We have measured the widths and energies of the $1s2s2p{}^2{P}_{1/2,3/2}\to 1s^{2}2s{}^2{S}_{1/2}$ transitions in lithiumlike sulfur and argon, as well as the energies of the forbidden $1s2s2p{}^4{P}_{5/2}\to 1s^{2}2s{}^2{S}_{1/2} M2$ transition in both elements. All measurements were performed with a double-flat-crystal spectrometer without the use of any reference line. The transition energy measurements have accuracies ranging from 2.3 to 6.4 ppm depending on the element and line intensity. The widths and the intensity ratios of the $1s2s2p{}^2{P}_{1/2,3/2}\to 1s^{2}2s{}^2{S}_{1/2}$ lines have also been measured. These are reference-free measurements of transitions in core-excited lithiumlike ions and have an accuracy comparable to the best relative measurements. We have also performed multiconfiguration Dirac-Fock calculations of the widths, energies, and intensity ratios. An extensive comparison between existing experimental results and theory is performed, and Bayesian techniques are employed to extract the energy of the $1s2p^2{}^4{P}_{1/2}\to 1s^{2}2p{}^2{P}_{1/2}$ transition in sulfur and identify contaminant transitions.

${}^3\mathrm{He}$ beam spin-echo experiments have been used to study surface morphology, molecular and atomic surface diffusion, phonon dispersions, phason dispersions, and phase transitions of ionic liquids. However, the interactions between ${}^3\mathrm{He}$ atoms and surfaces or their adsorbates are typically isotropic and weak. To overcome these limitations, one can use molecules instead of ${}^3\mathrm{He}$ in surface spin-echo experiments. The molecular degrees of freedom, such as rotation, may be exploited to provide additional insight into surfaces and the behavior of their adsorbates. Indeed, a recent experiment has shown that orthohydrogen can be used as a probe that is sensitive to the orientation of a Cu(115) surface [O. Godsi, G. Corem, Y. Alkoby, J. T. Cantin, R. V. Krems, M. F. Somers, J. Meyer, G.-J. Kroes, T. Maniv, and G. Alexandrowicz, Nat. Commun. 8, 15357 (2017)]. However, the additional degrees of freedom offered by molecules also pose a theoretical challenge: a large manifold of molecular states and magnetic-field-induced couplings between internal states. Here, we present a fully quantum-mechanical approach to model molecular surface spin-echo experiments and connect the experimental signal to the elements of the time-independent molecule-surface scattering matrix. We present a one-dimensional transfer-matrix method that includes the molecular hyperfine degrees of freedom and accounts for the spatial separation of the molecular wave packets due to the magnetic control fields. We apply the method to the case of orthohydrogen, show that the calculated experimental signal is sensitive to the scattering matrix elements, and perform a preliminary comparison to experiment. This paper sets the stage for Bayesian optimization to determine the scattering matrix elements from experimental measurements and for a framework that describes molecular surface spin-echo experiments to study dynamic surfaces.

The investigation of ultracold atomic or molecular collision is important for us to understand the essence of a chemical reaction. We present a simple analytical model of arbitrary-order partial-wave scatterings for ultracold molecular collision in the threshold regime. The threshold formulas of the transmission and reflection coefficients are given. For high-partial-wave collision, shape resonance will take place because of the existence of a centrifugal barrier. We investigate the effect of shape resonance and short-range interaction on molecular collisions. Moreover, we find a method to fit the loss parameter from the experimental data of ultracold bosonic molecular collision. As an applied example, we calculate the thermally average loss rate for the ultracold ${}^{87}\mathrm{Rb}{}^{133}\mathrm{Cs}+{}^{87}\mathrm{Rb}{}^{133}\mathrm{Cs}$ collision, our results are consistent with the recent experimental and theoretical results of Gregory et al. [Nat. Commun. 10, 3104 (2019)].

The engineering of topological nontrivial states of matter, using cold atoms, has made great progress in the last decade. Driven by experimental successes, it has become of major interest in the cold-atom community. In this work we investigate the time-reversal-invariant Hofstadter model with an additional confining potential. By calculating a local spin Chern marker we find that topologically nontrivial phases can be observed in all considered trap geometries. This holds also for spin-orbit-coupled fermions, where the model exhibits a quantum spin Hall regime at half filling. Using dynamical mean-field theory, we find that interactions compete against the confining potential and induce a topological phase transition depending on the filling of the system. A further effect of strong interactions yields a magnetic edge, which is localized through the interplay of the density distribution and the underlying topological band structure.

Optical orbital angular momentum transformation and corresponding azimuthal-mode selection rules have been studied exhaustively for various nonlinear optical interactions. However, nonlinear transformation of the radial mode has not been systematically studied since the pioneering work [Phys. Rev. A 56, 4193 (1997)]. In this paper, we theoretically study and experimentally verify the radial modal transitions of Laguerre-Gauss (LG) modes in parametric up-conversion. Specifically, we provide a general solution that describes the sum-frequency generation (SFG) field excited by two arbitrary LG modes. Based on the solution, one can predict the full spatial complex amplitude of SFG fields upon propagation precisely and readily obtain the associated full-field selection rule including both azimuthal and radial modes. This paper provides a theoretical basis for quantum and nonlinear optical research involving parametric up-conversion of complex structured light and paves the way for future work on full-field transformation of spatial modes in other nonlinear interactions.

When a laser beam passes through a rotating ground glass (RGG), the scattered light exhibits thermal statistics. This is extensively used in speckle imaging. This scattering process has not been addressed in the photon picture and is especially relevant if nonclassical light is scattered by the RGG. We develop the photon picture for the scattering process using the Bose statistics for distributing $N$ photons in $M$ pixels. We obtain an analytical form for the $P$ distribution of the output field in terms of the $P$ distribution of the input field. In particular, we obtain a general relation for the $n\mathrm{th}$-order correlation function of the scattered light, i.e., $g_{\text{out}}^{\left(n\right)}\simeq n!g_{\text{in}}^{\left(n\right)}$, which holds for any order $n$ and for arbitrary input states. This result immediately recovers the classical transformation of coherent light to pseudothermal light by RGG.

We present detailed studies on dark-bright vector solitons formed in dispersion- and birefringence-managed fiber lasers sustained by either incoherent or coherent cross-polarization coupling. We show that even under strong influence of the gain bandwidth limitation, coupled dark-bright solitons can still be formed in a fiber laser. Unique features of the dissipative dark-bright vector solitons are studied both numerically and experimentally, and compared with those of the vector solitons formed in nonlinear Schrödinger equation systems.

Recently, an inequality satisfied by noncontextual hidden-variable models and violated by quantum mechanics for all states of a four-level system has been derived based on an information-theoretic distance approach to nonclassical correlations. In this Rapid Communication, we experimentally demonstrate the violation of this inequality with single photons. Our experiment offers a method to study a distinction between quantum and classical correlations from an information-theoretic perspective.

We observe the parity-unfavored photoelectron emission in multiphoton single ionization of krypton atoms in an intense ultraviolet laser field. With systematic experiments of successively varying the light ellipticity and intensity, we identify that this parity-unfavored emission is associated with the first excited ionic state via a resonant pathway. We reveal an abnormal Coulomb asymmetry of the parity-unfavored photoelectron emission in elliptically polarized fields when the ellipticity is less than 0.3. The simulation using the time-dependent Schrödinger equation based on single-active-electron approximation fails to reproduce the experiment. We attribute the experimental observations to the angular momentum correlation between the resonantly excited electron and the spin-orbit excited ion. Our results demonstrate that the well-known picture of the angular momentum transfer in multiphoton ionization is broken and instead that the electron shares the spin angular momenta of photons with the ion as an effect of electron correlations.

Wave-chaotic systems underpin a wide range of research activities from fundamental studies of quantum chaos via electromagnetic compatibility up to more recently emerging applications, such as microwave imaging for security screening, antenna characterization, or wave-based analog computation. To implement a wave-chaotic system experimentally, traditionally cavities of elaborate geometries (bow tie shapes, truncated circles, or parallelepipeds with hemispheres) are employed because the geometry dictates the wave field's characteristics. Here, we propose and experimentally verify a conceptually different approach: a cavity of regular geometry but with tunable boundary conditions, experimentally implemented by leveraging a reconfigurable metasurface reflect array. This approach offers an alternative stirring mechanism and enables a fuller study of random matrix theory in connection with wave chaos.

The excitation probabilities, concurrence, and relative entropy of coherence for an array of three coupled two-level systems are studied analytically and numerically at various temperatures. We determine the excitation probabilities for each site, the concurrence between the outer sites, and the relative entropy of coherence of the system for three energy configurations of the trimer and various temperatures. We find that a well configuration (i.e., negative detuning of the inner site) at all temperatures localizes the excitation on the inner site, while a barrier configuration (i.e., positive detuning of the inner site) causes the excitation to be localized on the outer sites. We determine the decay times for the probabilities, concurrence, and relative entropy of coherence for all energy configurations at each temperature. The barrier configuration creates the largest concurrence at zero temperature; however, we find that the uniform configuration is able to resist the loss of quantum coherence and entanglement at higher temperatures more so than any of the other configurations.

We study the phase transitions induced by sequentially measuring a single qubit precessing under an external transverse magnetic field. Under projective quantum measurement, the probability distribution of the measurement outcomes can be mapped exactly to the thermodynamic probability distribution of a one-dimensional Ising model, whose coupling can be varied by the magnetic field from ferromagnetic to antiferromagnetic. For the general case of sequential quantum measurement,we develop a fast and exact algorithm to calculate the probability distribution function of the ferromagnetic order and antiferromagnetic order, and a phase diagram is obtained in the parameter space spanned by the measurement strength and magnetic field strength. The mapping to a long-range interacting Ising model is obtained in the limit of small measurement strength. Full counting statistical approach is applied to understand the phase diagram and to make connections with the topological phase transition that is characterized by the braid group. This work deepens our understanding of phase transitions induced by quantum measurement and may provide another method to characterize and steer the quantum evolution.

The ability of quantum devices to preserve or distribute entanglement is essential in employing quantum technologies. Such ability is described and guaranteed by the non-entanglement-breaking (nonEB) feature of participating quantum channels. For quantum information applications relying on entanglement, the certification of the nonEB feature is indispensable in designing, testing, and benchmarking quantum devices. Here, we develop a simple and direct approach for the certification of nonEB quantum channels. By utilizing the prepare-and-measure test, we derive a necessary and sufficient condition for witnessing the nonEB channels, which is applicable in almost all experimental scenarios. The approach not only unifies and simplifies existing methods in the standard scenario and the measurement-device-independent scenario, but also furthers the nonEB channel certification in the semi-device-independent scenario.

Stabilizer states constitute a set of pure states which plays a dominant role in quantum error correction, measurement-based quantum computation, and quantum communication. Central in these applications are the local symmetries of these states. We characterize all local, invertible (unitary and nonunitary) symmetries of arbitrary stabilizer states and provide an algorithm which determines them. We demonstrate the usefulness of these results by showing that the additional local symmetries find applications in entanglement theory and quantum error correction. More precisely, we study a central problem in entanglement theory, which is concerned with the existence of transformations via local operations among pure states. We demonstrate that the identified symmetries enable additional transformations from a stabilizer state to some other multipartite pure state. Furthermore, we demonstrate how the identified symmetries can be used to construct stabilizer codes with diagonal transversal gates.

Many quantum state preparation methods rely on a combination of dissipative quantum state initialization followed by unitary evolution to a desired target state. Here we demonstrate the usefulness of quantum measurement as an additional tool for quantum state preparation. Starting from a pure separable multipartite state, a control sequence, which includes rotation, spin squeezing via one-axis twisting, quantum measurement, and postselection, generates highly entangled multipartite states, which we refer to as projected squeezed (PS) states. Through an optimization method, we then identify parameters required to maximize the overlap fidelity of the PS states with the maximally entangled Greenberger-Horne-Zeilinger (GHZ) states. The method leads to an appreciable decrease in the state preparation time of GHZ states for successfully postselected outcomes when compared to preparation through unitary evolution with one-axis twisting only.

Finite-time Markovian channels, unlike their infinitesimal counterparts, do not form a convex set. As a particular instance of this observation, we consider the problem of mixing the three Pauli channels, conservatively assumed to be quantum dynamical semigroups, and fully characterize the resulting “Pauli simplex.” We show that neither the set of non-Markovian (completely positive indivisible) nor Markovian channels is convex in the Pauli simplex, and that the measure of non-Markovian channels is about 0.87. All channels in the Pauli simplex are P divisible. A potential application in the context of quantum resource theory is also discussed.

Resonant inelastic x-ray scattering (RIXS) spectra excited at the oxygen K edge of ${\mathrm{CO}}_2$ are presented and discussed. Although excitation from a gerade initial state to the intermediate $1s^{-1}{\pi }^{*}$ state breaks the inversion symmetry due to strong vibronic coupling, RIXS excited at the corresponding resonance exclusively populates gerade vibrations in the gerade electronic ground state. This observation constitutes an experimental confirmation of the prediction that the parity selection rule applies in RIXS on an inversion-symmetric polyatomic system, provided that the total electronic-vibronic wave function is considered. Parity selectivity is used for assigning spectra to the population of electronically excited final states, a procedure hampered only when symmetry-breaking vibronic coupling in the final states is prominent. A RIXS spectrum excited in the Rydberg region is tentatively assigned using a simplified quasi-two-step model in which it is assumed that the electron in the Rydberg orbital excited in the first step remains as a spectator during the second decay step.

A state-of-the-art approach for calculating the finite-nuclear-size correction to atomic energy levels and the bound-electron $g$ factor is introduced and demonstrated for a series of highly charged hydrogenlike ions. First, self-consistent mean-field calculations based on the Skyrme-type nuclear interaction are employed in order to produce a realistic nuclear proton distribution. In the second step, the obtained nuclear charge density is used to construct the potential of an extended nucleus, and the Dirac equation is solved numerically. The ambiguity in the choice of a Skyrme parametrization is suppressed by fine-tuning of only one parameter of the Skyrme force in order to accurately reproduce the experimental values of nuclear radii in each particular case. The homogeneously-charged-sphere approximation, the two-parameter Fermi distribution, and experimental nuclear charge distributions are used for comparison with our approach, and the uncertainties of the presented calculations are estimated. In addition, suppression of the finite-nuclear-size effect for the specific differences of $g$ factors is demonstrated.

Two recent experimental observations at the ATOMKI Institute of the Hungarian Academy of Sciences (regarding the angular emission pattern of electron-positron pairs from nuclear transitions from excited states in ${}^8\mathrm{Be}$ and ${}^4\mathrm{He}$) indicate the possible existence of a particle of a rest mass energy of roughly 17 MeV. The so-called X17 particle constitutes a virtual state in the process, preceding the emission of the electron-positron pair. Based on the symmetry of the nuclear transitions ($1^{+}\to 0^{+}$ and $0^{-}\to 0^{+}$), the X17 could either be a vector, or a pseudoscalar particle. Here, we calculate the effective potentials generated by the X17, for hyperfine interactions in simple atomic systems, for both the pseudoscalar as well as the vector X17 hypotheses. The effective Hamiltonians are obtained in a general form which is applicable to both electronic as well as muonic bound systems. The effect of virtual annihilation and its contribution to the hyperfine splitting also is considered. Because of the short range of the X17-generated potentials, the most promising pathway for the observation of the X17-mediated effects in bound systems concerns hyperfine interactions, which, for $S$ states, are given by modifications of short-range (Dirac-$\delta$) potentials in coordinate space. For the pseudoscalar hypothesis, the exchange of one virtual X17 quantum between the bound lepton and the nucleus leads to hyperfine effects, but does not affect the Lamb shift. Effects due to the X17 are shown to be drastically enhanced for muonic bound systems. Prospects for the detection of hyperfine effects mediated by X17 exchange are analyzed for muonic deuterium, muonic hydrogen, muonium, true muonium (${\mu }^{+}{\mu }^{-}$ bound system), and positronium.

The stopping power of protons through Hf foil has been studied both experimentally and theoretically. The measurements were performed at the Laboratory of Accelerators and X-Ray Diffraction in Lisbon by using the transmission method on self-supporting stopping material. The overall uncertainty of around $5\%$ was established over the protons energy range (0.6–2.5) MeV. The theoretical developments involved fully relativistic atomic structure calculations for Hf, which required the solution of the Dirac equation. The shell-wise local plasma approximation was used to describe the energy transferred to the bound $1s\text{−}4f$ electrons, and the outer four electrons were considered as a free electron gas. We found the relativistic description of the $4f$ shell and the screening between $4f$ and $5p$ electrons to be decisive around the stopping maximum. Present theoretical and experimental results are in very good agreement in the energy region of the new measurements. However, our theoretical stopping cross sections show substantial differences with the most used semiempirical models (SRIM2013 and ICRU-49) at intermediate to low energies. Our calculations suggest the stopping maximum to be higher and shifted to lower energies than these previous predictions. Future measurements around the maximum and below would be necessary for a better understanding of the stopping power of hafnium.