Across all scales of the physical world, dynamical systems can be usefully represented as abstract networks that encode the systems' units and interunit interactions. Understanding how physical rules shape the topological structure of those networks can clarify a system's function and enhance our ability to design, guide, or control its behavior. In the emerging area of quantum network science, a key challenge lies in distinguishing between the topological properties that reflect a system's underlying physics and those that reflect the assumptions of the employed conceptual model. To elucidate and address this challenge, we study networks that represent nonequilibrium quantum-electronic transport through quantum antidot devices—an example of an open, mesoscopic quantum system. The network representations correspond to two different models of internal antidot states: a single-particle, noninteracting model and an effective model for collective excitations including Coulomb interactions. In these networks, nodes represent accessible energy states and edges represent allowed transitions. We find that both models reflect spin conservation rules in the network topology through bipartiteness and the presence of only even-length cycles. The models diverge, however, in the minimum length of cycle basis elements, in a manner that depends on whether electrons are considered to be distinguishable. Furthermore, the two models reflect spin-conserving relaxation effects differently, as evident in both the degree distribution and the cycle-basis length distribution. Collectively, these observations serve to elucidate the relationship between network structure and physical constraints in quantum-mechanical models. More generally, our approach underscores the utility of network science in understanding the dynamics of quantum systems.

Photonic Greenberger-Horne-Zeilinger (GHZ) states serve as the central resource for a number of important applications in quantum information science, including secret sharing, sensing, and fusion-based quantum computing. The use of photon-emitter entangling gates is a promising approach to creating these states that sidesteps many of the difficulties associated with intrinsically probabilistic methods based on linear optics. However, the efficient creation of high-fidelity GHZ states of many photons remains an outstanding challenge due to both coherent and incoherent errors during the generation process. Here, we propose an entanglement concentration protocol that is capable of generating perfect GHZ states using only local gates and measurements on imperfect weighted graph states. We show that our protocol is both efficient and robust to incoherent noise errors.

We report an experimental and theoretical study of state-selective charge exchange processes in ${\mathrm{Ar}}^{8+}$ on He collisions at 1 and 3 keV/amu, benchmarking the fundamental electron capture dynamics under strong perturbations. The quantum-state selectivity has been experimentally resolved for one $1s$ electron of He capture into $4s$, $4p$, $4d+4f$, and $5s$ states of ${\mathrm{Ar}}^{7+}$ ion along with the corresponding scattering angle differential cross sections. By comparing to theoretical calculations with a two-active-electron semiclassical asymptotic-state close-coupling approach, we are able to verify the important role of electronic correlations during the collisions and that the impact parameter sensitive transition probability significantly mediates the state-selective specifics.

We report the development of a combined machine learning and high-throughput density functional theory (DFT) framework to accelerate the search for new ferroelectric materials. The framework can predict potential ferroelectric compounds using only elemental composition as input. A series of machine-learning algorithms initially predict the possible stable and insulating stoichiometries with polar crystal structures, necessary for ferroelectricity, within a given chemical composition space. A classification model then predicts the point groups of these stoichiometries. A subsequent series of high-throughput DFT calculations finds the lowest-energy crystal structure within the point group. As a final step, nonpolar parent structures are identified using group theory considerations, and the values of the spontaneous polarization are calculated using DFT. By predicting the crystal structures and the polarization values, this method provides a powerful tool to explore new ferroelectric materials beyond those in existing databases.

Transmon arrays are one of the most promising platforms for quantum information science. Despite being often considered simply as qubits, transmons are inherently quantum mechanical multilevel systems. Being experimentally controllable with high fidelity, the higher excited states beyond the qubit subspace provide an important resource for hardware-efficient many-body quantum simulations, quantum error correction, and quantum information protocols. Alas, dissipation and dephasing phenomena generated by couplings to various uncontrollable environments yield a practical limiting factor to their utilization. To quantify this in detail, we present here the primary consequences of single-transmon dissipation and dephasing to the many-body dynamics of transmon arrays. We use analytical methods from perturbation theory and quantum trajectory approach together with numerical simulations, and deliberately consider the full Hilbert space, including the higher excited states. The three main nonunitary processes are many-body decoherence, many-body dissipation, and heating/cooling transitions between different anharmonicity manifolds. Of these, the many-body decoherence—being proportional to the squared distance between the many-body Fock states—gives the strictest limit for observing effective unitary dynamics. Considering experimentally relevant parameters, including also the inevitable site-to-site disorder, our results show that the state-of-the-art transmon arrays should be ready for the task of demonstrating coherent many-body dynamics using the higher excited states. However, the wider utilization of transmons for ternary-and-beyond quantum computing calls for improving their coherence properties.

Two-dimensional (2D) Dirac materials are electronically and structurally very sensitive to proximity effects. We demonstrate, however, the opposite effect: that the deposition of a monolayer 2D material could exercise a substantial influence on the substrate electronic structure. Here we investigate TiC(111) and show that a graphene overlayer produces a proximity effect, changing the Fermi surface topology of the TiC from six electron pockets to one hole pocket on the depth of several atomic layers inside the substrate. In addition, the graphene electronic structure undergoes an extreme modification as well. While the Dirac cone remains gapless, it experiences an energy shift of 1.0 eV beyond what was recently achieved for the Lifshitz transition of overdoped graphene. Due to this shift, the antibonding ${\pi }^{*}$ band at the $\overline{\mathrm{M}}$ point becomes occupied and observable by photoemission.

Electrostatic charges placed near the interface between ordinary and topological insulators induce magnetic fields through the so-called topological magnetoelectric effect. Here we present a numerical implementation of the associated Maxwell equations. The resulting model is simple, fast, and quantitatively as accurate as the image charge method but with the advantage of providing easy access to elaborate geometries when pursuing specific effects. The model is used to study how magnetoelectric fields are influenced by the dimensions and the shape of the most common semiconductor nanostructures: quantum wells, quantum wires, quantum dots, and quantum rings. Pointlike charges give rise to magnetic fields of the order of mT, whose sign and spatial orientation are governed by the geometry of the nanostructure and the location of the charge. The results are rationalized in terms of the Hall currents induced on the surface, which constitute a simple yet valid framework for the deterministic design of magnetoelectric fields.

We present experimental data on the strong-field ionization of the argon dimer in a co-rotating two-color laser field. We observe a subcycle interference pattern in the photoelectron momentum distribution and infer the Wigner time delay using holographic angular streaking of electrons. We find that the Wigner time delay varies by more than 400 attoseconds as a function of the electron emission direction with respect to the molecular axis. The measured time delay is found to be independent of the parity of the dimercation and is in good agreement with our theoretical model based on the ionization of an aligned atomic $p$ orbital.

Manipulating quantum systems undergoing non-Gaussian dynamics in a fast and accurate manner is becoming fundamental to many quantum applications. Here, we focus on classical and quantum protocols transferring a state across a double-well potential. The classical protocols are achieved by deforming the potential, while the quantum ones are assisted by a counterdiabatic driving. We show that quantum protocols perform more quickly and accurately. Finally, we design a figure of merit for the performance of the transfer protocols—namely, the protocol grading—that depends only on fundamental physical quantities, and which accounts for the quantum speed limit, the fidelity, and the thermodynamics of the process. We test the protocol grading with classical and quantum protocols, and show that quantum protocols have higher protocol grading than the classical ones.

Dual scatterers preserve the helicity of an incident field, whereas antidual scatterers flip it completely. In this setting of linear electromagnetic scattering theory, we provide a completely general proof on the nonexistence of passive antidual scatterers. However, we show that scatterers fulfilling the refractive index matching condition flip the helicity of the fields very efficiently without being in contradiction with the law of energy conservation. Moreover, we find that this condition is paired with the impedance matching condition in several contexts of electromagnetism and, in particular, within Fresnel's and Mie's scattering problems. Finally, we show that index-matched media induce a resonant helicity mixing on the propagating electromagnetic waves. We reach this conclusion by identifying that the refractive index matching condition leads to the phenomenon of avoided crossing.

The ability of a gambling demon to extract work with a prescribed stopping threshold of cumulative work is analyzed theoretically and realized experimentally in the paradigm system of a Brownian particle in a time-dependent squeezing potential. The mechanism for the demon to beat the second law of thermodynamics (having a negative dissipative work or win) is sorted out in detail by analyzing the statistics of the stopped trajectories. The obtained insights, together with the associated nonequilibrium properties, further lead to improved strategies of the time-dependent stopping threshold to enhance the winning profit, which is confirmed experimentally.

X-ray free-electron lasers (XFELs) have brought new ways to probe and manipulate atomic and molecular dynamics with unprecedented spatial and temporal resolutions. A quantitative comparison of experimental results with their simulated theoretical counterpart, however, generally requires a precise characterization of the spatial and temporal x-ray pulse profile, providing a nonuniform photon distribution. The determination of the pulse profile constitutes a major, yet inevitable, challenge. Here, we propose a calibration scheme for intense XFEL pulses utilizing a set of experimental charge-state distributions of light noble gas atoms at a series of pulse energies in combination with first-principles simulations of the underlying atomic x-ray multiphoton ionization dynamics. The calibration builds on Bayesian optimization, which is a powerful, machine-learning-based tool particularly well suited for computationally expensive numerical optimization. We demonstrate the presented scheme to calibrate the pulse duration as well as the spatial fluence distribution profile of XFEL pulses. Our proposed method can serve as a comprehensive tool for characterizing ultraintense and ultrafast x-ray pulses.

In amorphous solids, there exists a universal phenomenon of vibrational anomaly, i.e., an excess of vibrational modes over the Debye level in the low-terahertz-frequency regime, which is termed a boson peak. Although the origin of the boson peak has been studied extensively for decades, quantitative prediction of its intensity remains elusive. In this paper, we try to quantify the intensity of the boson peak via an interpretable machine-learning strategy based on a dataset consisting of >1600 glass samples prepared with cooling rates spanning four orders of magnitude. We have attempted to extract information from a pure static structure and vibrational localization through four different feature inputs, among which the partial pair distribution function (PDF) yields the best predictive performance. It is found that the first several neighbor shells corresponding to a characteristic subnanometer length scale are important for capturing the structural genes of the boson peak in amorphous solids, regardless of the size of the glasses. Moreover, a higher boson peak relates to a more disordered atomic arrangement with lower peaks and shallower basins in the PDF. The obtained knowledge sheds light on rationalization of the boson peak in amorphous solids from pure structural information.