We show how preparation uncertainty relations that are formulated as sums of variances may be tightened by using the quantum Fisher information to quantify quantum fluctuations. We apply this to derive stronger angular momentum uncertainty relations, which in the case of spin-$1/2$ turn into equalities involving the purity. Using an analogy between pure-state decompositions in the Bloch sphere and the moment of inertia of rigid bodies, we identify optimal decompositions that achieve the convex- and concave-roof decomposition of the variance. Finally, we illustrate how these results may be used to identify the classical and quantum limits on phase estimation precision with an unknown rotation axis.

We present several inequalities related to the Robertson-Schrödinger uncertainty relation. In all these inequalities, we consider a decomposition of the density matrix into a mixture of states, and use the fact that the Robertson-Schrödinger uncertainty relation is valid for all these components. By considering a convex roof of the bound, we obtain an alternative derivation of the relation in Fröwis et al. [Phys. Rev. A 92, 012102 (2015)], and we can also list a number of conditions that are needed to saturate the relation. We present a formulation of the Cramér-Rao bound involving the convex roof of the variance. By considering a concave roof of the bound in the Robertson-Schrödinger uncertainty relation over decompositions to mixed states, we obtain an improvement of the Robertson-Schrödinger uncertainty relation. We consider similar techniques for uncertainty relations with three variances. Finally, we present further uncertainty relations that provide lower bounds on the metrological usefulness of bipartite quantum states based on the variances of the canonical position and momentum operators for two-mode continuous variable systems. We show that the violation of well-known entanglement conditions in these systems discussed in Duan et al. [Phys. Rev. Lett. 84, 2722 (2000)] and Simon [Phys. Rev. Lett. 84, 2726 (2000)] implies that the state is more useful metrologically than certain relevant subsets of separable states. We present similar results concerning entanglement conditions with angular momentum operators for spin systems.

The electromagnetic response of topological insulators is governed by axion electrodynamics, which features a topological magnetoelectric term in the Maxwell equations. As a consequence magnetic fields become the source of electric fields and vice versa, a phenomenon that is general for any material exhibiting a linear magnetoelectric effect. Axion electrodynamics has been associated with the possibility to create magnetic monopoles, in particular, by an electrical charge that is screened above the surface of a magnetoelectric material. Here we explicitly solve for the electromagnetic fields in this geometry and show that while vortexlike magnetic screening fields are generated by the electrical charge their divergence is identically zero at every point in space, which implies an absence of induced magnetic monopoles. Nevertheless magnetic image charges can be made explicit in the problem, and even if no bound state with electric charges yielding a dyon arises, a dyonlike angular momentum follows from our analysis. Because of its dependence on the dielectric constant this angular momentum is not quantized, which is consistent with a general argument that precludes magnetic monopoles to be generated in Maxwell magnetoelectrics. We also solve for topologically protected zero modes in the Dirac equation induced by the point charge. Since the induced topological defect on the topological insulator's surface carries an electric charge as a result of the axion term, these zero modes are not self-conjugated.

Charge migration (CM) is the electronic response that immediately follows localized ionization or excitation in a molecule, before the nuclei have time to move. It typically unfolds on subfemtosecond time scales and most often corresponds to dynamics far from equilibrium, involving multielectron interactions in a complex chemical environment. While CM has been documented experimentally and theoretically in multiple organic and inorganic compounds, the general mechanism that regulates it remains unsettled. In this work we use tools from nonlinear dynamics to analyze CM that takes place along the backbone of conjugated hydrocarbons, which we simulate using time-dependent density-functional theory. In this electron-density framework we show that CM modes emerge as attosecond solitons and demonstrate the same type of solitary-wave dynamics in both simplified model systems and full three-dimensional molecular simulations. We show that these attosecond-soliton modes result from a balance between dispersion and nonlinear effects tied to time-dependent multielectron interactions. Our soliton-mode mechanism, and the nonlinear tools we use to analyze it, pave the way for understanding migration dynamics in a broad range of organic molecules. For instance, we demonstrate the opportunities for chemically steering CM via molecular functionalization, which can alter both the initially localized electron perturbation and its subsequent time evolution.

Quantum multicritical points (QMCPs) emerge at the junction of two or more quantum phase transitions due to the interplay of disparate fluctuations, leading to novel universality classes. While quantum critical points have been well characterized, our understanding of QMCPs is much more limited, even though they might be less elusive to study experimentally than quantum critical points. Here, we characterize the QMCP of an interacting heterogeneous quantum system in two and three dimensions, the ferromagnetic random transverse-field Ising model (RTIM). The QMCP of the RTIM emerges due to both geometric and quantum fluctuations, studied here numerically by the strong disorder renormalization group method. The QMCP of the RTIM is found to exhibit ultraslow, activated dynamic scaling, governed by an infinite disorder fixed point. This ensures that the obtained multicritical exponents tend to the exact values at large scales, while also being universal—i.e., independent of the form of disorder—providing a solid theoretical basis for future experiments.

Understanding looping probabilities, including the particular case of ring closure or cyclization, of fluctuating polymers (e.g., DNA) is important in many applications in molecular biology and chemistry. In a continuum limit the configuration of a polymer is a curve in the group $\mathit{SE}$(3) of rigid body displacements, whose energy can be modeled via the Cosserat theory of elastic rods. Cosserat rods are a more detailed version of the classic wormlike-chain (WLC) model, which we show to be more appropriate in short-length scale, or stiff, regimes, where the contributions of extension and shear deformations are not negligible and lead to noteworthy high values for the cyclization probabilities (or $J$-factors). We therefore observe that the Cosserat framework is a candidate for gaining a better understanding of the enhanced cyclization of short DNA molecules reported in various experiments, which is not satisfactorily explained by WLC-type models. Characterizing the stochastic fluctuations about minimizers of the energy by means of Laplace expansions in a (real) path integral formulation, we develop efficient analytical approximations for the two cases of full looping, in which both end-to-end relative translation and rotation are prescribed, and of marginal looping probabilities, where only end-to-end translation is prescribed. For isotropic Cosserat rods, certain looping boundary value problems admit nonisolated families of critical points of the energy due to an associated continuous symmetry. For the first time, taking inspiration from (imaginary) path integral techniques, a quantum mechanical probabilistic treatment of Goldstone modes in statistical rod mechanics sheds light on $J$-factor computations for isotropic rods in the semiclassical context. All the results are achieved exploiting appropriate Jacobi fields arising from Gaussian path integrals and show good agreement when compared with intense Monte Carlo simulations for the target examples.

Natural selection is a general and powerful concept to explain evolutionary processes in biological organisms and to design engineering systems such as genetic algorithms. In conventional biology, it is assumed that changes in an organism occur solely from random mutations, which are independent of its ancestors' experiences. However, there is accumulating evidence that organisms transmit their information to the next generation via epigenetic states or other methods of information transfer. This information may enable descendants to learn adaptive traits without relying on selection and may accelerate evolution when combined with natural selection. The combination of natural selection and individual learning is also becoming important for engineering applications to improve genetic algorithms or reinforcement learning. While acceleration of the evolutionary process by learning has previously been suggested, no theoretical foundation is available to support it. To accelerate evolutionary processes by learning, individuals should be able to learn behaviors to optimize fitness, which is a trait of the population rather than the individual. It has not yet been clarified whether and how individual learning can improve fitness and thereby accelerate evolution. We also lack a methodology to quantify acceleration and thus enable us to understand, verify, and predict the impacts of learning. In this work, we show that agents can accelerate the evolutionary process by ancestral learning, which employs the information transmitted only from their ancestors (ancestral information). We then clarify that acceleration occurs because ancestral information enables agents to estimate the gradient of fitness. Finally, to quantify acceleration, we extend Fisher's fundamental theorem for natural selection to ancestral learning. The extended Fisher's fundamental theorem relates acceleration to the variation of individual fitness, and thus enables a quantitative understanding of when, how, and why learning is beneficial.

We study transport properties of a single electron transistor based on elastic nanotube. Assuming that an external compressive force is applied to the nanotube, we focus on the vicinity of the Euler buckling instability. We demonstrate that in this regime the transport through the transistor is extremely sensitive to elastic disorder. In particular, built-in curvature (random or regular) leads to the “elastic curvature blockade”: appearance of threshold bias voltage in the $I\text{−}V$ curve which can be larger than the Coulomb-blockade-induced one. In the case of a random curvature, an additional plateau in the dependence of the average current on a bias voltage appears.

Understanding phonons in $\alpha \text{−}{\mathrm{RuCl}}_3$ is critical to analyze the controversy around the observation of the half-integer thermal quantum Hall effect. While many studies have focused on the magnetic excitations in $\alpha \text{−}{\mathrm{RuCl}}_3$, its vibrational excitation spectrum has remained relatively unexplored. We investigate the phonon structure of $\alpha \text{−}{\mathrm{RuCl}}_3$ via inelastic neutron-scattering experiments and density-functional-theory calculations. Our results show excellent agreement between experiment and first-principles calculations. After validating our theoretical model, we extrapolate the low-energy phonon properties. We find that the phonons in $\alpha \text{−}{\mathrm{RuCl}}_3$ that either propagate or vibrate in the out-of-plane direction have significantly reduced velocities and therefore have the potential to dominate the observability of the elusive half-integer plateaus in the thermal Hall conductance. In addition, we use low-energy interlayer phonons to resolve the low-temperature stacking structure of our large crystal of $\alpha \text{−}{\mathrm{RuCl}}_3$, which we find to be consistent with that of the $R\overline{3}$ space group, in agreement with neutron diffraction.

We investigate the effect of electron-rotation coupling ($\mathbf{R}-\mathrm{\Omega }$ coupling) on fs UV photodissociation dynamics of aligned diatomic molecules. We consider the showcase of ground-state $\mathrm{Mg}{\mathrm{H}}^{+}$ (${}^1{\mathrm{\Sigma }}^{+}$) pumped by an fs IR pulse, which initiates rotational dynamics leading to field-free molecular alignment. A time-delayed fs UV pulse probes the degree of alignment of the rotational wave packet in the framework of photodissociation spectroscopy. The molecular alignment correlates directly with the angular distribution of the photofragments in the dissociative ${}^1\mathrm{\Pi }$ state, as it is shown in our simulations comparing the cases when the $\mathbf{R}-\mathrm{\Omega }$ coupling is included and ignored. We show how the angular distribution of the photofragment is strongly affected by the $\mathbf{R}-\mathrm{\Omega }$ coupling at various delay times with specific molecular alignment. It was shown that increases of the fs UV pulse intensity and the degree of alignment enhance the effect of $\mathbf{R}-\mathrm{\Omega }$ coupling on the angular distribution of the photofragments. On the contrary, an increase of the initial temperature tends to reduce the effect of $\mathbf{R}-\mathrm{\Omega }$ coupling, which is explained by the fact that such an effect turns smaller as the increasing of magnetic state $|{\mathrm{M}}_0|$ for each initial rotational state ${\mathrm{J}}_0$; furthermore, higher excited rotational state ${\mathrm{J}}_0$ contains more magnetic states ${\mathrm{M}}_0$, and the results have been averaged over all degenerated ${\mathrm{M}}_0$ states.

We report on experimental results on laser-driven proton acceleration using high-intensity laser pulses. We present power law scalings of the maximum proton energy with laser pulse energy and show that the scaling exponent $\xi$ strongly depends on the scale length of the preplasma, which is affected by the temporal intensity contrast. At lower laser intensities, a shortening of the scale length leads to a transition from a square root toward a linear scaling. Above a certain threshold, however, a significant deviation from this scaling is observed. Two-dimensional particle-in-cell simulations show that, in this case, the electric field accelerating the ions is generated earlier and has a higher amplitude. However, since the acceleration process starts earlier as well, the fastest protons outrun the region of highest field strength, ultimately rendering the acceleration less effective. Our investigations thus point to a principle limitation of the proton energy in the target normal sheath acceleration regime, which would explain why a significant increase of the maximum proton energy above the limit of $100\mathrm{MeV}$ has not yet been achieved.

Nothing can physically travel faster than light in vacuum. There are several ways proposed to bypass the light barrier and produce Čerenkov radiation (ČR) in vacuum. In this paper, we theoretically predict ČR in vacuum from a spatiotemporally modulated boundary. We consider the modulation of traveling wave type and apply a uniform electrostatic field on the boundary to generate electric dipoles. Since the induced dipoles stick to the interface, they travel at the modulation speed. When the grating travels faster than light, it emits ČR. In order to quantitatively examine this argument, we need to calculate the field scattered at the boundary. We utilize a dynamical differential method, which we developed in a previous paper, to quantitatively evaluate the field distribution in such a situation. We can confirm that all scattered fields are evanescent if the modulation speed is slower than light while some become propagating if the modulation is faster than light.

A comprehensive model of processes in a discharge capillary is required in order to obtain nominal parameters of a preformed plasma channel suitable for the laser wake-field acceleration. We present three-dimensional magnetohydrodynamics simulations of a hydrogen gas filling process and discharge plasma formation in a short square shaped capillary with gas supply channels. Time evolution of the gas pressure and the plasma density in the capillary channel for a chosen discharge current profile is analyzed. Performed simulations provide distributions of the electric current, the magnetic field, and the electron density along the whole channel, taking into account gas supply areas as well as areas outside of the capillary. Obtained results show that the presence of gas supplies leads to the inhomogeneous plasma density distribution along the capillary channel which has to be taken into account for generating an optimal laser-driven electron beam.

Frustrated magnets with highly degenerate ground states are at the heart of hunting exotic states of matter. Recent studies in spin-orbit coupled honeycomb magnets have generated immense interest in bond-dependent interactions, appreciating a symmetric off-diagonal $\mathrm{\Gamma }$ interaction which exhibits a macroscopic degeneracy in the classical limit. Here, we study a generic spin model and discover a chiral-spin (CS) ordering with spontaneously broken time-reversal symmetry near the dominant $\mathrm{\Gamma }$ region. The CS phase is demonstrated to possess a staggered chirality relation in different sublattices, and it exhibits gapless excitations, as revealed by the vanishing energy gap and the finite central charge on cylinders. Although there is a vestige of a tiny peak in the corner of the second Brillouin zone, the magnetic order is likely to vanish as the system size increases. Finally, we also attempt to gain insight into the possible topological signature of the CS phase by calculating the dynamic structure factor and the modular $\mathcal{S}$ matrix.

We introduce an active matter model composed of sterically interacting particles, which absorb resources from a substrate and move in response to resource gradients. For varied ratios of absorption rate to substrate recovery rate, we find a variety of regimes including periodic waves, partial clustering, stochastic motion, and a frozen state. If passive particles are added, they can form crystalline clusters in an active fluid. This model could be implemented using colloidal systems on feedback landscapes and can provide a soft matter realization of excitable media and ecological systems.

Most of the fundamental characteristics of quantum mechanics, such as nonlocality and contextuality, are manifest in discrete, finite-dimensional systems. However, many quantum information tasks that exploit these properties cannot be directly adapted to continuous variable systems. To access these quantum features, continuous quantum variables can be made discrete by binning together their different values, resulting in observables with a finite number, $d$, of outcomes. While direct measurement indeed confirms their manifestly discrete character, here we employ a salient feature of quantum physics known as mutual unbiasedness to show that such coarse-grained observables are in a sense neither continuous nor discrete. Depending on $d$, the observables can reproduce either the discrete or the continuous behavior, or neither. To illustrate these results, we present an example for the construction of such measurements and employ it in an optical experiment confirming the existence of four mutually unbiased measurements with $d=3$ outcomes in a continuous variable system, surpassing the number of mutually unbiased continuous variable observables.

We report the existence of an enhanced operating regime for a high-frequency, low-pressure capacitively coupled plasma (CCP) discharge in the presence of a weak magnetic field applied parallel to the electrodes. Our particle-in-cell simulations show that the discharge operates at significantly higher plasma density and ion flux when the electron-cyclotron frequency equals half the applied RF frequency at a given voltage. The physical mechanism responsible for this behavior is a resonance between the oscillatory motion of the sheath edge and the electron bounce in the cyclotron motion, which is half of the cyclotron period. Hence we call this resonance the electron bounce cyclotron resonance. In each collision with the sheath the electrons gain a substantial amount of energy eventually sufficient to produce higher ionization near the sheath leading to increase in the ion flux. The effect is observed at a relatively weak magnetic field, about 10G at 60 MHz. The proposed effect can be used for enhancing the operational performance of CCP devices in industrial applications.

We study experimentally and theoretically the temperature dependence of transverse magnetic routing of light emission from hybrid plasmonic-semiconductor quantum well structures where the exciton emission from the quantum well is routed into surface plasmon polaritons propagating along a nearby semiconductor-metal interface. In II-VI and III-V direct-band semiconductors the magnitude of routing is governed by the circular polarization of exciton optical transitions, that is induced by a magnetic field. For structures comprising a (Cd,Mn)Te/(Cd,Mg)Te diluted magnetic semiconductor quantum well we observe a strong directionality of the emission up to $15\%$ at low temperature of $20\mathrm{K}$ and magnetic field of $485\mathrm{mT}$ due to giant Zeeman splitting of holes mediated via the strong exchange interaction with ${\mathrm{Mn}}^{2+}$ ions. For increasing temperatures towards room temperature the magnetic susceptibility decreases and the directionality strongly drops to $4\%$ at about 65 K. We also propose an alternative design based on a nonmagnetic (In,Ga)As/(In,Al)As quantum well structure, suitable for higher temperatures. According to our calculations, such structure can demonstrate emission directionality up to $5\%$ for temperatures below 200 K and moderate magnetic fields of 1 T.

We propose a realistic regime to detect the light-induced topological band gap in graphene via time-resolved angle-resolved photoelectron spectroscopy (trARPES), which can be achieved with current technology. The direct observation of Floquet-Bloch bands in graphene is limited by low-mobility, Fourier-broadening, laser-assisted photoemission (LAPE), probe-pulse energy-resolution bounds, space-charge effects, and more. We characterize a regime of low driving frequency and high amplitude of the circularly polarized light that induces an effective band gap at the Dirac point that exceeds the Floquet zone. This circumvents limitations due to energy resolutions and band broadening. The electron distribution across the Floquet replicas in this limit allows for distinguishing LAPE replicas from Floquet replicas. We derive our results from a dissipative master equation approach that gives access to two-point correlation functions and the electron distribution relevant for trARPES measurements.