Table of contents

Inferring causal relations from empirical data is a central task in any scientific inquiry. To that aim, a mathematical theory of causality has been developed, not only stating under which conditions such inference becomes possible but also offering a formal framework to reason about cause and effect within quantum theory. This perspective article provides an overview of the latest results in the growing field of quantum causality, with a special focus on experimental implementations and concepts that only recently have entered in the quantum information dictionary. Ideas like bilocality, instrumental variables and interventions will be discussed from both a fundamental and an experimental perspective.

We consider the family of singularity-free rotating black hole solutions in Einstein's conformal gravity found in Bambi C. et al., JCAP, 05 (2017) 003 and we constrain the value of the conformal parameter L from the analysis of a $30\ \text{ks}$ NuSTAR observation of the stellar-mass black hole in GS 1354–645 during its outburst in 2015. Our new constraint is much stronger than that found in previous work. Here we obtain $L/M < 0.12$ (99% confidence level, statistical uncertainty only).

The phenomenon of a chiral-symmetry protected thermalization gap in Hermitian photonic systems is counterintuitive as it implies that the photon coherence can be continuously improved by disorders towards an asymptotic limit. We show that the phenomenon disappears in time-independent, non-Hermitian photonic systems even when the chiral symmetry is well preserved. In fact, the degree of thermalization generally increases with the disorder strength, in agreement with intuition. As non-Hermitian characteristics (e.g., weak gain and loss) can be expected in realistic physical situations, the phenomenon of a thermalization gap may be observed but only in well-controlled experiments with high-quality materials.

We demonstrate that it is possible to implement a quantum perceptron with a sigmoid activation function as an efficient, reversible many-body unitary operation. When inserted in a neural network, the perceptron's response is parameterized by the potential exerted by other neurons. We prove that such a quantum neural network is a universal approximator of continuous functions, with at least the same power as classical neural networks. While engineering general perceptrons is a challenging control problem —also defined in this work— the ubiquitous sigmoid-response neuron can be implemented as a quasi-adiabatic passage with an Ising model. In this construct, the scaling of resources is favorable with respect to the total network size and is dominated by the number of layers. We expect that our sigmoid perceptron will have applications also in quantum sensing or variational estimation of many-body Hamiltonians.

Under a rotation by an angle ϑ, both the right- and left-handed Weyl spinors pick up a phase factor $\exp(\pm i \vartheta/2)$. The upper sign holds for the positive helicity spinors, while the lower sign holds for the negative helicity spinors. For $\vartheta = 2\pi$ radians this produces the famous minus sign. However, the four-component spinors are built from a direct sum of the indicated two-component spinors. The effect of the rotation by $2\pi$ radians on the eigenspinors of the parity – that is, the Dirac spinors – is the same as on Weyl spinors. It is because for these spinors the right- and left-transforming components have the same helicity. And the rotation-induced phases, being the same, factor out. But for the eigenspinors of the charge conjugation operator, i.e., Elko, the left- and right-transforming components have opposite helicities, and, therefore, they pick up opposite phases. As a consequence the behaviour of the eigenspinors of the charge conjugation operator (Elko) is more subtle: for $0<\vartheta<2\pi$ a self-conjugate spinor becomes a linear combination of the self- and antiself-conjugate spinors with ϑ -dependent superposition coefficients –and yet the rotation preserves the self-/antiself-conjugacy of these spinors! This apparently paradoxical situation is fully resolved. This new effect, to the best of our knowledge, has never been reported before. The purpose of this communication is to present this result and to correct an interpretational error of a previous version.

By considering the intrinsic anisotropy, present in almost all magnetic systems, as a perturbation to the usual Zeeman term, we show that the spin-spin dipolar interaction also known as zero-field splitting (ZFS) leads to an extra geometrical phase in addition to the conventional Berry's phase. Furthermore, we suggest some ways to observe the energy shift in electron paramagnetic resonance spectra due to Berry's phase and how we can separate it from the conventional Zeeman Berry's phase.

One of the authors (MM) dedicates this work to the memory of his mother, Djabou Zoulikha, who died on 3 February 2019.

We theoretically study the geometric effect of quantum dynamical evolution in the presence of a nonequilibrium noisy environment. We derive the expression of the time-dependent geometric phase in terms of the dynamical evolution and the overlap between the time evolved state and initial state. It is shown that the frequency shift induced by the environmental nonequilibrium feature plays a crucial role in the geometric phase and evolution path of the quantum dynamics. The nonequilibrium feature of the environment makes the length of the evolution path become longer and reduces the dynamical decoherence and non-Markovian behavior in the quantum dynamics.

The consequences of imposing boundary conditions to extra spacetime coordinates on a gauge field theory are considered. In a toy model, a larger spacetime symmetry described by a Lorentz-Poincaré SO(p, q) group is broken to a symmetry group of the form $SO(p,q-n)\times O(n)$ due to boundary conditions in the extra dimensions, leading to an effectively massive gauge field coupled to a "scalar" field in the observable spacetime coordinates.

The molecular alignment induced by external fields is of significance in manipulating a chemical reaction. In this paper, the field-free alignment of a FCN molecule steered by a terahertz (THz) half-cycle laser pulse (HCP) is investigated by solving the density matrix equation. It is shown that a high degree of molecular alignment can be achieved by adjusting the carrier envelope phase (CEP) of the THz HCP. The appearance of a saturation threshold for the molecular alignment by a THz HCP when the field amplitude is increased arises from the 1st-order dipole moment interaction. The effect of the central frequency on the molecular alignment, which is manipulated by a THz HCP, is discussed in detail. By changing the matching number of the THz laser pulse at different CEPs, it is found that under the same conditions, a THz HCP has a certain advantage over a few-cycle laser pulse in improving the FCN molecular alignment. Comparing with a double THz HCP, a better result of the molecular alignment can be obtained by a single THz HCP at the same rotational temperature of molecules.

As for the acoustic radiation calculation of vibrating structures in the free field, less attention has been paid to the time domain analysis than frequency domain analysis. Nevertheless time domain sound calculation is essential for applications in which the dynamic process should be carefully addressed. Previous researchers tried hard to improve the efficiency of transient acoustic radiation calculation and have made many progresses. However, until now the transient acoustic radiation in the free field still suffers from insufficient computational resources and low efficiency when the number of discrete elements is large and the temporal sample sequence is long. In order to solve these problems, a modal expansion and spatial delay based transient sound field calculation method is proposed. By constructing the DMATM (Delayed Modal Acoustic Transfer Matrix) in the proposed method, the physical coordinates of structural nodes are transferred to low-dimensional modal coordinates and the spatial delay information of different nodes is preserved. To describe this method in detail, the acoustic radiation process of the impact sound synthesis of a cylinder is investigated. The results show that the proposed method is more efficient than previous methods under the same accuracy.

We numerically studied the realization of plasmon-induced transparency (PIT) in an InSb waveguide coupled with two resonators of different lengths in the terahertz (THz) region. In the two different methods of the finite-difference time domain method (FDTD) and coupling mode theory (CMT), the physical mechanism of the PIT effect of the waveguide system is numerically simulated and theoretically analyzed. The results of numerical simulations and the theoretical calculations were found to be perfectly consistent. Most importantly, a novel modulation mechanism for the PIT effect is found in the waveguide. The conductivity of the photoactive silicon is controlled by adjusting the intensity of the pump light, thereby realizing the dynamic modulation of the PIT window. In addition, the resonance frequency of the PIT spectrum can be controlled by adjusting the coupling distance between the resonators or the temperature of the environment. This plasmonic waveguide system may have interesting potential applications for integrating optical circuits, such as optical switches, slow light components, and thermally tuned filters.

Given the increasing interest for non-reciprocal materials, we propose a novel acoustic imaging method for layered non-reciprocal media. The method we propose is a modification of the Marchenko imaging method, which handles multiple scattering between the layer interfaces in a data-driven way. We start by reviewing the basic equations for wave propagation in a non-reciprocal medium. Next, we discuss Green's functions, focusing functions, and their mutual relations, for a non-reciprocal horizontally layered medium. These relations form the basis for deriving the modified Marchenko method, which retrieves the wave field inside the non-reciprocal medium from reflection measurements at the boundary of the medium. With a numerical example we show that the proposed method is capable of imaging the layer interfaces at their correct positions, without artefacts caused by multiple scattering.

Microperforated panels (MPPs) play important roles in sound absorbing systems. The classical Maa theory for the MPPs is modified to account for the effect of roughness on the surface of microperforations on sound absorption. Correspondingly, the relative acoustic resistance and relative acoustic mass of the system are determined theoretically. Full numerical simulations with the method of finite elements are performed on the roughened MPP to validate the modified theory, with good agreement achieved. It is demonstrated that surface roughness decreases resonant frequency and promotes viscous dissipation, thus enhancing the sound absorbing capability of the MPP. This work extendes Maa's theory for the sound absorption of MPP from smooth perforations to rough perforations. The modified theory for MPP with roughened perforations has a great significance in sound absorption field, since it is more applicable for the realistic situations.

The circular dichroism (CD) effect characterized by different optical responses between left and right circularly polarized lights is widely applied for polarization-resolved detection and imaging. The tunable CD effect is of substantial importance in improving the detection sensitivity and imaging resolution. In this paper, we show that planar Z-shaped composite metamaterial embedded with VO₂ that exhibits insulator-metal transition (IMT) can enable thermally tunable chirality. Simulated by the finite element method, the tunable CD effect can be achieved by changing the environment temperature to initiate the IMT of VO₂. We also demonstrate that the underlying mechanism of the CD effect generation is the electric multipole oscillation response in the vicinity of the VO₂ gap in vertical direction. These findings not only provide a new strategy to change chirality and tune the effect of CD dynamically but also broaden its potential applications in polarization-resolved detection and imaging.

X-ray phase-contrast imaging (XPCI) is a versatile technique with applications in many fields, including fundamental physics, biology and medicine. Where X-ray absorption radiography requires high density ratios for effective imaging, the image contrast for XPCI is a function of the density gradient. In this letter, we apply XPCI to the study of laser-driven shock waves. Our experiment was conducted at the Petawatt High-Energy Laser for Heavy Ion EXperiments (PHELIX) at GSI. Two laser beams were used: one to launch a shock wave and the other to generate an X-ray source for phase-contrast imaging. Our results suggest that this technique is suitable for the study of warm dense matter (WDM), inertial confinement fusion (ICF) and laboratory astrophysics.

Many biological systems form colonies at high density. Passive granular systems will be jammed at such densities, yet for the survival of biological systems it is crucial that they are dynamic. We construct a phase diagram for a system of active particles interacting via Vicsek alignment, and vary the density, self-propulsion force, and orientational noise. We find that the system exhibits four different phases, characterized by transitions in the effective diffusion constant and in the orientational order parameter. Our simulations show that there exists an optimal noise such that particles require a minimal force to unjam, allowing for rearrangements.

We propose the existence of a symmetry-protected topological Dirac nodal-line (DNL) magnonic phase in layered honeycomb collinear antiferromagnets even in the presence of spin-orbit Dzyaloshinskii-Moriya interaction. We show that the magnon spin Nernst effect, predicted to occur in strictly two-dimensional (2D) honeycomb collinear antiferromagnets cancels out in the layered honeycomb collinear antiferromagnets. In other words, the magnon spin Nernst effect in each 2D antiferromagnetic layer cancels out the succeeding layer. Hence, the Berry curvature vanishes in the entire Brillouin zone due to the combination of time-reversal and space-inversion $(\mathcal{PT})$ symmetry. However, upon symmetry breaking by an external magnetic field, we show that a non-vanishing Berry curvature and Chern number protected topological magnon bands are induced in the non-collinear spin structure. This leads to an experimentally accessible magnon thermal Hall effect in the $\mathcal{PT}$ symmetry-broken topological DNL magnonic phase of layered honeycomb antiferromagnets. We propose that the current predicted results can be experimentally investigated in the layered honeycomb antiferromagnets CaMn₂Sb₂, BaNi₂V₂O₈, and Bi₃Mn₄O₁₂(NO₃).

We report on the effect of 80 keV Xe⁺ ion irradiation on the commercially available, sectioned 10B pencil-graphite specimens subjected to normal and oblique angle incidence of ions (θ_i = 0°–70°) and considering a fixed ion fluence of $1\times 10^{14}\ \text{ions/cm}^{2}$. The pristine and irradiated samples were characterized by powder x-ray diffraction, atomic force microscopy and Raman spectroscopy studies. Unlike pristine graphite and normal incidence cases, several ripple patterns can be visualized in the atomic force micrographs of the specimens irradiated with an oblique angle incidence. Moreover, apart from conventional G-band located at ${\sim}1580\ \text{cm}^{-1}$, a distinct diamond-like carbon (DLC) phase has been witnessed at ${\sim}1330\ \text{cm}^{-1}$ in the Raman spectra of the graphitic specimens subjected to ion impact and for $\theta_{\mathrm{i}}$ values of 50° and 70°. The formation of the DLC phase along with surface ripple formation would find numerous scopes while dealing with properties such as surface transport, field emission and mechanically hard coating in miniaturized devices.

We investigate the Andreev reflection and 0-π transition in the graphene-based antiferromagnetic superconducting junctions on the SiC substrate, respectively. The differential conductance of Andreev reflection is reduced in the presence of a gap induced by the antiferromagnet or the substrate. Interestingly, although the gap induced by the antiferromagnet is the same to the one induced by the substrate, their differential conductances of the Andreev reflection are absolutely different, which can be used to detect antiferromagnetism in experiment. Although the interaction between the antiferromagnet and the substrate cannot show special differential conductance, their interaction can bring the 0-π transition. This breaks up the conventional wisdom that the antiferromagnetism-induced 0-π transition shows a rigorous atomic-scale dependence on the interlayer thickness. Furthermore, compared with the conventional atomic-scale dependent 0-π transition, our finding can be realized more easily in the experiment.

The orbital distribution of the spin fluctuation in the iron-based superconductors (IBSs) is the key information needed to understand the magnetism, superconductivity and electronic nematicity in these multi-orbital systems. In this work, we propose that the resonant inelastic X-ray scattering (RIXS) technique can be used to probe selectively the spin fluctuation on different Fe 3d orbitals. In particular, the spin fluctuation on the three t_2g orbitals, namely, the 3d_xz, 3d_yz and the 3d_xy orbital, can be selectively probed in the $\sigma\rightarrow\pi'$ scattering geometry by aligning the direction of the outgoing photon in the y-, x- and z-direction. Such orbital-resolved information on the spin fluctuation is invaluable for the study of the orbital-selective physics in the IBSs and can greatly advance our understanding of the relation between orbital ordering and spin nematicity in the IBSs and the orbital-selective pairing mechanism in these multi-orbital systems.

A common thread in designing electrochemically based renewable energy devices comprises materials that exploit nano-scale morphologies, e.g., supercapacitors, batteries, fuel cells, and bulk heterojunction organic photovoltaics. In these devices, however, Coulomb forces often influence the fine nano-details of the morphological structure of active layers leading to a notorious decrease in performance. By focusing on bulk heterojunction organic photovoltaics as a case model, a self-consistent mean-field framework that combines binary (bi-stable) and ternary (tri-stable) morphologies with electrokinetics is presented and analyzed, i.e., undertaking the coupling between the spatiotemporal evolution of the material and charge dynamics along with charge transfer at the device electrodes. Particularly, it is shown that tri-stable composition may stabilize stripe morphology that is ideal bulk heterojuction. Moreover, since the results rely on generic principles they are expected to be applicable to a broad range of electrically charged amphiphilic-type mixtures, such as emulsions, polyelectrolytes, and ionic liquids.

More than sixty years ago Morris Swadesh proposed to measure the similarity between two languages by considering their overlap, i.e., the percentage of cognates. The overlap was estimated by comparing two ad hoc lists with words corresponding to same meanings for the two languages. The notable assumption of Swadesh was that replacements in a vocabulary occur at a universal constant rate so that the time distance from the eventual common ancestor can be determined. In his view only replacements are relevant while horizontal transfers (borrowings from other languages) are less important and their effect can be eventually taken into account assuming that the divergence is diminished by contact. Later, the mainstream of glottochronology adopted the point of view that the effect of loanwords could be eliminated by careful work devoted to their identification so that they would not affect any measure of distance between languages. The aim of this paper is to show by experimental evidence that horizontal transfers, on the contrary, are a primary aspect of languages evolution since their effect on the vocabulary is at least as important as that of spontaneous replacements. We finally show that this phenomenon severely and unavoidably limits the possibility to fully reconstruct a proto-language. This limitation is fundamental, i.e., it gives a bound which cannot be infringed, independently of the method used for the reconstruction.

X-ray computed tomography (CT) reconstruction suffers from beam-hardening artefacts caused by the polychromaticity of virtually all lab-based X-ray sources. A method to correct for beam-hardening is a direct, pixel-wise signal-to-thickness calibration (STC). We compare reconstructions of conventionally flat-field corrected as well as STC preprocessed measurements of various samples performed on a commercial microCT device based on a flat-panel detector. We show that a good estimate between the transmission signal and the respective material thickness can be given by multiple exponential functions. We further compare the exponential interpolation approach to a hyperbolic model, which reduces the number of necessary calibration measurements significantly. Our method shows that typical beam-hardening artefacts like cupping and filling can be almost completely suppressed and a significant contrast increase is gained. The method can be applied with little additional calibration and computation effort and allows shorter acquisition times since beam filtration can be reduced or omitted.

Original article: EPL, 125 (2019) 10010.