In recent years, quantum battery (QB) models have emerged as crucial platforms for exploring the connections between quantum effects and energy within the framework of quantum thermodynamics. Inspired by the physical implementation of quantum switch (QS), we, in this work, present a simple QB model in which a two-level system as battery, under a series of coherent chargers, performs the indefinite causal order (ICO) charging by introducing a control qubit as QS. Within the energy-preserving collision interaction framework, we analyze the charging ICO and conventional definite causal order (DCO) dynamics, respectively, and discover the underlying connection between the final charged states of battery in ICO and DCO charging modes. That is, the ICO charged state can be understood as a split of the DCO charged state, accompanied by an offset caused by the interference component characterizing the ICO effect. That provides direct theoretical evidence that the ICO effect positively influences the ergotropy (i.e., the maximum extractable work) of QB only when quantum coherence is present in the charging process, and it is independent of the average stored energy of the QB. By combining numerical simulations, we examine that quantum coherence in our model is a strong quantum resource contributing to the ergotropy of QB, while the ICO effect plays a catalyst-like role, assisting quantum coherence in further enhancing the ergotropy of the battery. In addition, we also demonstrate that increasing the number of chargers also benefits the optimal ergotropy of QB. Finally, we reveal that, when the coherent chargers are subject to the dephasing channel, the ICO charging mode is more robust against dephasing noise than the DCO charging mode. This investigation deepens our understanding of quantum coherence and the ICO effect in the context of quantum thermodynamics.

Analog quantum computing with Rydberg atoms is seen as an avenue to solve hard graph optimization problems, because they naturally encode the maximum independent set (MIS) problem on unit disk (UD) graphs, a problem that admits rather efficient approximation schemes on classical computers. Going beyond UD-MIS to address generic graphs requires embedding schemes, typically with chains of ancilla atoms, and an interpretation algorithm to map results back to the original problem. However, interpreting approximate solutions obtained with realistic quantum computers proves to be a difficult problem. As a case study, we evaluate the ability of two interpretation strategies to correct errors in the recently introduced crossing lattice embedding. We find that one strategy, based on finding the closest embedding solution, leads to very high qualities, albeit at an exponential cost. The second strategy, based on ignoring defective regions of the embedding graph, is polynomial in the graph size, but it leads to a degradation of the solution quality which is prohibitive under realistic assumptions on the defect generation. Moreover, more favorable defect scalings lead to a contradiction with well-known approximability conjectures. Therefore, it is unlikely that a scalable and generic improvement in solution quality can be achieved with Rydberg platforms—thus moving the focus to heuristic algorithms.

Charge-exchange processes in collisions of ${\mathrm{C}}^{3+}\left(1s^{2}2\mathit{s}{}^{2}S\right)$ ions with hydrogen and deuterium atoms are investigated employing the full quantum-mechanical molecular-orbital close-coupling method in the large energy range of ${10}^{-5}$–10 keV/u. The required molecular potential curves and coupling matrix elements are calculated by the ab initio multireference single- and double-excitation configuration interaction method, and large basis sets with full active electrons are employed to obtain accurate asymptotic energy differences. Total and state-selective electron capture cross sections and rate coefficients are obtained and show satisfactory agreement with the experimental data. Total cross sections are found to increase as the collision energy decreases with the dominance of $2s3s$ ${}^{3}S$ and $2p^2{}^{1}S$ channels in the energy region below 0.1 keV/u, and then increase as the collision energy increases above 0.1 keV/u due to the contributions of higher states. Meanwhile, the total cross sections exhibit complex oscillatory behaviors in various energy regions. Below 1 eV/u, the dense oscillatory structures arise from quasibound vibrotational states in the entrance channel, and the isotope effect for hydrogen and deuterium targets is noticeable in this energy region. Around 10 eV/u, the oscillatory patterns originate from the interference between the $2p^2{}^{1}S$ channel and the entrance channel, while in the 0.5–3.0 keV/u energy range, the oscillatory structures primarily arise from the multichannel interference among the $2s3s$, $2s3p$, $2p3s$, and $2s4s$ states in both the triplet and singlet states of ${\mathrm{C}}^{2+}$ ions. The ratios of cross sections for electron capture to the triplet- and singlet-state manifolds are found to deviate from the spin-statistics rule bacause of the significant differences in electron capture to $2p^2$ and $2s3s$ electronic configurations between triplet and singlet states.

We present a comprehensive study of the electronic transitions in neutral cadmium (Cd i) with a focus on forbidden transitions, motivated by recent advances in laser technology and the growing relevance of cadmium in quantum gas research, precision metrology, and atom trapping. General analytic expressions are derived for transition matrix elements of all multipolar orders, formulated to be applicable for experimental use. Using configuration interaction combined with many-body perturbation theory, we calculate not only the previously reported contributions from electric dipole ($E1$) transitions, but also the electric quadrupole ($E2$), electric octupole ($E3$), magnetic dipole ($M1$), and magnetic quadrupole transitions ($M2$) that have not yet been investigated for cadmium. These matrix elements are then employed to determine the lifetimes of key excited states, particularly those pertinent to laser cooling and optical frequency standards, and to evaluate the long-range dispersion coefficient $C_6$. The linewidths of the strongest transitions, along with the atomic energy levels, are compared with available experimental data to validate the accuracy of the simulations. Overall, the results are in good agreement, with the calculated energy levels exhibiting an average relative deviation of $0.3\%$ from experiments. These values serve as benchmarks for both bosonic and fermionic isotopes, providing a foundation for future experimental and theoretical work in cadmium-based precision spectroscopy and cold-collision studies.

We investigate the phenomenon of electron-positron pair production resulting from the interaction of neutral xenon gas with a high-intensity laser field. The pairs are generated through a combination of nonlinear Compton scattering and the Breit-Wheeler mechanism. We examine the pair-production process in a single circularly polarized laser pulse and in a combination of two counterpropagating pulses with opposite circular polarizations. Special focus is placed on analyzing the role of an additional external magnetic field. It is demonstrated that one can significantly enhance the positron yield by introducing a constant magnetic field, the strength of which is of the same order as the laser amplitude. We show that the magnetic field modifies the electron dynamics and gives rise to much larger values of the corresponding nonlinearity parameters for the particles. Within the framework of quantum electrodynamics, it is well known that these quantities predominantly govern the rates of photon emission and pair production; therefore, the magnetic field can considerably stimulate the mechanism of positron creation. In this context, we also discuss experimentally relevant scenarios where such strong magnetic fields may arise and prospects for determining laser intensity by measuring the number of positrons produced.

This study reports the design and optimization of a single-layer, all-dielectric chiral metasurface based on Mie resonances. The metasurface comprises dielectric cylinders with varying radii and exhibits strong performance in the near-infrared region, showing pronounced circular dichroism and optical activity. Its performance is analyzed using exact Mie resonance decomposition to calculate the scattering cross section and the phase of the electric dipole mode. The metasurface achieves a transmittance difference of 0.644 between right- and left-circularly polarized light at 1520 nm and a maximum optical activity of 150 °/λ, despite a thickness of only 580 nm (about one-third of the target wavelength). These results demonstrate that a single-layer metasurface can deliver optical performance comparable to more complex chiral structures, highlighting its efficiency and potential for practical optical applications. Moreover, the design strategy can be extended to other chiral metasurfaces by varying the arrangement and spacing of the dielectric cylinders, offering advantages in fabrication simplicity and cost effectiveness. This work advances the development of next-generation chiral devices for sensing, polarization control, and nonlinear optical applications.

We analyze the non-Markovian spontaneous-emission dynamics of a two-level test atom placed in a cavity formed by two atomic arrays in a waveguide quantum electrodynamics (QED) setup. We demonstrate a crossover from single-mode to multimode strong-coupling cavity QED as the cavity length $\sim d$ becomes comparable to the coherence length associated with collective spontaneous emission $\sim v/\left(N\gamma \right)$. The resulting non-Markovian dynamics of the test atom and the emergent spectral density of the field are analyzed as a function of various tunable atomic array parameters: the number of atoms, length of the atomic cavity, and resonance frequency of the atoms forming the atomic mirrors. Our results show limitations to cooperatively enhanced light-matter coupling in the presence of time-delayed feedback. We further illustrate that the non-Markovian system dynamics can be efficiently approximated in terms of a few modes of the emergent spectral density of the field.

A study of an open nondegenerate parametric oscillator is presented below threshold in the undepleted pump regime with time-delayed coherent feedback. The figure of merit is the two-mode squeezing spectrum measuring the strength of continuous-variable Einstein-Podolsky-Rosen-type entanglement between the down-converted modes in the output field as a function of frequency. Exploring different areas of the parameter space reveals the possibility of significantly enhanced entanglement with tunable characteristic frequency. This is a result of the emerging dynamical features that are only accessible due to delayed feedback. The system is very sensitive to phase matching, which can be adjusted by various parameters of the system and the feedback loop. In this way, sharp, resonancelike features are obtained that show a level of robustness against extraneous losses.

Time-independent scattering methods are widely employed to analyze transport in non-Hermitian systems. Their application, however, rests on a critical yet often overlooked assumption: that an incident wave is a pure superposition of scattering states. In practice, any physically realistic, spatially localized wave packet will generally have a nonzero overlap with the system's bound states, thereby violating this premise. While this violation is inconsequential in Hermitian systems, it can invalidate the conventional scattering picture in their non-Hermitian counterparts. The underlying cause is the emergence of time-growing bound states, which manifest as poles of the scattering matrix ($S$ matrix) in the first quadrant of the complex wave-number plane. Any initial overlap with these states becomes exponentially amplified, eventually dominating the long-time dynamics. Consequently, the actual evolution of a wave packet diverges dramatically from the conventional scattering picture, rendering the transmission and reflection coefficients derived from time-independent scattering methods unphysical. Using tight-binding models with non-Hermiticity introduced via imaginary on-site potentials or asymmetric hopping, we demonstrate that parameter regimes supporting such growing states are common. We therefore conclude that an analysis of $S$-matrix poles is an indispensable step to confirm the physical relevance of time-independent scattering calculations in non-Hermitian systems.

Quantum teleportation using continuous-variable cluster states is a promising method for quantum state transmission. Experiments show photon subtraction improves the fidelity of two-mode-entangled state-based teleportation, but its effect on universal quantum teleportation with cluster states is unclear. This study clarifies this issue. We create a characteristic function-based representation for universal teleportation, combining it with the Hermite polynomial representation to analyze the impact of photon subtraction. Our analysis finds that the enhancement depends on the photon subtraction modes and the initial squeezing parameter of cluster states. Contrary to the two-mode case, subtracting photons from all cluster state modes does not improve fidelity. By analyzing the equivalent squeezing of initial states after photon subtraction, we determine that squeezing improvement decides teleportation quality. A remarkable finding reveals that performing photon subtraction on two modes of a multirail cluster state can enhance equivalent squeezing of all modes, offering new possibilities for quantum information processing tasks.

We investigate the dynamics of entanglement in dissipative fermionic and bosonic Su-Schrieffer-Heeger (SSH) models and discover that they exhibit a revival dynamics of entanglement when the decoherence channel preserves the chiral symmetry. This behavior is only observable in the topological phase, and the visibility of the revival diminishes to zero at the phase boundary. Furthermore, the revival acquires a universal character, meaning that the entire time-evolution profile remains independent of system size, provided that the size exceeds the localization length of the edge modes associated with the topological phase. Our findings suggest that the universal entanglement revival originates from the topological properties of the SSH model. These dynamical properties may be experimentally accessible, for instance, by utilizing photonic quantum computers.

In Phys. Rev. A 108, L060402 (2023), we introduced a Bayesian measurement error mitigation algorithm, which leveraged complete information from the readout signal, and validated the protocol on a quantum device with five superconducting qubits. Here, we present an improved algorithm's implementation, tailored for multiqubit experiments on near-term superconducting qubit quantum devices. In particular, we provide a detailed algorithm workflow, from calibrating the detector response functions to the postprocessing of measurement outcomes, offering a computationally efficient solution for the output size typical of current quantum computing devices. We show how the numerical representation of the noise function affects the performance of the error mitigation algorithm and test the convergence criteria. We benchmark our protocol on actual quantum computers with superconducting qubits, where the readout signal encodes the measurement information as unprocessed analog data before qubit state assignment. Finally, we compare the performance of our algorithm against other measurement error mitigation methods, such as iterative Bayesian unfolding and the Mthree method, and show how our method can be integrated on top of other readout error mitigation protocols.

Angular entanglement is investigated theoretically in various realizable regimes of SPDC. It is shown, in particular, that in the sufficiently well pronounced, noncollinear regime the degree of angular entanglement grows with increasing degree of noncollinearity. A rather specific dependence is found for the degree of entanglement in its dependence on the varying influence of the so-called walk-off effect of the pump in the SPDC crystal. A joint influence of the walk-off and noncollinearity is found to partly suppress the degree of angular entanglement.

We investigate the hierarchy of quantum correlations in a quadratic bosonic parity-time-symmetric system (PTSS) featuring distinct dissipation and amplification channels. The hierarchy includes global nonclassicality, entanglement, asymmetric quantum steering, and Bell nonlocality. We elucidate the interplay between the system physical nonlinearity (which serves as a source of quantumness) and the specific dynamics of bosonic PTSSs, which are qualitatively influenced by their damping and amplification characteristics. Using a set of quantifiers (including local and global nonclassicality depths, negativity, steering parameters, and the Bell parameter) we demonstrate that the standard PTSS typically exhibits weaker quantumness than its counterparts affected solely by damping or solely by amplification. Both the maximum values attained by these quantifiers and the speed and duration of their generation are generally lower in the standard PTSS. A comparative analysis of three two-mode PTSSs (standard, passive, and active) with identical eigenvectors and real parts of eigenfrequencies, but differing in their damping and amplification strengths, reveals the crucial role of quantum fluctuations associated with gain and loss. Among them, the passive PTSS yields the most strongly nonclassical states. Nevertheless, under suitable conditions, the standard PTSS can also generate highly nonclassical states. The supremacy of the passive PTSS is further supported by its fundamental advantages in practical realizations.

We study a quantized, discrete, and drifting version of the Harper Hamiltonian, also called the finite almost Mathieu operator, which resembles the pendulum Hamiltonian but, in phase space, is confined to a torus. Spacing between pairs of eigenvalues of the operator spans many orders of magnitude, with nearly degenerate pairs of states at energies that are associated with circulating orbits in the associated classical system. When parameters of the system slowly vary, both adiabatic and diabatic transitions can take place at drift rates that span many orders of magnitude. Only under an extremely negligible drift rate would all transitions into superposition states be suppressed. The wide range of energy-level spacings could be a common property of quantum systems with nonlocal potentials that are related to resonant classical dynamical systems. Notions for adiabatic drift are discussed for quantum systems that are associated with classical ones with a divided phase space.

Understanding dissipation in open quantum systems is crucial for the development of robust quantum technologies. In this work, we introduce a transformer-based machine learning framework to infer time-dependent dissipation rates in systems governed by the Lindblad master equation. Our method requires only time series of observable quantities, such as $\langle {\sigma }_x\left(t\right)\rangle$, $\langle {\sigma }_y\left(t\right)\rangle$, and $\langle {\sigma }_z\left(t\right)\rangle$, and does not require prior knowledge of the initial quantum state or the system Hamiltonian during inference. We demonstrate the effectiveness of our approach across a hierarchy of open quantum models, including single-qubit systems with time-independent and time-dependent jump rates, two-qubit interacting systems (e.g., Heisenberg and transverse Ising models), and the Jaynes-Cummings model with cavity loss and time-dependent decay. Our method accurately reconstructs both fixed and time-varying dissipation profiles from observable trajectories. We further show that under reasonable assumptions, the jump rates in these models are uniquely determined by a small set of measurable observables. The results suggest that transformer-based learning provides a scalable and data-driven route for identifying environmental effects in open quantum systems.

A single excitation in a quantum spin network described by the Heisenberg model can affect a variety of continuous-time quantum walks on unweighted graphs, including those governed by the discrete Laplacian, adjacency matrix, and signless Laplacian. In this paper, we show that the Heisenberg model can affect these three quantum walks on signed weighted graphs, as well as a generalized Laplacian equal to the discrete Laplacian plus a real-valued multiple of the degree matrix, for which the standard Laplacian, adjacency matrix, and signless Laplacian are special cases. We explore the algorithmic consequence of this generalized Laplacian quantum walk when searching a weighted barbell graph consisting of two equal-sized, unweighted cliques connected by a single signed weighted edge or bridge, with the search oracle constituting an external magnetic field in the spin network. We prove that there are two weights for the bridge (which could both be positive, both negative, or one of each, depending on the multiple of the degree matrix) that allow amplitude to cross from one clique to the other—except for the standard and signless Laplacians that respectively only have one negative or positive weight bridge—boosting the success probability from 0.5 to 0.820 or 0.843 for each weight. Moreover, one of the weights leads to a two-stage algorithm that further boosts the success probability to 0.996.

The complex Rydberg structure of ytterbium atoms is shaped by multiple low-lying ion-core-excited states and strong channel interactions, which presents both opportunities and challenges for quantum information processing and precision metrology. In this work, we extend high-resolution microwave spectroscopy and multichannel quantum defect theory (MQDT) modeling of singly excited $6sn\ell$ Rydberg states in ${}^{174}\mathrm{Yb}$ and ${}^{171}\mathrm{Yb}$ to include the $\ell =3$ ($f$) and $\ell =4$ ($g$) series. Our measurements reveal $p\text{−}f$ mixing in odd-parity Rydberg states of ${}^{171}\mathrm{Yb}$, which we incorporate by combined MQDT models for $6snp$ and $6snf$ series. Additionally, we observe that for $\ell =4$ the spin-orbit interaction dominates over the exchange interaction, such that the $6sng$ states are more accurately described in a $jj$-coupled basis. We validate our models by comparing the predicted Landé $g$ factors and static dipole polarizabilities with experimental measurements, finding excellent agreement. These results provide important input for designing high-fidelity entangling gates with ytterbium atoms.