Neurons and other excitable systems can release energy suddenly given a small stimulus. Excitability has recently drawn increasing interest in optics, as it is key to realize all-optical artificial neurons enabling speed-of-light information processing. However, the realization of all-optical excitable units and networks remains challenging. Here we demonstrate how laser-driven optical cavities with memory in their nonlinear response can sustain excitability beyond the constraints of memoryless systems. First we demonstrate different classes of excitability and spiking, and their control in a single cavity with memory. This single-cavity excitability is limited to a narrow range of memory times commensurate with the linear dissipation time. To overcome this limitation, we explore coupled cavities with memory. We demonstrate that this system can exhibit excitability for arbitrarily long memory times, even when the intercavity coupling rate is smaller than the dissipation rate. Our coupled-cavity system also sustains spike trains—a hallmark of neurons—that spontaneously break mirror symmetry. Our predictions can be readily tested in thermo-optical cavities, where thermal dynamics effectively give memory to the nonlinear optical response. The huge separation between thermal and optical timescales in such cavities is promising for the realization of artificial neurons that can self-organize to the edge of a phase transition, like many biological systems do.

Based on the covariant underdamped and overdamped Langevin equations with Stratonovich coupling to multiplicative noises and the associated Fokker-Planck equations on a Riemannian manifold, we present the first law of stochastic thermodynamics on the trajectory level. The corresponding fluctuation theorems are also established, with the total entropy production of the Brownian particle and the heat reservoir playing the role of dissipation function.

Group-equivariant neural networks have emerged as an efficient approach to model complex data, using generalized convolutions that respect the relevant symmetries of a system. These techniques have made advances in both the supervised learning tasks for classification and regression, and the unsupervised tasks to generate new data. However, little work has been done in leveraging the symmetry-aware expressive representations that could be extracted from these approaches. Here, we present holographic-(variational) autoencoder [H-(V)AE], a fully end-to-end SO(3)-equivariant (variational) autoencoder in Fourier space, suitable for unsupervised learning and generation of data distributed around a specified origin in 3D. H-(V)AE is trained to reconstruct the spherical Fourier encoding of data, learning in the process a low-dimensional representation of the data (i.e., a latent space) with a maximally informative rotationally invariant embedding alongside an equivariant frame describing the orientation of the data. We extensively test the performance of H-(V)AE on diverse datasets. We show that the learned latent space efficiently encodes the categorical features of spherical images. Moreover, the low-dimensional representations learned by H-VAE can be used for downstream data-scarce tasks. Specifically, we show that H-(V)AE's latent space can be used to extract compact embeddings for protein structure microenvironments, and when paired with a random forest regressor, it enables state-of-the-art predictions of protein-ligand binding affinity.

Time and frequency transfer plays an important role in the fields of navigation and positioning, remote sensing, and fundamental physics. The performance of long-haul frequency transfer is ultimately limited by accumulated phase and amplitude noise in fiber propagation. In this work we overcome these limitations and demonstrate a frequency transfer system over 3000 km of indoor spooled fibers via repetition-frequency-locked frequency combs, which benefit from the extremely high signal-to-noise ratio. With the help of the digital phase discrimination and compensation method and active power stabilization, the dramatic phase and amplitude variations are sufficiently suppressed, respectively, which results in the residual instability $7.5\times {10}^{-14}$ at 1 s and $1.0\times {10}^{-16}$ at 10 000 s, without optical-electrical-optical recovery by the regeneration system. Our results mark a breakthrough in building large-scale, high-precision synchronization networks.

Integrated photonic systems provide a flexible platform where artificial lattices can be engineered in a reconfigurable fashion. Here, we show that one-dimensional photonic arrays with engineered losses allow the realization of topological excitations stemming from non-Hermiticity and bulk mode criticality. We show that a generalized modulation of the local photonic losses allows the creation of topological modes both in the presence of periodicity and even in the quasiperiodic regime. We demonstrate that a localization transition of all the bulk photonic modes can be engineered in the presence of a quasiperiodic loss modulation, and we further demonstrate that such a transition can be created in the presence of both resonance frequency modulation and loss modulation. We finally address the robustness of this phenomenology to the presence of next to the nearest neighbor couplings and disorder in the emergence of criticality and topological modes. Our results put forward a strategy to engineer topology and criticality solely from engineered losses in a photonic system, establishing a potential platform to study the impact of nonlinearities in topological and critical photonic matter.

We present results of the coefficient of thermal expansion for the frustrated quasi-two-dimensional molecular conductor $\theta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{RbZn}{\left(\mathrm{SCN}\right)}_4$ for temperatures $1.5\le T\le 290$ K. A pronounced first-order phase transition anomaly is observed at the combined charge-order/structural transition at 215 K. Furthermore, clear evidence is found for two separate glasslike transitions at $T_{\mathrm{g}}=90$–100 K and $T_{\mathrm{g}}^{†}=120$–130 K, similar to previous findings for $\theta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{CsZn}{\left(\mathrm{SCN}\right)}_4$ and $\theta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{CsCo}{\left(\mathrm{SCN}\right)}_4$ reported by Thomas et al. [Phys. Rev. B 105, L041114 (2022)], both of which lack the charge-order/structural transition. Our findings indicate that these glasslike transitions are common features for the $\theta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_{2}M{M}^{\prime }{\left(\mathrm{SCN}\right)}_4$ family, with $M=(\mathrm{Rb}, \mathrm{Cs})$ and $M^{\prime }=(\mathrm{Co}, \mathrm{Zn})$, irrespective of the presence or absence of charge order. These results are consistent with our model calculations on the glasslike dynamics associated with the flexible ethylene end groups of the BEDT-TTF molecules for various $\theta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_{2}M{M}^{\prime }{\left(\mathrm{SCN}\right)}_4$ salts, predicting two different conformational glass transitions. Moreover, calculations of the hopping integrals show a substantial degree of dependence on the end groups' conformation, suggesting a significant coupling to the electronic degrees of freedom. Our findings support the possibility that the glassy freezing of the ethylene end groups could drive or enhance glassy charge dynamics.

We present a security proof for variable-length QKD in the Renner framework against IID collective attacks. Our proof can be lifted to coherent attacks using the postselection technique. Our first main result is a theorem to convert a sequence of security proofs for fixed-length protocols satisfying certain conditions to a security proof for a variable-length protocol. This conversion requires no new calculations, does not require any changes to the final key lengths or the amount of error-correction information, and at most doubles the security parameter. Our second main result is the description and security proof of a more general class of variable-length QKD protocols, which does not require characterizing the honest behavior of the channel connecting the users before the execution of the QKD protocol. Instead, these protocols adaptively determine the length of the final key, and the amount of information to be used for error correction, based upon the observations made during the protocol. We apply these results to the qubit BB84 protocol, and show that variable-length implementations lead to higher expected key rates than the fixed-length implementations.

We study various quantum quench processes induced by the Möbius/sine-square deformation of the Hamiltonian in two-dimensional conformal field theories starting from the thermofield double state in the two copies of the Hilbert space. These quantum quenches, some of which are directly related to the operator entanglement of the time-evolution operators, allow us to study scrambling and recovery of quantum information. In particular, under the sine-square deformed time evolution, we show from the time dependence of mutual information that the Bell pairs, initially shared by the subsystems of the two Hilbert spaces, may revive even after the mutual information for small subsystems is completely destroyed by quantum information scrambling dynamics. This mutual information is robust against the strong scrambling dynamics. As a consequence, the steady state has a nonlocal correlation shared not by any of two parties but by three parties. In the holographic dual description, a wormhole connecting the two Hilbert spaces may nonlinearly grow with time during the quantum quenches. We also propose effective pictures that describe the dynamics of mutual information during the time evolution by inhomogeneous Hamiltonians.

We investigate irradiated high-mobility two-dimensional electron systems (2DES) under low or moderated magnetic fields. These systems present microwave-induced magnetoresistance oscillations (MIRO) which, as we demonstrate, reveal the presence of coherent states of the quantum harmonic oscillator. We also show that the principle of minimum uncertainty of coherent states is at the heart of MIRO and zero-resistance states (ZRS). Accordingly, we are able to explain, based on coherent states, important experimental evidence of these photo-oscillations, such as their physical origin, their periodicity with the inverse of the magnetic field and their peculiar oscillations minima and maxima positions in regards of the magnetic field. Thus, remarkably enough, we come to the conclusion that 2DES, under low magnetic fields, become a system of quasiclassical states or coherent states and MIRO would be the smoking gun of the existence of these peculiar states in these systems.

We propose and demonstrate a general method to calibrate the frequency-dependent response of self-compensating noble-gas–alkali-metal comagnetometers to arbitrary spin perturbations. This includes magnetic and nonmagnetic perturbations such as rotations and exotic spin interactions. The method is based on a fit of the magnetic field response to an analytical model. The frequency-dependent response of the comagnetometer to arbitrary spin perturbations can be inferred using the fit parameters. We demonstrate the effectiveness of this method by comparing the inferred rotation response to an experimental measurement of the rotation response. Our results show that experiments relying on zero-frequency calibration of the comagnetometer response can over- or underestimate the comagnetometer sensitivity by orders of magnitude over a wide frequency range. Moreover, this discrepancy accumulates over time as operational parameters tend to drift during comagnetometer operation. The demonstrated calibration protocol enables accurate prediction and control of comagnetometer sensitivity to, for example, ultralight bosonic dark-matter fields coupling to electron or nuclear spins, as well as accurate monitoring and control of the relevant system parameters.

Autoresonant phase locking of the plasma wakefield to the beat frequency of two driving lasers offers advantages over conventional wakefield acceleration methods, since it requires less demanding laser parameters and is robust to variations in the target plasma density. Here, we investigate the kinetic and nonlinear processes that come into play during autoresonant plasma beat-wave acceleration of electrons, their impact on the field amplitude of the accelerating structure, and on acceleration efficiency. Particle-in-cell simulations show that the process depends on the plasma density in a nontrivial way but can be reliably modeled under specific conditions. Beside recovering previous fluid results in the deeply underdense plasma limit, we demonstrate that robust field excitation can be achieved within a fully kinetic self-consistent modeling. By adjusting the laser properties, we can amplify the electric field to the desired level, up to wave breaking, and efficiently accelerate particles; we provide suggestions for optimized laser and plasma parameters. This versatile and efficient acceleration scheme, producing electrons from tens to hundreds of $\mathrm{MeV}$ energies, holds promise for a wide range of applications in research industry and medicine.

Geometry can be used to explain many properties commonly observed in real networks. It is therefore often assumed that real networks, especially those with high average local clustering, live in an underlying hidden geometric space. However, it has been shown that finite-size effects can also induce substantial clustering, even when the coupling to this space is weak or nonexistent. In this paper, we study the weakly geometric regime, where clustering is absent in the thermodynamic limit but present in finite systems. Extending Mercator, a network embedding tool based on the $\mathrm{popularity}\times \mathrm{similarity}$ ${\mathbb{S}}^1/{\mathbb{H}}^2$ static geometric network model, we show that, even when the coupling to the geometric space is weak, geometric information can be recovered from the connectivity alone for networks of any size. The fact that several real networks are best described in this quasigeometric regime suggests that the transition between nongeometric and geometric networks is not a sharp one.

Wave phenomena in bianisotropic media have been broadly scrutinized in classical electrodynamics because these media offer additional degrees of freedom to engineer electromagnetic waves. However, the majority of investigations concerning such systems have so far been limited to stationary (time-invariant) media. Temporally varying the magnetoelectric coupling manifesting bianisotropy engenders a unique prospect to manipulate wave-matter interactions in new ways. In this paper, we theoretically contemplate electromagnetic effects in weakly dispersive bianisotropic media of all classes when the corresponding magnetoelectric coupling parameter suddenly jumps in time, creating a time interface in spatially uniform bianisotropic media. We investigate scattering effects at such time interfaces, revealing novel polarization- and direction-dependent phenomena. Some of these phenomena are validated through the use of simulation software. We anticipate that our work paves the road for further exploration of time-varying bianisotropic metamaterials (metasurfaces) and bianisotropic photonic time crystals, thus opening up interesting possibilities to control wave polarization and amplitude in reciprocal and nonreciprocal manners.

Shock-augmented ignition (SAI) has recently been proposed as a viable approach to optimize the shock ignition scheme of laser fusion, which may substantially reduce laser power and intensity requirements and, as a result, greatly inhibit the growth of laser-plasma instabilities. However, the nonlocal thermal transport effect, which is believed to play a significant role in shock ignition, has not been considered in evaluation of the SAI scheme yet. Here, by self-consistently including modeling of the space-time-dependent nonlocal thermal transport into the radiation hydrodynamic simulations, we revisit the whole implosion and ignition dynamics of SAI numerically and theoretically. We find that, due to the nonlocal effect, on the one hand, the time-delay window between the igniter spike and the compression pulse in the drive laser for achieving high-gain fusion is much broadened from about 0.45 to 0.7 ns, relaxing the difficulty of laser pulse shaping; on the other hand, both the coast time and the fusion burn duration are significantly reduced by, respectively, 0.22 ns and 47 ps, in favor of increasing the hot-spot pressure around the stagnation stage.

The Ising model describes collective behaviors such as phase transitions and critical phenomena in various physical, biological, economical, and social systems. It is well known that spontaneous phase transition at finite temperature does not exist in the Ising model with short-range interactions in one dimension. Yet, little is known about whether this forbidden phase transition can be approached arbitrarily closely—at fixed finite temperature. Here I use symmetry analysis of the transfer matrix to reveal the existence of spontaneous ultranarrow phase crossover (UNPC) at finite temperature in one class of one-dimensional Ising models on decorated two-leg ladders, in which the crossover temperature $T_0$ is determined solely by on-rung interactions and decorations, while the crossover width $2\delta T$ is independently, exponentially reduced ($\delta T=0$ means a genuine phase transition) by on-leg interactions and decorations. These findings establish a simple ideal paradigm for realizing an infinite number of one-dimensional Ising systems with spontaneous UNPC at desirable $T_0$, which would be characterized in routine laboratory measurements as a genuine first-order phase transition with large latent heat thanks to the ultranarrow $\delta T$ (say less than one nanokelvin), paving a way to push the limit in our understanding of phase transitions and the dynamical actions of frustration arbitrarily close to the forbidden regime.

Quantum information processing architectures typically only allow for nearest-neighbor entanglement creation. In many cases, this prevents the direct generation of $\mathrm{GHZ}$ states, which are commonly used for many communication and computation tasks. Here, we show how to obtain $\mathrm{GHZ}$ states between nodes in a network that are connected in a straight line, naturally allowing them to initially share linear cluster states. We prove a strict upper bound of $\lfloor (n+3)/2\rfloor$ on the size of the set of nodes sharing a $\mathrm{GHZ}$ state that can be obtained from a linear cluster state of $n$ qubits, using local Clifford unitaries, local Pauli measurements, and classical communication. Furthermore, we completely characterize all selections of nodes below this threshold that can share a $\mathrm{GHZ}$ state obtained within this setting. Finally, we demonstrate these transformations on the IBMQ Montreal quantum device for linear cluster states of up to $n=19$ qubits.

The control of material loss mechanisms is playing an increasingly important role for improving coherence times of superconducting quantum devices. Such material losses can be characterized through the measurement of planar superconducting resonators, which reflect losses through the resonance's quality factor $Q_l$. The resonance quality factor consists of both internal (material) losses as well as coupling losses when resonance photons escape back into the measurement circuit. The combined losses are then described as $Q_l^{-1}=\mathrm{Re}\left\{Q_c^{-1}\right\}+Q_i^{-1}$, where $Q_c$ and $Q_i$ reflect the coupling and internal quality factors of the resonator, respectively. To separate the relative contributions of $Q_i$ and $Q_c$ to $Q_l$, diameter-correcting circle fits use algebraic or geometric means to fit the resonance signal on the complex plane. However, such circle fits can produce varied results, so to address this issue, we use a combination of simulation and experiment to determine the reliability of a fitting algorithm across a wide range of quality factor values from $Q_i\ll Q_c$ to $Q_c\ll Q_i$. In addition, we develop a measurement protocol that can not only reduce fitting errors by factors $\gtrsim 2$ but also mitigates the influence of the measurement background on the fit results. This technique can be generalized for other resonance systems beyond superconducting resonators.

The formation of dispersive shock waves in one-dimensional Bose gas represents a limitation of Generalized Hydrodynamics (GHD) due to the coarse-grained nature of the theory. Nevertheless, GHD accurately captures long-wavelength behavior, thus indicating an implicit knowledge of the underlying microscopic physics. Such representations are already known through the Whitham modulation theory, where dispersionless equations describe the evolution of the slowly varying shock wave parameters. Here we study the correspondence between Whitham's approach to the Gross-Pitaevskii equation and GHD in the semiclassical limit and beyond. Our findings enable the recovery of the shock wave solution directly from GHD simulations, which we demonstrate for both zero and finite temperature. Additionally, we study how free expansion protocols affect the shock wave density and their implications for experimental detection. The combined picture of Whitham and GHD lends itself to additional physical interpretation regarding the formation of shock waves.

In the extracellular matrix, eukaryotic cells exert forces that deform their surroundings. By doing so, they can perform mechanosensation: Cells measure the mechanics of their environment, and adapt their behavior accordingly. Extracellular matrices are, however, disordered nonlinear media: How can a mechanosensor at the cellular scale reliably measure the surroundings mechanics through local probing? Here, we develop a model for nonlinear mechanosensation in disordered fiber networks. At low forces, the linear response of the matrix combined with its extreme mechanical heterogeneity precludes reliable mechanosensation. In contrast, we find that this heterogeneity is strongly suppressed in the physiologically relevant nonlinear mechanical regime where fibers buckle. Conceptually, nonlinearity increases the range of mechanosensation, thereby enhancing disorder averaging and providing more accurate nonlinear mechanical measurements. We support our model using microrheology experiments and show theoretically that this nonlinear mechanosensation is generic to all fiber networks. This contrasts with the collagen-specific observation that nonlinear macroscopic elastic moduli are independent of network density, which we show to originate from the fiber's constitutive nonlinearity. Together, our theoretical study disentangles the micro- and macrorheological nonlinearities of fiber networks, and shows how mechanosensors such as cells can take advantage of these nonlinearities to robustly measure their mechanical environment despite heterogeneities.