The appearance of nontrivial phases in Kitaev materials exposed to an external magnetic field has recently been a subject of intensive studies. Here, we elucidate the relation between the field-induced ground states of the classical and quantum spin models proposed for such materials, by using the infinite density matrix renormalization group (iDMRG) and the linear spin wave theory (LSWT). We consider the $K\mathrm{\Gamma }{\mathrm{\Gamma }}^{\prime }$ model, where $\mathrm{\Gamma }$ and ${\mathrm{\Gamma }}^{\prime }$ are off-diagonal spin exchanges on top of the dominant Kitaev interaction $K$. Focusing on the magnetic field along the [111] direction, we explain the origin of the nematic paramagnet, which breaks the lattice-rotational symmetry and exists in an extended window of magnetic field, in the quantum model. This phenomenon can be understood as the effect of quantum order-by-disorder in the frustrated ferromagnet with a continuous manifold of degenerate ground states discovered in the corresponding classical model. We compute the dynamical spin structure factors using a matrix operator based time evolution and compare them with the predictions from LSWT. We, thus, provide predictions for future inelastic neutron scattering experiments on Kitaev materials in an external magnetic field along the [111] direction. In particular, the nematic paramagnet exhibits a characteristic pseudo-Goldstone mode, which results from the lifting of a continuous degeneracy via quantum fluctuations.

A significant number of servers that constitute the Internet are to provide private data via private communication channels to mutually anonymous registered users. Such are the servers of banks, hospitals that provide cloud storage and many others. Replacing communication channels by maximally entangled states is a promising idea for the quantum-secured Internet (QI). While it is an important idea for large distances secure communication, for the case of the mentioned class of servers pure entanglement based solution is not only unnecessary but also opens a threat. A crack stimulating a node to generate secure connections via entanglement swapping between two hackers can cause uncontrolled consumption of resources. Turning into positive a recently proven no-go result by S. Bäuml et al. [Nat. Commun. 6, 6908 (2015)], we propose a natural countermeasure against this threat. The solution bases on connections between hub-nodes and end-users realized with states that contain secure key but do not allow for swapping of this key. We then focus on the study of the quantum memory cost of such a scheme and prove a fundamental lower bound on its memory overhead. In particular, we show that to avoid the possibility of entanglement swapping, it is necessary to store at least twice as much memory than it is the case in standard quantum-repeater-based network design. For schemes employing either states with positive partial transposition that approximates certain privates states or private states hardly distinguishable from their attacked versions, we derive much tighter lower bounds on required memory. Our considerations yield upper bounds on a two-way repeater rate for states with positive partial transposition (PPT), which approximates strictly irreducible private states. As a byproduct, we provide a lower bound on the trace distance between PPT and private states, shown previously only for private bits.

Controlling the helicity and directionality of the scattered light by dielectric particles is paramount to a variety of phenomenology of interest in all-dielectric optics and photonics. In this paper, we demonstrate that the helicity of an incoming beam is not preserved when several multipoles contribute to the scattering. This inhibits the usual link between duality restoration and zero optical backscattering condition. In addition, we show that the helicity conservation in the scattering by high-refractive index dielectric Mie spheres can be used as a probe of pure-multipolar spectral regions, particularly of dipolar nature, beyond its presumed spectral interval. This finding reveals that the dipolar behavior is not necessarily limited to small particles, in contrast to the current understanding. Interestingly, our formalism allows us to demonstrate that the optimum forward light scattering condition, predicted for a particular nanosphere, is fulfilled for an infinite number of refractive indexes in these dipolar regimes.

We perform high-field magnetization measurements on the triangular lattice antiferromagnet ${\mathrm{Fe}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$. We observe a plateau in the magnetization centered at approximately half the saturation magnetization over a wide range of temperature and magnetic field. From density functional theory calculations, we determine a likely set of magnetic exchange constants. Incorporating these constants into a minimal Hamiltonian model of our material, we find that the plateau and the ${\mathbb{Z}}_3$ symmetry-breaking ground state both arise from the competition of interplane and intraplane exchange interactions. These findings are pertinent to the magnetoelectric properties of ${\mathrm{Fe}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$, which allow electrical switching of antiferromagnetic textures at relatively low current densities.

We address the problem of reconstructing quantum theory from the perspective of an agent who makes bets about the outcomes of possible experiments. We build a general Bayesian framework that can be used to organize the agent's beliefs and update them when new information becomes available. Our framework includes as special cases classical and quantum probability theory, as well as other forms of probabilistic reasoning that may arise in future physical theories. Building on this framework, we develop a notion of an ideal experiment, which in quantum theory coincides with the notion of projective measurement. We then prove that, in every general Bayesian theory, ideal experiments must satisfy the exclusivity principle, a property of projective measurements that plays a central role in the characterization of quantum correlations. Our result suggests that the set of quantum correlations may be completely characterized in terms of Bayesian consistency conditions.

We present a simple modification of the standard $\text{U}\left(1\right)$ slave boson construction for the single band $t\text{−}J$ model which accounts for two-particle bound states of spinons and holons. This construction naturally gives rise to fractionalized Fermi-liquid ground states, featuring small, hole-like pocket Fermi surfaces with an anisotropic quasiparticle weight in the absence of broken symmetries. In a specific parameter regime our approach maps the square lattice $t\text{−}J$ model to a generalized quantum dimer model, which was introduced as a toy model for the metallic pseudogap phase in hole-doped cuprates in [Proc. Natl. Acad. Sci. USA 112, 9552 (2015)]. Our slave boson construction captures essential features of the nodal-antinodal dichotomy and straightforwardly describes sharp, Fermi arc-like features in the electron spectral function. Moreover, it allows us to study quantum phase transitions between fractionalized Fermi-liquid phases and superconductors or ordinary Fermi liquids.

Neodymium is a remarkable active component in numerous magnetic alloys that are used in various applications. However, the application of a bare neodymium thin film is limited due to the lack of information about its electrical and magnetic properties. We report a synergistic study of an Nd thin film using experimental and theoretical techniques of polarized neutron reflectometry, magnetoresistance measurement, and density functional theory. Unlike bulk Nd, the thin film specimen is a very poor electrical conductor. Also, as grown, the thin film on silicon substrate does not exhibit any magnetism in a zero field. However, moderate in-plane field application of $H=1.2\mathrm{T}$ tends to induce weak magnetism in the system at a low temperature of $T<$ 18 K, which coincides with an unusual cross-over behavior in magnetoresistance. The study provides important insight in the physical characteristics of the Nd thin film that are atypical for a magnetic system.

Several studies have pointed out an excess in the AMS-02 antiproton spectrum at rigidities of 10–20 GV. Its spectral properties were found to be consistent with a dark-matter particle of mass 50–100 GeV which annihilates hadronically at roughly the thermal rate. In this paper, we reinvestigate the antiproton excess, including all relevant sources of systematic errors. Most importantly, we perform a realistic estimate of the correlations in the AMS-02 systematic error which could potentially “fake” a dark-matter signal. The dominant systematics in the relevant rigidity range originate from uncertainties in the cross sections for absorption of cosmic rays within the detector material. We calculate their correlations within the Glauber-Gribov theory of inelastic scattering. The AMS-02 correlations enter our spectral search for dark matter in the form of covariance matrices, which we make publicly available for the cosmic-ray community. We find that the global significance of the antiproton excess is reduced to below 1 $\sigma$ once all systematics, including the derived AMS-02 error correlations, are taken into account. No significant preference for a dark-matter signal in the AMS-02 antiproton data is found in the mass range 10–10 000 GeV.

We introduce the idea of collisional models for Brownian particles, in which a particle is sequentially placed in contact with distinct thermal environments and external forces. Thermodynamic properties are exactly obtained, irrespective of the number of reservoirs involved. In the presence of external forces, the entropy production presents a bilinear form in which Onsager coefficients are exactly calculated. Analysis of Brownian engines based on sequential thermal switchings is proposed and considerations about their efficiencies are investigated, taking into account distinct external forces protocols. Our results shed light to an alternative route for obtaining efficient thermal engines based on finite times Brownian machines.

In the Kitaev honeycomb model, the quantum spin fractionalizes into itinerant Majorana fermion and gauge flux upon cooling, leading to rich experimental ramifications at finite temperature and an upsurge of research interest. However, accurate modelings of the Kitaev materials by including realistic couplings beyond the pure Kitaev model constitute a major challenge for the community. With recently developed exponential tensor-network approach, we clear the pathway and perform finite-temperature simulations of the extended Kitaev model with additional interactions common in real materials. At intermediate temperature, we find an emergent Curie law of magnetic susceptibility and a stripy spin-structure factor characterizing the robust Kitaev fractional liquid. With this insight, we revisit the susceptibility measurements of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ and $\alpha \text{−}{\mathrm{RuCl}}_3$ and find evidence of ferromagnetic Kitaev coupling and finite-temperature fractionalization. Bridging the gap between theories and experiments with the state-of-the-art tensor-network simulations, our findings provide guidance for future experimental exploration of the spin liquids in Kitaev materials by thermodynamic measurements and spin-resolved structure factor probes.

We investigate the relaxation dynamics of a single artificial atom interacting, via multiple coupling points, with a continuum of bosonic modes (photons or phonons) in a one-dimensional waveguide. In the non-Markovian regime, where the traveling time of a photon or phonon between the coupling points is sufficiently large compared to the inverse of the bare relaxation rate of the atom, we find that a boson can be trapped and form a stable bound state. As a key discovery, we further find that a persistently oscillating bound state can appear inside the continuous spectrum of the waveguide if the number of coupling points is more than two since such a setup enables multiple bound modes to coexist. This opens up prospects for storing and manipulating quantum information in larger Hilbert spaces than available in previously known bound states.

The rare-earth magnet $\mathrm{Tm}\mathrm{Mg}\mathrm{Ga}{\mathrm{O}}_4$ is proposed to be an intrinsic quantum Ising magnet described by the antiferromagnetic transverse field Ising model (TFIM) on a triangular lattice, where the relevant degrees of freedom are the nondegenerate dipole-multipole doublets of the ${\mathrm{Tm}}^{3+}$ ions and the transverse field has an intrinsic origin from the weak splitting of the doublet. We compare this special doublet of ${\mathrm{Tm}}^{3+}$ with the dipole-octupole Kramers doublet. We study the proposed effective model for the Tm-based triangular lattice and consider the effects of external magnetic fields and finite temperatures. From the orthogonal operator approach, we show that the TFIM with the three-sublattice intertwined ordered state agrees with the experiments and further clarify the discrepancy in the numbers of the magnetic sublattices and the measured magnon branches. We make specific predictions for the evolution of the magnetic properties with the external magnetic field. Furthermore, we demonstrate that an emergent U(1) symmetry emerges in thermal melting of the underlying orders and at the criticality, and summarize the previously known signatures related to the finite-temperature Berezinskii-Kosterlitz-Thouless physics. We discuss the broad relevance of intrinsic quantum Ising magnets to many other systems, especially Tm-based materials.

We show that two-dimensional band insulators, with vanishing bulk polarization, obey bulk-and-edge to corner charge correspondence, stating that the knowledge of the bulk and the two corresponding ribbon band structures uniquely determines a fractional part of the corner charge irrespective of the corner termination. Moreover, physical observables related to macroscopic charge density of a terminated crystal can be obtained by representing the crystal as collection of polarized edge regions with polarizations ${\vec{P}}_{\alpha }^{\text{edge}}$, where the integer $\alpha$ enumerates the edges. We introduce a particular manner of cutting a crystal, dubbed “Wannier cut,” which allows us to compute ${\vec{P}}_{\alpha }^{\text{edge}}$. We find that ${\vec{P}}_{\alpha }^{\text{edge}}$ consists of two pieces: the bulk piece expressed via quadrupole tensor of the bulk Wannier functions' charge density and the edge piece corresponding to the Wannier edge polarization—the polarization of the edge subsystem obtained by Wannier cut. For a crystal with $n$ edges, out of $2n$ independent components of ${\vec{P}}_{\alpha }^{\text{edge}}$, only $2n-1$ are independent of the choice of Wannier cut and correspond to physical observables: corner charges and edge dipoles.

A fundamental problem in quantum information processing is the discrimination among a set of quantum states of a system. In this paper, we address this problem on an open quantum system described by a graph, whose evolution is defined by a quantum stochastic walk. In particular, the structure of the graph mimics those of neural networks, with the quantum states to discriminate encoded on input nodes and with the discrimination obtained on the output nodes. We optimize the parameters of the network to obtain the highest probability of correct discrimination. Numerical simulations show that after a transient time the probability of correct decision approaches the theoretical optimal quantum limit. These results are confirmed analytically for small graphs. Finally, we analyze the robustness and reconfigurability of the network for different set of quantum states and show that this architecture can pave the way to experimental realizations of our protocol as well as novel quantum generalizations of deep learning.

The ultrafast dynamics of short-pulse-laser-excited fused silica is investigated by measuring polarization-resolved reflectances of a wavelength-tunable probe pulse following a near-infrared pump pulse. These data, which should, in principle, allow the determination of both real and imaginary parts of the time-dependent electric permittivity, turn out to become incompatible with an isotropic optical response after $\sim 200$ fs. This is reconciled with a uniaxial permittivity tensor, which predicts greatly enhanced absorption of $p$-polarized light, suggesting that an absorption channel is missing in the standard optical models for strongly excited dielectrics. The timescale for the excitation-induced variations in the optical properties exhibits a wavelength dependence that can be explained qualitatively by Pauli blocking in the electronic band structure.

Quantum magnets with a large $\mathrm{SU}\left(N\right)$ symmetry are a promising playground for the discovery of new forms of exotic quantum matter. Motivated by recent experimental efforts to study $\mathrm{SU}\left(N\right)$ quantum magnetism in samples of ultracold fermionic alkaline-earth-like atoms in optical lattices, we study here the temperature dependence of spin correlations in the $\mathrm{SU}\left(N\right)$ Heisenberg spin model in a wide range of temperatures. We uncover a sizable regime in temperature, starting at $T=\infty$ down to intermediate temperatures and for all $N\ge 2$, in which the correlations have a common spatial structure on a broad range of lattices, with the sign of the correlations alternating from one Manhattan shell to the next, while the amplitude of the correlations is rapidly decreasing with distance. Focussing on the one-dimensional chain and the two-dimensional square and triangular lattice for certain $N$, we discuss the appearance of a disorder and a Lifshitz temperature, separating the commensurate Manhattan high-$T$ regime from a low-$T$ incommensurate regime. We observe that this temperature window is associated to an approximately $N$-independent entropy reduction from the $ln\left(N\right)$ entropy at infinite temperature. Our results are based on high-temperature series arguments and as well as large-scale numerical full diagonalization results of thermodynamic quantities for $\mathrm{SU}\left(3\right)$ and $\mathrm{SU}\left(4\right)$ square lattice samples, corresponding to a total Hilbert space of up to $4\times {10}^9$ states.

A deficiency in the superfluid density in overdoping cuprates is an unconventional feature of keen interest in superconductivity with an electron correlation and, likewise, it was previously suggested that pressure, which weakens the electron correlation, induces an increased deficiency in the superfluid density in an organic superconductor with a fixed band filling. Here, we report our magnetic penetration depth measurements of a layered organic superconductor situated near a Mott transition under various pressures to revisit the issue of superfluid density. The analyses of the results find no meaningful variation in superfluid density with increasing pressure, which greatly suppresses the superconducting transition temperature.

Doppler harmonic generation of a high-power laser on a relativistic plasma mirror is a promising path to produce bright attosecond light bursts. However, a major challenge has been to find a way to generate isolated attosecond pulses, better suited to timed-resolved experiments, rather than trains of pulses. A promising technique is the attosecond lighthouse effect, which consists in imprinting different propagation directions to successive attosecond pulses of the train, and then spatially filtering one pulse in the far field. However, in the relativistic regime, plasma mirrors get curved by the radiation pressure of the incident laser and thus focus the generated harmonic beams. This increases the harmonic beam divergence and makes it difficult to separate the attosecond pulses angularly. In this article, we propose two novel techniques readily applicable in experiments to significantly reduce the divergence of Doppler harmonics, and achieve the generation of isolated attosecond pulses from the lighthouse effect without requiring few-cycle laser pulses. Their validity is demonstrated using state-of-the-art simulations, which show that isolated attosecond pulses with $10\text{TW}$ peak power in the XUV range can be generated with PW-class lasers. These techniques can equally be applied to other generation mechanisms to alleviate the constraints on the duration on the laser pulses needed to generate isolated attosecond pulses.

Entropy production provides a general way to state the second law of thermodynamics for nonequilibrium scenarios. In open quantum system dynamics, it also serves as a useful quantifier of the degree of irreversibility. In this work we shed light on the relation between correlations, initial preparation of the system, and non-Markovianity by studying a system of two harmonic oscillators independently interacting with their local baths. Their dynamics, described by a time-local master equation, is solved to show, both numerically and analytically, that the global purity of the initial state of the system influences the behavior of the entropy production rate and that the latter depends algebraically on the entanglement that characterizes the initial state.

We study the effect of localized attacks on a multiplex network, where each layer is a network of communities embedded in space. We assume that nodes are densely connected within a community and sparsely connected to the nodes in the neighboring communities. To investigate percolation processes in this realistic system we develop an analytical scheme, applying the finite-element method. We find, both by simulation and theory, that in many cases there is a critical size of localized damage above which it will spread and the entire system will collapse. In addition, we show that for a constant number of links, networks with less connectivity between communities are surprisingly more robust.