered in this context. Many materials, however, exhibit strong optical nonlinear response, which prompts further evaluation of dispersive forces beyond linear response. Here we present a discrete coupled nonlinear dipole approach that takes into account linear and nonlinear properties of all dipolar nanoparticles in a given system. This method is based on a Hamiltonian for nonlinear dipoles, which we apply in different systems uncovering a complex interplay of distance, anisotropy, polarizabilities, and hyperpolarizabilities in the vdW energy. This investigation broadens our basic understanding of dispersive interactions, especially in the context of nonlinear materials.

In this work, we numerically demonstrate and classify the intermittently emerged multiscale defect clusters (DCs) composed of few to tens of disclinations, in the two-stage solid-hexatic-liquid melting transition of a two-dimensional Yukawa system. We uncover the topological pathways for their formation and how the crystalline-ordered domains (CODs) with different lattice orientations nearby DCs can be formed, which affect the structural order variations in the transitions. It is found that the six basic processes: the pair generation (I), dissociation (II), scattering (III), propagation (IV), recombination (V), and pair annihilation (VI) of dislocations, each through a single bond-breaking-replacement process, govern DC evolutions. Small DCs composed of antiparallel dislocations through process I by localized particle-shear motion dominate in the solid phase. The further successive combination of processes II to IV, through successive bond-breaking-replacement rotations by stringlike cooperative hopping of several particles, are responsible for the formation of open and ring-shaped stringlike DCs composed of connected dislocations, free dislocations, free disclinations, and CODs enclosed by stringlike DCs, dominated in the hexatic phase. The spreading of neighboring small DCs or the emergence of new small DCs nearby can generate larger DCs with tens of dislocations. The combinations of reversed basic processes can break or diminish a DC. Even though the two-stage transitions can also be signified by the onsets of the rapid rises of the free disclinations and free dislocations successively, the increases of the number and averaged size of multiscale DCs enclosing CODs play the key roles for the successive losses of translational and orientational orders in the two-stage phase transitions.

Driven or self-propelling particles moving in viscoelastic fluids recently emerged as a novel class of active systems showing a complex yet rich set of phenomena due to the non-Newtonian nature of the dispersing medium. Here we investigate the one-dimensional growth of clusters made of active colloidal shakers, which are realized by oscillating magnetic rotors dispersed within a viscoelastic fluid and at different concentrations of the dissolved polymer. These magnetic particles when actuated by an oscillating field display a flow profile similar to that of a shaker force dipole, i.e., without any net propulsion. We design a protocol to assemble clusters of colloidal shakers and induce their controlled expansion into elongated zigzag structures. We observe a power law growth of the mean chain length and use theoretical arguments to explain the measured $1/3$ exponent. These arguments agree well with both experiments and particle based numerical simulations.

The rate of adaptation in theoretical models of biological evolution generally increases with the mutation rate. However, there is a threshold beyond which mutations into lethal states lead to extinction. It would be logical to assume that eliminating such lethal states could be advantageous for evolution. Here, we demonstrate that lethal mutations actually accelerate adaptation on rugged fitness landscapes with multiple peaks and valleys in the presence of competition for resources. We investigate a modified stochastic version of the quasispecies model, incorporating two types of genotypes—viable and lethal—and show that higher rates of lethal mutations result in shorter evolution times towards the best-fit genotype. This phenomenon can be attributed to an increased frequency of traversing fitness valleys, facilitated by reduced selection pressure against less-fit variants.

One-dimensional quantum optical models usually rest on the intuition of large-scale separation or frozen dynamics associated with the different spatial dimensions, for example when studying quasi one-dimensional atomic dynamics, potentially resulting in the violation of $(3+1)$-dimensional Maxwell's theory. Here, we provide a rigorous foundation for this approximation by means of the light-matter interaction. We show how the quantized electromagnetic field can be decomposed—exactly—into an infinite number of subfields living on a lower-dimensional subspace and containing the entirety of the spectrum when studying axially symmetric setups, such as with an optical fiber, a laser beam, or a waveguide. The dimensional reduction approximation then corresponds to a truncation in the number of such subfields that in turn, when considering the interaction with for instance an atom, corresponds to a modification to the atomic spatial profile. We explore under what conditions the standard approach is justified and when corrections are necessary in order to account for the dynamics due to the neglected spatial dimensions. In particular we examine what role vacuum fluctuations and structured laser modes play in the validity of the approximation.

We investigate a system composed of two coupled oscillators subject to stochastic fluctuations in its internal parameters. In particular, we answer the question whether the well-known classical analogy of the quantum dynamics of two-level systems (TLSs), i.e., qubits, provided by two coupled oscillators can be extended to simulate the dynamics of dissipative quantum systems. In the context of nanomechanics, the analogy in the dissipation-free case has already been tested in multiple experimental setups, e.g., doubly clamped or cantilever string resonators and optically levitated particles. A well-known result of this classical analogy is that the relaxation and decoherence times of the analog quantum system must be equal, i.e., $T_1=T_2$, in contrast to the general case of quantum TLSs. We show that this fundamentally quantum feature, i.e., $T_1\ne T_2$, can be implemented as well in the aforementioned classical systems by adding stochastic fluctuations in their internal parameters. Moreover, we show that these stochastic contributions can be engineered in the control apparatus of those systems, discussing, in particular, the application of this theory to levitated nanoparticles and to nanostring resonators.

Cosmological simulations describing the evolution of density perturbations of a self-gravitating collisionless dark matter (DM) fluid in an expanding background provide a powerful tool to follow the formation of cosmic structures over wide dynamic ranges. The most widely adopted approach, based on the $N$-body discretization of the collisionless Vlasov-Poisson (VP) equations, is hampered by an unfavorable scaling when simulating the wide range of scales needed to cover at the same time the formation of single galaxies and of the largest cosmic structures. On the other hand, the dynamics described by the VP equations is limited by the rapid increase of the number of resolution elements (grid points and/or particles) which is required to simulate an ever growing range of scales. Recent studies showed an interesting mapping of the six-dimensional $+$ 1 ($\text{6D}+1$) VP problem into a more amenable $\text{3D}+1$ nonlinear Schrödinger-Poisson (SP) problem for simulating the evolution of DM perturbations. This opens up the possibility of improving the scaling of time propagation simulations using quantum computing. In this paper, we introduce a quantum algorithm for simulating the Schrödinger-Poisson (SP) equation by adapting a variational real-time evolution approach to a self-consistent, nonlinear, problem. To achieve this, we designed a novel set of quantum circuits that establish connections between the solution of the original Poisson equation and the solution of the corresponding time-dependent Schrödinger equation. We also analyzed how nonlinearity impacts the variance of observables. Furthermore, we explored how the spatial resolution behaves as the SP dynamics approaches the classical limit ($\hslash /m\to 0$) and discovered an empirical logarithmic relationship between the required number of qubits and the scale of the SP equation ($\hslash /m$). This entire approach holds the potential to serve as an efficient alternative for solving the Vlasov-Poisson (VP) equation by means of classical algorithms.

We present a universal parameter-free quantum Monte Carlo (QMC) algorithm designed to simulate arbitrary spin-$1/2$ Hamiltonians. To ensure the convergence of the Markov chain to equilibrium for every conceivable case, we devise a clear and simple automated protocol that produces QMC updates that are provably ergodic and satisfy detailed balance. We demonstrate the applicability and versatility of our method by considering several illustrative examples, including the simulation of the XY model on a triangular lattice, the toric code, and random $k$-local Hamiltonians.

Digital-analog quantum computing is a computational paradigm which employs an analog Hamiltonian resource together with single-qubit gates to reach universality. Here, we design a new scheme which employs an arbitrary two-body source Hamiltonian, extending the experimental applicability of this computational paradigm to most quantum platforms. We show that the simulation of an arbitrary two-body target Hamiltonian of $n$ qubits requires $\mathcal{O}\left(n^2\right)$ analog blocks with guaranteed positive times, providing a polynomial advantage compared to the previous scheme. Additionally, we propose a classical strategy which combines a Bayesian optimization with a gradient descent method, improving the performance by $\sim 55\%$ for small systems measured in the Frobenius norm.

To go beyond quantum optics in free-space setups, atom-light interfaces with structured photonic environments are often employed to realize unconventional quantum electrodynamics (QED) phenomena. However, when employed as quantum buses, those long-distance nanostructures are limited by fabrication disorders. In this work, we alternatively propose to realize structured light-matter interactions by engineering multiple coupling points of hybrid giant atom–conventional environments without any periodic structure. We present a general optimization method to obtain the real-space coupling sequence for multiple coupling points. We report a broadband chiral emission for frequency-tunable giant emitters, with no analog in other quantum setups. Moreover, we show that the QED phenomena in the band-gap environment, such as fractional atomic decay and dipole-dipole interactions mediated by a bound state, can be observed in our setup. Numerical results indicate that our proposal is robust against fabrication disorders of the coupling sequence. Our work opens up a route for realizing unconventional light-matter interactions.

The role of quantum fluctuations in modifying the critical behavior of nonequilibrium phase transitions is a fundamental but unsolved question. In this study, we examine the absorbing state phase transition of a 1D chain of qubits undergoing a contact process that involves both coherent and classical dynamics. We adopt a discrete-time quantum model with states that can be described in the stabilizer formalism, and therefore allows for an efficient simulation of large system sizes. The extracted critical exponents indicate that the absorbing state phase transition of this Clifford circuit model belongs to the directed percolation universality class. This suggests that the inclusion of quantum fluctuations does not necessarily alter the critical behavior of nonequilibrium phase transitions of purely classical systems. Finally, we extend our analysis to a non-Clifford circuit model, where a tentative scaling analysis in small systems reveals critical exponents that are also consistent with the directed percolation universality class.

In this paper, we explore the two-dimensional Green's function problem above an instantaneous time-modulated dielectric half-space. In particular, we focus on studying the excitation dynamic with modulation that is carried out near the critical angle and the total internal reflection regime, where an impinging spectrum of waves expects the so-called lateral wave excitation on the interface. We start by analyzing the reflection of a plane wave, with detailed attention given to the difference between two cases: a slow quasistatic modulation versus a relatively fast modulation that leads to substantial excitation of intermodulation frequencies. Next, we provide a space-time spectral representation to the space-domain Green's function, and later, we move to evaluate the spectral integral both in a brute-force numerically exact manner and using integration along the steepest descent path and the branch cuts, which unravel the distinction between different wave phenomena associated with the excitation problem. Thus, we identify different wave species that can be associated with reflected rays and waves that resemble the known head wave in the classical problem of stationary stratified media. We examine the unique time-dependent behavior of each wave species that arises as a consequence of the interface between the time-harmonics of the reflected space-time plane wave spectrum. Lastly, we demonstrate the broader validity of our analytical predictions, also in the case of a dispersive and finite time-modulated medium. To that end, we use a full-wave simulation of a source above a finite time-modulated thick layer that is implemented using a time-varying wire medium. These full-wave results are compared with our current analytical model with effective (homogenized) parameters of the time-modulated wire medium taken from Phys. Rev. Appl. 16, 054003 (2021).

The antiferromagnetic kagome lattice compound FeGe has been revealed to host an emergent charge-density-wave (CDW) state which manifests complex interplay between the spin, charge, and lattice degrees of freedom. Here, we present a comprehensive study of the de Haas–van Alphen effect by measuring torque magnetometry under magnetic fields up to 45.2 T to map Fermi surfaces in this unusual CDW state. For a field along the $c$ direction, we resolve four cyclotron orbits, with the largest one roughly corresponding to the area of the $2\times 2$ folded Brillouin zone. Three smaller orbits are characterized by light effective cyclotron masses in the range of $(0.18–0.30)m_e$. Angle-resolved measurements identify one Fermi surface segment with weak anisotropy. Combined with band structure calculations, our results suggest that features of unfolded Fermi surfaces are robust against CDW reconstruction, corroborating the unconventional effect of a short-ranged CDW on the electronic structure.

Harvesting free energy from the environment is essential for the operation of many biological and artificial systems. We use techniques from stochastic thermodynamics to investigate the maximum rate of harvesting achievable by optimizing a set of reactions in a Markovian system, possibly under various kinds of topological, kinetic, and thermodynamic constraints. This question is relevant for the optimal design of new harvesting devices as well as for quantifying the efficiency of existing systems. We first demonstrate that the maximum harvesting rate can be expressed as a constrained convex optimization problem. We illustrate it on bacteriorhodopsin, a light-driven proton pump from Archaea, which we find is close to optimal under realistic conditions. In our second result, we solve the optimization problem in closed-form in three physically meaningful limiting regimes. These closed-form solutions are illustrated on two idealized models of unicyclic harvesting systems.

We study the energetic efficiency of navigating microswimmers by explicitly taking into account the geometry of their body. We show that, whereas arguments based solely on propulsion efficiency lead one to conclude that needle-like swimmers are most energetically efficient, disk-like swimmers rotated by flow gradients naturally follow time-optimal trajectories. The coupling between body geometry and hydrodynamics thus leads to a generic trade-off between the energetic costs associated with propulsion and navigation, which is accompanied by the selection of a finite optimal aspect ratio. We derive from optimal control theory the steering policy ensuring overall minimum energy dissipation, and characterize how navigation performances vary with the swimmer shape. Our results highlight the important role of the swimmer geometry in realistic navigation scenarios.

Recent studies have revealed a deep connection between the asymmetry of cross-correlations and thermodynamic quantities in the short-time limit. In this study, we address the finite-time domain of the asymmetry for both open classical and quantum systems. Focusing on Markovian dynamics, we show that the asymmetry observed in finite-time cross-correlations is upper bounded by dissipation. We prove that, for classical systems in a steady state with arbitrary operational durations, the asymmetry exhibits, at most, linear growth over time, with the growth speed determined by the rates of entropy production and dynamical activity. In the long-time regime, the asymmetry exhibits exponential decay, with the decay rate determined by the spectral gap of the transition matrix. Remarkably, for quantum cases, quantum coherence is equally important as dissipation in constraining the asymmetry of correlations. We demonstrate an example where only quantum coherence bounds the asymmetry while the entropy production rate vanishes. Furthermore, we generalize the short-time bounds on correlation asymmetry, as reported by Shiraishi [Phys. Rev. E 108, L042103 (2023)] and Ohga et al. [Phys. Rev. Lett. 131, 077101 (2023)], to encompass finite-time scenarios. These findings offer insights into the thermodynamic aspects of correlation asymmetry.

Spatiotemporal control encompasses a variety of techniques for producing laser pulses with dynamic intensity peaks that move independently of the group velocity. This controlled motion of the intensity peak offers a new approach to optimizing laser-based applications and enhancing signatures of fundamental phenomena. Here, we demonstrate spatiotemporal control with a plasma optic. A chirped laser pulse focused by a plasma lens exhibits a moving focal point, or “flying focus,” that can travel at an arbitrary, predetermined velocity. Unlike currently used conventional or adaptive optics, a plasma lens can be located close to the interaction region and can operate at an orders of magnitude higher, near-relativistic intensity.

Classical optimization problems can be solved by adiabatically preparing the ground state of a quantum Hamiltonian that encodes the problem. The performance of this approach is determined by the smallest gap encountered during the evolution. Here, we consider the maximum independent set problem, which can be efficiently encoded in the Hamiltonian describing a Rydberg atom array. We present a general construction of instances of the problem for which the minimum gap decays superexponentially with system size, implying a superexponentially large time to solution via adiabatic evolution. The small gap arises from locally independent choices which cause the system to initially evolve and localize into a configuration far from the solution in terms of Hamming distance. We investigate remedies to this problem. Specifically, we show that quantum quenches in these models can exhibit signatures of quantum many-body scars, which in turn, can circumvent the superexponential gaps. By quenching from a suboptimal configuration, states with a larger ground-state overlap can be prepared, illustrating the utility of quantum quenches as an algorithmic tool.