The Born series applied to the Lippmann-Schwinger equation is a straightforward method for solving optical scattering problems, which however diverges except for very weak scatterers. Replacing the Born series by Padé approximants is a solution of this problem. However, in some cases it is rather difficult to obtain an accurate Padé approximant. In this paper we aim to understand the cause by studying the scattering by a cylinder. We find that there is a strong connection between eigenvalues of the Lippmann-Schwinger operator that are close to the real axis, the occurrence of a near-resonance, and the problematic behavior of the Padé approximant. The determination of these eigenvalues provides a general method to obtain, for any given geometry, materials for which near-resonances occur.

The generation of elliptically polarized attosecond pulses has become a crucial issue due to the growing interest in its application to probing circular dichroism and chirality in matter. Here, we propose a method capable of generating isolated attosecond pulses (IAPs) with near-circular polarization. Through the combination of the mixed gases scheme with a polarization gating laser field, we obtain an IAP with an ellipticity of 0.91 and a pulse duration of 290 as. Our analysis indicates that the polarization of attosecond pulses is predominantly influenced by the mixed gases, specifically by the selective interference between the recombination dipole moments of different gas components. In contrast to polarization, the temporal profile of attosecond pulses mainly depends on the laser field. Our approach achieves independent control of the polarization and temporal properties of IAPs by adjusting the gas mixture and the polarization gating laser field, respectively, thus paving an effective and convenient way to generate and control elliptically polarized IAPs.

Quantum technology is highly relevant to manipulating photon states, for which the quantum waveguide serves as a primary building block. It is a vital task to explore the statistics of tens-of-photons states in quantum waveguides coupled to quantum emitters, such as optical cavities (OCs), two-level atoms (TLAs), and Jaynes-Cummings emitters (JCEs). However, the related theoretical framework has not been established. Here, we use matrix-product-state theory and show that although OCs do not change the second-order photon-photon correlation $g^{\left(2\right)}$, they can tune the occupation ratio of bunching photons $\eta$. What is more, the states scattered by TLAs and JCEs exhibit extremely different statistical behaviors in tens-of-photons cases with respect to those in few-photon ones. The scattering effects from JCEs tend to those from OCs as photon number $n\to \infty$, and photon-photon correlation due to JCEs reaches a maximum value as $n$ increases. We anticipate these distinguishable results for tens-of-photons states will be a starting point for multiphoton manipulation in quantum waveguides.

The distinguishability between two quantum states can be defined in terms of their trace distance. The operational meaning of this definition involves a maximization over measurement projectors. Here we introduce an alternative definition of distinguishability which, instead of projectors, is based on maximization over normalized states (density matrices). It is shown that this procedure leads to a distance (between two states) that in contrast to the usual approach based on a 1-norm, is based on an infinite-norm. Properties such as convexity, monotonicity, and invariance under unitary transformations are fulfilled. Equivalent operational implementations based on maximization over classical probabilities and hypothesis-testing scenarios are also established. When considering the action of completely positive transformations, contractivity is only granted for unital maps. This feature allows us to introduce a measure of the quantumness of nonunital maps that can be written in terms of the proposed distinguishability measure and corresponds to the maximal possible deviation from contractivity. Particular examples sustain the main results and conclusions.

Quantum reservoir computing algorithms recently emerged as a standout approach in the development of successful methods for the noisy intermediate-scale quantum (NISQ) era because of its superb performance and compatibility with current quantum devices. By harnessing the properties and dynamics of a quantum system, quantum reservoir computing effectively uncovers hidden patterns in data. However, the design of the quantum reservoir is crucial to this end in order to ensure an optimal performance of the algorithm. In this work, we introduce a precise quantitative method with strong physical foundations based on the Krylov evolution to assess the wanted good performance in machine-learning tasks. Our results show that the Krylov approach to complexity strongly correlates with quantum reservoir performance, making it a powerful tool in the quest for optimally designed quantum reservoirs, which will pave the road to the implementation of successful quantum machine-learning methods.

Postcollision interaction (PCI) in near-threshold photoionization of the $K$ shell of atomic Xe is investigated with hard-x-ray synchrotron radiation. Applying Auger electron spectroscopy, line-shape distortion is monitored in the $K\text{−}{L}_2\to L_2{M}_{4,5}{M}_{4,5}$ decay channel, where the initial $K\text{−}{L}_2$ ($K{\alpha }_2$) x-ray-emission step mediates the emission of the two continuum electrons. The $K$-edge result is compared to similar data obtained on direct $L_2{M}_{4,5}{M}_{4,5}$ decay at the $L_2$ edge without x-ray-emission. It is found that the evolution of the PCI shift with excess photon energy is very similar at both edges, but a subtle decrease in the PCI shift is observable at the $K$ edge due to the effect of the ultrafast x-ray emission step delaying the PCI energy exchange by a few attoseconds.

We investigate theoretically the high-order harmonic generation of the molecular ion ${\text{H}}_3{}^{2+}$ by numerically solving the two-dimensional time-dependent Schrödinger equation. The results show that a resonant harmonic with large ellipticity is generated and the harmonic efficiency is significantly enhanced when the infrared laser field is combined with a time-delayed attosecond extreme-ultraviolet (XUV) laser pulse. The generation of resonant harmonics can be understood as the electron transition between the ground and excited states through the absorption of XUV photons. By varying the time delay and amplitudes of the two laser pulses, we can obtain the harmonics close to circular polarization in the resonant region. The input XUV pulse is linearly polarized, while the generated resonant harmonics are elliptically polarized, which means that the input XUV pulse can be reshaped. Moreover, we investigate the dependence of the resonant harmonic on the laser polarization, which shows the sensitivity of the harmonic polarization to the laser polarization and the resonance state can also be changed by changing the direction of polarization of the laser field.

Helical filamentation of optical fields has gained significant attention due to its potential physical properties, complexity, and practicality. However, conventional methods of manipulating filament formation face a major challenge in achieving controllable helical filamentation. We propose a method to manipulate helical collapse based on a nonlinearity-related Gouy phase shift. This method is triggered by the nonlinear interaction between vector vortex beams and anisotropic Kerr media. It is uncovered that, in the absence of the vortex, the radially polarized beam can only form the multifilament at specific locations in a ${\mathrm{BaF}}_2$ crystal. Nevertheless, once the vortex is attached, the beam can rotate based on the multifilament. The direction and angle of the rotation can be controlled by varying the topological charge number of the vortex phase. In addition, spin-orbit coupling and a rotational nonlinear refractive phase shift are generated during the helical collapse process. Finally, the physical mechanism behind this helical collapse is illustrated by a nonlinearity-related Gouy phase shift. This research presents a different approach to controlling optical field collapse and filamentation, with potential applications in various fields, including optical communications, optical manipulation, and optical microprocessing.

Finding ways to transform a quantum state to another is fundamental to quantum information processing. In this paper, we apply the sparse matrix approach to the quantum state transformation problem. In particular, we present an approach for searching for unitary matrices for quantum state transformation by directly optimizing the objective problem using the alternating direction method of multipliers. Moreover, we consider the use of group sparsity as an alternative sparsity choice in quantum state transformation problems. Our approach incorporates sparsity constraints into quantum state transformation by formulating it as a nonconvex problem. It establishes a useful framework for efficiently handling complex quantum systems and achieving precise state transformations.

Nonresonant two-photon Raman transitions with $\mathrm{\Delta }J=\pm 2$ selection rules induce molecular alignment by altering the rotational quantum number $J$, whereas three-photon Raman transitions with $\mathrm{\Delta }J=\pm 1$ cause molecular orientation. Here, we perform a combined numerical and theoretical investigation to explore how these Raman transitions depend on laser intensity and to discern multiorder Raman transitions in laser-induced alignment and orientation going beyond two- and three-photon processes. Our results demonstrate the presence of higher-order transitions in molecular alignment and shed light on how alignment affects subsequent three-photon rotational Raman transitions as revealed by the alignment and orientation spectra and the Dyson series expansion of the unitary operator. As the laser pulse strength increases, multiorder transitions become directly observable, which can be utilized to enhance the alignment and orientation of molecules through the excitation of multiple rotational states.

While there exist theories that have states more strongly entangled than quantum theory, in the sense that they show Clauser-Horne-Shimony-Holt (CHSH) values above Tsirelson's bound, all known examples of such theories have a strictly smaller set of measurements. Therefore, in tasks that require both bipartite states and measurements, they do not perform better than quantum mechanics. One of the simplest information processing tasks involving both bipartite states and measurements is that of entanglement swapping. In this paper, we study entanglement swapping in generalized probabilistic theories (GPTs). In particular, we introduce the iterated CHSH game, which measures the power of a GPT to preserve nonclassical correlations, in terms of the largest CHSH value obtainable after $n$ rounds of entanglement swapping. Our main result is the construction of a GPT that achieves a CHSH value of 4 after an arbitrary number of rounds. This addresses a question about the optimality of quantum theory for such games recently raised by Weilenmann and Colbeck. One challenge faced when treating this problem is that there seems to be no general framework for constructing GPTs in which entanglement swapping is a well-defined operation. Therefore, we introduce an algorithmic construction that turns a bipartite GPT into a multipartite GPT that supports entanglement swapping, if consistently possible.

We provide an architecture for a multimode quantum battery based on the framework of continuous-variable systems. We examine the performance of the battery by using a generic class of multimode initial states whose parameters can be tuned to produce separable as well as entangled states and that can be charged locally as well as globally by Gaussian unitary operations. Analytical calculations show that a separable state is equally advantageous as an entangled one for two- and three-mode batteries when taking the figures of merit as the second moments of the change in energy. In order to produce a stable quantum battery consisting of an arbitrary number of modes, we derive compact analytical forms of the energy fluctuations and prove that for a multimode separable Gaussian initial state, fluctuations decrease as the number of modes increases, thereby obtaining a scaling analysis. Moreover, we demonstrate that local displacement as a charger is better for minimizing the fluctuations in energy than that involving the squeezing unitary operation.

A normalizing flow computes a bijective mapping from an arbitrary distribution to a predefined (e.g., normal) distribution. Such a flow can be used to address different tasks, e.g., anomaly detection, once such a mapping has been learned. In this work we introduce normalizing flows for quantum architectures, describe how to model and optimize such a flow, and evaluate our method on example datasets. Our proposed models show competitive performance for anomaly detection compared to classical methods, especially those ones where there are already quantum inspired algorithms available. In the experiments we compare our performace to isolation forests (IF), the local outlier factor (LOF), or single-class SVMs.

Secure multiparty quantum computation (MPQC) protocol is a cryptographic primitive allowing error-free distributed quantum computation to a group of $n$ mutually distrustful quantum nodes even when some quantum nodes disobey the instructions of the protocol. Here we suggest a modified MPQC protocol that adopts unconventional quantum error-correcting codes and as a consequence reduces the number of qubits required for the protocol execution. In particular, the replacement of the self-dual Calderbank-Shor-Steane quantum error-correcting codes with triply even ones permits us to avoid the previously indispensable but resource-intensive procedure of the “magic-state” verification. Besides, since every extra qubit reduces the credibility of physical devices, our suggestion makes the MPQC protocol more accessible for the near-future technology by reducing the number of necessary qubits per quantum node from $n^2+\mathrm{\Theta }\left(r\right)n$, where $r$ is the security parameter, to $n^2+3n$.

Using the Bethe ansatz (BA), we rigorously obtain nonequilibrium dynamics of an impurity with a large initial momentum $Q$ in the one-dimensional interacting bosonic medium. We show that magnonlike and excitonlike states obtained from the BA equations drastically determine the oscillation nature of the quantum flutter with the periodicity given by ${\tau }_{\mathrm{QF}}=2\pi /\left[\right|{\varepsilon }_{\mathrm{c}}\left(0\right)|-|{\varepsilon }_{\mathrm{s}}\left(0\right)\left|\right]$, where the charge and spin dressed energies ${\varepsilon }_{\mathrm{c},\mathrm{s}}\left(0\right)$ are precisely given by the thermodynamical BA equations. While we further find a persistent revival dynamics of the impurity with a larger periodicity ${\tau }_L=L/[v_{\mathrm{c}}(Q-k^{*})-v_{\mathrm{s}}\left(k^{*}\right)]$ than ${\tau }_{\mathrm{QF}}$, manifesting a quantum reflection induced by the periodic boundary conditions of a finite length $L$, here $v_{\mathrm{c},\mathrm{s}}$ are the sound velocities of charge and spin excitations, respectively, and $k^{*}$ is a characteristic momentum of the impurity to the Fermi point. Finally, we study the application of such a magnon impurity as a quantum resource for measuring the gravitational force.

We investigate the ground-state properties of the quasi-one-dimensional dipolar gases using continuous matrix product states techniques. Making use of the first- and second-order correlation functions, we find that the system supports the superfluid, super-Tonks-Girardeau, and quasicrystal phases according to the Luttinger liquid theory. We also map out the phase diagram on the parameter plane consisting the contact and dipolar interaction strengths. Furthermore, we compute the Luttinger parameter, the structure factor, and the momentum distribution of the system. Finally, we show that the predicted dipolar effect can potentially be observed in quasi-one-dimensional gases of polar molecules.