Motivated by the recent prediction of anisotropy in piezoresistance of ballistic graphene along longitudinal and transverse directions, we investigate the angular gauge factor of graphene in the ballistic and diffusive regimes using highly efficient quantum transport models. It is shown that the angular gauge factor in both ballistic and diffusive graphene between $0^{\circ }$ to ${90}^{\circ }$ bears a sinusoidal relation with a periodicity of $\pi$ due to the reduction of sixfold symmetry into a twofold symmetry as a result of applied strain. The angular gauge factor is zero at critical angles ${20}^{\circ }$ and ${56}^{\circ }$ in ballistic and diffusive regimes, respectively. Based on these findings, we propose a graphene-based ballistic nanosensor which can be used as a reference piezoresistor in a Wheatstone bridge readout technique. The reference sensors proposed here are unsusceptible to inherent residual strain present in strain sensors and unwanted strain generated by the vapors in explosives detection. The theoretical models developed in this paper can be applied to explore similar applications in other two-dimensional Dirac materials. The proposals made here potentially pave the way for implementation of nanoelectromechanical strain sensors based on the principle of ballistic transport, which will eventually replace conventional microelectromechanical piezoresistance sensors with a decrease in feature size. The presence of strain-insensitive “critical angle” in graphene may be useful in flexible wearable electronics also.

The classical methods of identifying significant slow components (modes) in a strongly fluctuating signal usually require strict stationarity. A notable exception is the procedure called empirical mode decomposition (EMD), which is designed to work well for nonstationary and nonlinear (quasiperiodic) time series. However, EMD has some well-known limitations such as the end divergence effect, mode mixing, and the general problem of interpreting the modes. Methods to overcome these limitations, such as ensemble EMD or complete ensemble EMD with adaptive noise, promise an exact reconstruction of the original signal and a better spectral separation of the intrinsic mode functions (IMFs). All these variants share the feature that the decomposition runs from the top to the bottom: The first few IMFs represent the noise contribution and the last is a long-term trend. Here we propose a decomposition from the bottom to the top, by the introduction of smoothness-controlled cubic spline fits. The key tool is a systematic scan by cubic spline fits with an input parameter controlling the smoothness, essentially the number of knots. Regression qualities are evaluated by the usual coefficient of determination $R^2$, which grows monotonically when the number of knots increases. In contrast, the growth rate of $R^2$ is not monotonic: When an essential slow mode is approached, the growth rate exhibits a local minimum. We demonstrate that this behavior provides an optimal tool to identify strongly quasiperiodic slow modes in nonstationary signals. We illustrate the capability of our method by reconstruction of a synthetic signal composed of a chirp, a strong nonlinear background, and a large-amplitude additive noise, where all EMD-based algorithms fail spectacularly. As a practical application, we identify essential slow modes in daily ice extent anomalies at both the Arctic and the Antarctic. Our analysis demonstrates the distinct freezing-melting dynamics on the two poles, where apparently different factors are determining the time evolution of ice sheets. Thus, we believe that our methodology offers a competitive tool to identify modes in strongly fluctuating data and advances significantly the state of the art regarding the decomposition of nonlinear time series.

We reanalyze gravitational waves from binary-neutron-star mergers GW170817 and GW190425 using a numerical-relativity (NR) calibrated waveform model, the TF2+_KyotoTidal model, which includes nonlinear tidal terms. For GW170817, by imposing a uniform prior on the binary tidal deformability $\tilde{\mathrm{\Lambda }}$, the symmetric $90\%$ credible interval of $\tilde{\mathrm{\Lambda }}$ is estimated to be ${481}_{-359}^{+436}$ and ${402}_{-279}^{+465}$ for the case of $f_{\max }=1000$ and $2048\mathrm{Hz}$, respectively, where $f_{\max }$ is the maximum frequency in the analysis. We also reanalyze the event with other waveform models: two post-Newtonian waveform models (TF2_PNTidal and TF2+_PNTidal), the TF2+_NRTidal model that is another NR calibrated waveform model, and its upgrade, the TF2+_NRTidalv2 model. While estimates of parameters other than $\tilde{\mathrm{\Lambda }}$ are broadly consistent among various waveform models, our results indicate that estimates of $\tilde{\mathrm{\Lambda }}$ depend on waveform models. However, the difference is smaller than the statistical error. For GW190425, we can only obtain little information on the binary tidal deformability. The systematic difference among the NR calibrated waveform models will become significant to measure $\tilde{\mathrm{\Lambda }}$ as the number of detectors and events increase and sensitivities of detectors are improved.

In an oscillatory medium, a region which oscillates faster than its surroundings can act as a source of outgoing waves. Such pacemaker-generated waves can synchronize the whole medium and are present in many physical and biological systems, where they are a means of transmitting information. Through numerical simulations, we quantify how the properties of the pacemaker and the underlying limit cycle determine the wave speed, as well as the speed with which they overtake the medium. We compare oscillators based on two of the main mechanisms that generate oscillations in biochemical systems: bistability and time delay. We show that these mechanisms produce oscillations whose wave propagation properties differ markedly. While both types of oscillatory media admit waves that propagate linearly outwards, the dependence of the wave speed on a timescale separation parameter is different between the two. If timescale separation is lost, waves no longer spread linearly. Finally, we quantify the effects of pacemaker size, the frequency difference with the surroundings, and the diffusion strength.

Infrared femtosecond laser pulses are important tools both in strong-field physics, driving x-ray high-harmonic generation, and as the basis for widely tunable, if inefficient, ultrafast sources in the visible and ultraviolet. Although anomalous material dispersion simplifies compression to few-cycle pulses, attosecond pulses in the infrared have remained out of reach. We demonstrate soliton self-compression of 1800-nm laser pulses in hollow capillary fibers to subcycle envelope duration (2 fs) with 27-GW peak power, corresponding to attosecond field transients. In the same system, we generate wavelength-tunable few-femtosecond pulses from the ultraviolet (300 nm) to the infrared (740 nm) with energy up to $25\mu \mathrm{J}$ and efficiency up to 12%, and experimentally characterize the generation dynamics in the time-frequency domain. A compact second stage generates multi-microjoule pulses from 210 to 700 nm using less than $200\mu \mathrm{J}$ of input energy. Our results significantly expand the toolkit available to ultrafast science.

Entanglement has become central for the characterization of quantum matter both in and out of equilibrium. In a dynamic context, entanglement exhibits universal linear temporal growth in generic systems, which stems from the underlying linear light cones as they occur in planar geometries. Inhomogeneous space-times can lead, however, to strongly bent trajectories. While such bent trajectories crucially impact correlation spreading and therefore the light-cone structure, it has remained elusive how this influences the entanglement dynamics. In this work, we investigate the real-time evolution of the entanglement entropy in one-dimensional quantum systems after quenches that change the underlying space-time background of the Hamiltonian. Concretely, we focus on the Rindler space describing the space-time in close vicinity to a black hole. As a main result, we find that entanglement grows sublinearly in a generic fashion both for interacting and noninteracting quantum matter. We further observe that the asymptotic relaxation becomes exponential, as opposed to algebraic for planar Minkowski space-times, and in the vicinity of the black hole, the relaxation time for large subsystems becomes independent of the subsystem size. We study entanglement dynamics both for the case of noninteracting fermions, allowing for exact numerical solutions, and for random unitary circuits representing a paradigmatic class of ergodic systems.

The magneto-optical and opto-magnetic effects describe the interaction of light with a magnetic medium. The most prominent examples are the Faraday and Cotton-Mouton effects that modify the transmission of light through a medium, and the inverse Faraday and inverse Cotton-Mouton effects that produce effective magnetic fields for the spin in the material. Here, we introduce the phenomenology of the analogous magneto-phononic and phono-magnetic effects, in which vibrational quanta take the place of the light quanta. We show, using a combination of first-principles calculations and phenomenological modeling, that the effective magnetic fields exerted by the phonon analogs of the inverse Faraday and inverse Cotton-Mouton effects on the spins of antiferromagnetic nickel oxide yield magnitudes comparable to and potentially larger than those of the opto-magnetic originals.

It has been shown that, despite being local, a perturbation applied to a single site of the one-dimensional $XXZ$ model is enough to bring this interacting integrable spin-1/2 system to the chaotic regime. Here, we show that this is not unique to this model, but happens also to the Ising model in a transverse field and to the spin-1 Lai-Sutherland chain. The larger the system is, the smaller the amplitude of the local perturbation for the onset of chaos. We focus on two indicators of chaos, the correlation hole, which is a dynamical tool, and the distribution of off-diagonal elements of local observables, which is used in the eigenstate thermalization hypothesis. Both methods avoid spectrum unfolding and can detect chaos even when the eigenvalues are not separated by symmetry sectors.

Resonant inelastic x-ray scattering (RIXS) is a powerful spectroscopic technique that offers an elemental- and orbital-selective probe of the electronic excitations over a huge energy range. We present a many-body approach to determine RIXS spectra in solids, yielding an intuitive expression for the RIXS cross section in terms of pathways between intermediate many-body states containing a core hole, and final many-body states containing a valence hole. Explicit excited many-body states are obtained from the diagonalization of the Bethe-Salpeter equation in an all-electron framework. For the paradigmatic example of the fluorine $K$ edge of LiF, we show how the excitation pathways determine the spectral shape of the emission, and demonstrate the nontrivial role of electron-hole correlation in the RIXS spectra.

We propose and experimentally demonstrate a quantum state tomography protocol that generalizes and improves upon the Wallentowitz-Vogel-Banaszek-Wódkiewicz point-by-point Wigner function reconstruction. The full density operator of an arbitrary quantum state is efficiently reconstructed in the Fock basis, using semidefinite programming, after interference with a small set of calibrated coherent states. This protocol is resource- and computationally efficient, is robust against noise, does not rely on approximate state displacements, and ensures the physicality of results.

Certain patterns of symmetry fractionalization in (2+1)-dimensional [(2+1)D] topologically ordered phases of matter can be anomalous, which means that they possess an obstruction to being realized in purely (2+1)D. In this paper we demonstrate how to compute the anomaly for symmetry-enriched topological states of bosons in complete generality. We demonstrate how, given any unitary modular tensor category (UMTC) and symmetry fractionalization class for a global symmetry group $G$, one can define a (3+1)-dimensional [(3+1)D] topologically invariant path integral in terms of a state sum for a $G$-symmetry-protected topological (SPT) state. We present an exactly solvable Hamiltonian for the system and demonstrate explicitly a (2+1)D $G$-symmetric surface termination that hosts deconfined anyon excitations described by the given UMTC and symmetry fractionalization class. We present concrete algorithms that can be used to compute anomaly indicators in general. Our approach applies to general symmetry groups, including anyon-permuting and antiunitary symmetries. In addition to providing a general way to compute the anomaly, our result also shows, by explicit construction, that every symmetry fractionalization class for any UMTC can be realized at the surface of a (3+1)D SPT state. As a by-product, this construction also provides a way of explicitly seeing how the algebraic data that defines symmetry fractionalization in general arises in the context of exactly solvable models. In the case of unitary orientation-preserving symmetries, our results can also be viewed as providing a method to compute the ${\mathcal{H}}^4(G,U\left(1\right))$ obstruction that arises in the theory of $G$-crossed braided tensor categories, for which no general method has been presented to date.

The infamous sign problem leads to an exponential complexity in Monte Carlo simulations of generic many-body quantum systems. Nevertheless, many phases of matter are known to admit a sign-problem-free representative, allowing efficient simulations on classical computers. Motivated by long-standing open problems in many-body physics, as well as fundamental questions in quantum complexity, the possibility of intrinsic sign problems, where a phase of matter admits no sign-problem-free representative, was recently raised but remains largely unexplored. Here we establish the existence of an intrinsic sign problem in a broad class of gapped, chiral, topological phases of matter. Within this class, we exclude the possibility of stoquastic Hamiltonians for bosons (or “qudits”) and of sign-problem-free determinantal Monte Carlo algorithms for fermions. The intrinsically sign-problematic class of phases we identify is defined in terms of topological invariants with clear observable signatures: the chiral central charge and the topological spins of anyons. We obtain analogous results for phases that are spontaneously chiral, and present evidence for an extension of our results that applies to both chiral and nonchiral topological matter.

The investigation of novel electronic phases in low-dimensional quantum materials demands for the concurrent development of new measurement techniques that combine surface sensitivity with high spatial resolution and high measurement accuracy. We propose a quantum sensing imaging modality based on superconducting charge qubits to study dissipative charge-carrier dynamics with nanometer spatial and better than nanosecond temporal resolution. Using analytical and numerical calculations, we show that superconducting-charge-qubit microscopy (SCQM) has the potential to resolve temperature and resistivity changes in a sample as small as $\mathrm{\Delta }T\le 0.1\mathrm{mK}$ and $\mathrm{\Delta }\rho \le 1\times {10}^4\mathrm{\Omega }\mathrm{cm}$, respectively. Among other applications, SCQM will be especially suited to study the microscopic mechanisms underlying interaction driven quantum phase transitions, to investigate the boundary modes found in novel topological insulators and, more broadly, to visualize dissipative charge-carrier dynamics occurring in mesoscopic and nanoscale devices.

Two-dimensional Heisenberg antiferromagnets play a central role in quantum magnetism, yet the nature of dynamic correlations in these systems at finite temperature has remained poorly understood for decades. We solve this long-standing problem by using a quantum-classical duality to calculate the dynamic structure factor analytically and, paradoxically, find a broad frequency spectrum despite the very long quasiparticle lifetime. The solution reveals multiscale physics whereby an external probe creates a classical radiation field containing infinitely many quanta. Crucially, it is the multiscale nature of this phenomenon which prevents a conventional renormalization group approach. We also challenge the common wisdom on static correlations and perform Monte Carlo simulations which demonstrate excellent agreement with our theory.

We theoretically investigate the resonance of third harmonic generation (THG) that has been observed at frequency being half of the superconducting gap in a multiband disordered superconductor NbN. The central question is whether the dominant contribution to the THG resonance comes from the Higgs mode (the collective amplitude mode of the superconducting order parameter) or quasiparticle excitations. To resolve this issue, we analyze a realistic three-band model with effective intraband and interband phonon-mediated interactions together with nonmagnetic impurity scatterings. Using the first-principles estimate of the ratio between the intraband and interband pairing interactions with multiband impurity scattering rates being varied from clean to dirty regimes, we calculate the THG susceptibility for NbN in a channel-resolved manner by means of the BCS and self-consistent Born approximations. In the dirty regime, which is close to the experimental situation, the leading contribution is given by the paramagnetic channel of the Higgs mode having almost no polarization-angle dependence, while the second leading contribution comes from the paramagnetic channel of quasiparticles generally showing significant polarization-angle dependence. The result is consistent with the recent experimental observation of no polarization-angle dependence of THG, giving firm evidence that the Higgs mode dominantly contributes to the THG resonance in NbN superconductors.

An optimal filter for Poisson observations is developed as a variant of the traditional Kalman filter. Poisson distributions are characteristic of infectious diseases, which model the number of patients recorded as presenting each day to a health care system. We develop both a linear and a nonlinear (extended) filter. The methods are applied to a case study of neonatal sepsis and postinfectious hydrocephalus in Africa, using parameters estimated from publicly available data. Our approach is applicable to a broad range of disease dynamics, including both noncommunicable and the inherent nonlinearities of communicable infectious diseases and epidemics such as from COVID-19.

Restricted-valence random sequential adsorption is studied in its pure and disordered versions on the square and triangular lattices. For the simplest case (pure on the square lattice) we prove the absence of percolation for maximum valence $V_{\max }=2$. In other cases, Monte Carlo simulations are used to investigate the percolation threshold, universality class, and jamming limit. Our results reveal a continuous transition for the majority of the cases studied. The percolation threshold is computed through finite-size scaling analysis of seven properties; its value increases with the average valency. Scaling plots and data-collapse analyses show that the transition belongs to the standard percolation universality class even in disordered cases.

Most of the DNA that composes a complex organism is noncoding and defined as junk. Even the coding part is composed of genes that affect the phenotype differently. Therefore a random mutation has an effect on the specimen fitness that strongly depends on the DNA region where it occurs. Understanding how this heterogeneous composition influences the fitness evolution of individuals is hampered by the complexity of the problem and a clear picture is missing. Here we study a minimal model for the evolution of an ecosystem where two antagonist species struggle for survival on a lattice. Each specimen has a unique toy genome that codes for its phenotype. The gene pool of populations changes in time due to the effect of random mutations on genes (entropic force) and of interactions with the environment and between individuals (natural selection). We prove that the relevance of each gene in the manifestation of the phenotype is a key feature for evolution. In the presence of a uniform gene relevance, a mutational meltdown is observed. Natural selection acts to quench the ecosystem in a nonequilibrium state that slowly drifts, decreasing the fitness and leading to the extinction of the species. Conversely, if a specimen is provided with a heterogeneous gene relevance, natural selection wins against entropic forces and the species evolves, increasing its fitness. We finally show that heterogeneity together with spatial correlations are responsible for spontaneous sympatric speciation.

Majorana fermions feature non-Abelian exchange statistics and promise fascinating applications in topological quantum computation. Recently, second-order topological superconductors (SOTSs) have been proposed to host Majorana fermions as localized quasiparticles with zero excitation energy, pointing out a new avenue to facilitate topological quantum computation. We provide a minimal model for SOTSs and systematically analyze the features of Majorana zero modes with analytical and numerical methods. We further construct the fundamental fusion principles of zero modes stemming from single or multiple SOTS islands. Finally, we propose concrete schemes in different setups formed by SOTSs, enabling us to exchange and fuse the zero modes for non-Abelian braiding and holonomic quantum gate operations.

Hayden and Preskill proposed a thought experiment in which Bob can recover the information Alice throws into a black hole if he has a quantum computer entangled with the black hole, and for which Yoshida and Kitaev recently proposed a concrete decoding scheme. In the context of quantum many-body physics, the parallel question is that after a small system is thermalized with a large system, how can one decode the initial state information with the help of two entangled many-body systems? Here, we propose to realize this decoding protocol in a physical system of two Dicke models, with two cavity fields prepared in a thermofield double state. We show that the Yoshida-Kitaev protocol allows us to read out the initial spin information after it is scrambled into the cavity. We show that the readout efficiency reaches a maximum when the model parameters are tuned to the regime where the system is the most chaotic, characterized by the shortest scrambling time in the out-of-time-ordered correlation function. Our proposal opens up the possibility of discussing this profound thought experiment in a realistic setting.