Precision measurement of ${\mathrm{Mg}}^{+}$ ion $D_1$ and $D_2$ doublet transition frequencies plays a significant part in the study of space-time variation of fundamental physical constant $\alpha$. Here, we report a precision measurement of ${}^{25}\mathrm{Mg}^{+}$ ion $D_1$ and $D_2$ doublet transition frequencies using the decoherence-assisted spectroscopy method with the full use of spontaneous emission signals to improve the detection sensitivity. We obtain the $D_1\text{and}{D}_2$ transition frequencies of ${}^{25}\mathrm{Mg}^{+}$ with uncertainties of 0.12 and 0.24 MHz, respectively, which are one third of that in [A. Ozawa et al., Nat. Commun. 8, 44 (2017)]. Simultaneously, we obtain the experimental and theoretical values of the ${}^{25}\mathrm{Mg}^{+}$ hyperfine structure constants in this work. The differences between the measured magnetic dipole constants $A_{P_{1/2}}=-102.87\left(8\right)$ MHz, $A_{P_{3/2}}=-18.74\left(9\right)$ MHz and the theoretical values $A_{P_{1/2}}=-102.02\left(94\right)$ MHz, $A_{P_{3/2}}=-19.46\left(45\right)$ MHz are within $2\sigma$, while the measured electric quadrupole constant $B_{P_{3/2}}=24.54\left(45\right)$ MHz and the theoretical value of $B_{P_{3/2}}=22.67\left(32\right)$ MHz shows a difference of more than $2\sigma$, indicating that considerations of more physical effects in theoretical calculation, such as the possible capture of electron correlations, is required.

We express the photoionization cross section of an atomic or molecular shape resonance in the form $\sigma \propto {sin}^2{\delta }_{\ell }$ which connects the cross section with the photoelectron scattering phase in the dominant partial wave $\ell$. This expression allows us to relate the cross section with the photoelectron group delay (Wigner time delay) ${\tau }_{\mathrm{W}}=\partial {\delta }_{\ell }/\partial E$ which is measurable by recently developed laser interferometric techniques. Such a direct connection is a fundamental property of shape resonances which provides a comprehensive test of time delay measurements against a large body of older synchrotron cross-section data.

We investigate the question of whether there are any appreciable polarization contributions to the total bremsstrahlung in electron-impact solid thin-film experiments. Our results definitively show no evidence of a polarization contribution. The bremsstrahlung double-differential cross sections at a ${90}^{\circ }$ photon emission angle for collisions of (10–25)-keV electrons with four medium- and high-$Z$ thin-film solid targets are measured, in which the photon energy range is extended down to approximately 3 keV for the solid thin-film experiments. Comparisons of the experimental values with two theories, i.e., ordinary bremsstrahlung (OB) and stripping approximation including polarization bremsstrahlung, are made and good agreement with the OB theory is found. The suppression mechanism of polarization bremsstrahlung in the solid-target experiments will need further theoretical studies.

We report on the existence and stability of $\pi$-mode solitons in both one-dimensional (1D) and two-dimensional (2D) nonlinear Su-Schrieffer-Heeger (SSH) arrays with periodic longitudinal modulation that mimics temporal periodic driving in Floquet systems. The SSH array is a paradigmatic example of the topological insulator, where edge states appear for the proper ratio of the intra- and intercell couplings. When the SSH array is additionally periodically driven due to longitudinal oscillations of waveguide centers, so that for half of the driving cycle it is in trivial phase, while on other half it is in topological phase, a new type of anomalous topological $\pi$-mode emerges at the edges of the driven lattice. We consider $\pi$-mode solitons with propagation constants in the gap of this equivalent Floquet system bifurcating under the action of nonlinearity from anomalous linear $\pi$-mode states. In the 1D case such periodically oscillating solitons become more robust with an increase of the amplitude of oscillations of waveguide positions and survive over hundreds of longitudinal lattice periods. We also found that they can be very robust in the 2D equivalent Floquet SSH arrays. Furthermore, we show that $\pi$-mode solitons can be directly excited by Gaussian beams launched into the array at the proper distance. Our results suggest a framework for experimental observation of the $\pi$-mode solitons, including in higher-order topological Floquet systems.

Resonances between core-hole states have been long hypothesized to understand nonlinear responses of atoms and molecules exposed to intense x-ray free-electron laser (XFEL) pulses, but evaded clear identification in previous studies due to their transient nature. Here we report clear evidence of core-to-core resonances by state-of-the-art electron-ion coincidence spectroscopy on Kr atoms. The observed $3p\text{−}3d$ resonance manifested the significance in multiple ionization to highly charged states against the ultrafast core-hole decay. The present study, elucidating the role of the core-to-core resonance in electronic responses of matter to high-frequency laser fields, will advance our understanding of nonlinear spectroscopy and ultrafast imaging by ultrashort intense EUV and x-ray pulses.

The combination of driving and dissipation guides a quantum system into a nonequilibrium steady state (NESS). It is an intriguing question, in how far this principle can be exploited for the robust preparation of interesting many-body target states beyond the strict constraints of thermal equilibrium. We consider an open system of ultracold bosonic atoms coupled to a heat bath and show that, counterintuitively, the interplay of bath-induced dissipation and controlled Floquet (i.e., driving-induced) heating can give rise to nonequilibrium Bose condensation in a quantum-scar-like mode protected from the drive. In particular, we use Floquet-Born-Markov theory to microscopically derive kinetic equations for a one-dimensional system of bosonic atoms in an optical lattice of finite extent coupled to a three-dimensional thermal bath of weakly interacting bosons treated in Bogoliubov theory. The bath temperature $T$ is assumed to lie well above the crossover temperature, below which the majority of the system's particles form a (finite-size) Bose condensate in the single-particle ground state in equilibrium. However, when a strong local potential modulation is switched on, which resonantly excites the system, a nonequilibrium Bose condensate is formed in an excited state that decouples from the drive. This strategy of engineering the NESS of an open quantum system via tailored Floquet heating is likely to find applications also for different systems and target states.

We theoretically study circular dichroism of chiral molecules embedded inside a helicity-preserving Fabry-Pérot cavity. We find an increase of the intrinsic chiroptical response of the molecules by 2 orders of magnitude and report the first clear signature of chiral cavity polaritons upon entering the regime of strong light-matter coupling. We study a cavity design based on two dielectric photonic crystal mirrors acting, in a narrow frequency range, as efficient polarization cross-converters in transmission for one polarization and almost perfect reflectors for the other polarization. We show that a Pasteur medium hosted inside such a cavity can couple efficiently to both the outside of the cavity and to the helicity-preserving mode, inheriting an enhanced chiral character. We expect such a device to be useful in the future to design ultrasensitive chiral sensors for optics and stereochemistry.

We investigate the depletion of deep-lying single-electron states in the ${\mathrm{N}}_2$ dimer under the influence of very short extreme-ultraviolet (XUV) pulses. We find, first, a marked occupation inversion for a certain window of XUV energies around 50 eV, where depletion of the deepest bound valence electron state is much larger than for any other state, and second, that this occupation inversion drives a dipole instability, i.e., a spontaneous reappearance of the dipole signal long after the laser pulse is over and the initial dipole oscillations have died out. As a tool for this study, we use time-dependent density functional theory with a self-interaction correction solved on a coordinate-space grid with absorbing boundary conditions. Key observables are state-specific electron emission (depletion) and photoelectron spectra (PES). The dipole instability generates additional electron emission, leading to a specific low-energy structure in PES, a signal which could be used to identify the dipole instability experimentally. The here reported dedicated depletion of a deep lying electron state by a well-tuned XUV pulse has also been found in other atoms and molecules. It provides a practicable realization of an instantaneously produced deep hole state, a situation which is often assumed ad hoc in numerous theoretical studies of energetic ultrafast processes. Moreover, the identification of the subsequent dipole instability by PES will allow one to study basic decay channels of hole states in detail.

Recent experimental breakthroughs in non-Hermitian ultracold atomic lattices have dangled tantalizing prospects in realizing exotic, hitherto unreported, many-body non-Hermitian quantum phenomena. In this work, we propose and study an experimental platform for a radically different non-Hermitian phenomenon dubbed polaron squeezing. We find that it is marked by a dipolelike accumulation of fermions arising from an interacting impurity in a background of non-Hermitian reciprocity-breaking hoppings. We computed their spatial density and found that, unlike Hermitian polarons which are symmetrically localized around impurities, non-Hermitian squeezed polarons localize asymmetrically in the direction opposite to conventional non-Hermitian pumping and nonperturbatively modify the entire spectrum, despite having a manifestly local profile. We investigated their time evolution and found that, saliently, they appear almost universally in the long-time steady state, unlike Hermitian polarons which only exist in the ground state. In our numerics, we also found that, unlike well-known topological or skin localized states, squeezed polarons exist in the bulk, independently of boundary conditions. Our findings could inspire the realization of many-body states in ultracold atomic setups, where a squeezed polaron can be readily detected and characterized by imaging the spatial fermionic density.

The hyperfine structures of the $2{}^{3}S_1$ and $2{}^{3}P_J$ states of ${}^7\mathrm{Be}^{2+}$ and ${}^9\mathrm{Be}^{2+}$ are investigated within the framework of the nonrelativistic quantum electrodynamics, including relativistic and radiative corrections up to order $m{\alpha }^6$. The uncertainties of the calculated hyperfine splittings are on the order of tens of ppm, and for ${}^9\mathrm{Be}^{2+}$ our results improve the previous theoretical and experimental values by at least two orders of magnitude. The improved sensitivity of the hyperfine splittings of ${}^{7,9}\mathrm{Be}^{2+}$ to the nuclear Zemach radius and electric quadrupole moment opens the way to future measurements to extract the atomic physics values of these two nuclear properties to an accuracy of 5% or better.

In studies of ion-induced molecular fragmentation, the challenging measurement of the velocity distribution of fragments emitted below 1-eV kinetic energy is rarely achieved, although most fragments have an energy below this value. Here, we study ${\mathrm{H}}^{+}$ fragment emission in collisions of 10-keV ${\mathrm{O}}^{+}$ ions with ${\mathrm{H}}_2$ molecules using a field-free time-of-flight technique developed specifically to detect sub-eV fragments. We find that, in the velocity map, the binary ridge due to direct ${\mathrm{H}}^{+}$ knockout is split into two parts arising from the rotational motion of the ${\mathrm{H}}_2$ molecule, and that this split scales with rotational velocity. The velocity distribution of the nuclei in the original molecule is determined and the thermally populated $J=1$ rotational level is found to be the dominant contributor, although asymmetry in the split indicates projectile-induced rotational transitions between $M$ sublevels. These rotation effects influence fragment emission probabilities, thus carrying important consequences for the radiation-induced hydrogen loss and ${\mathrm{H}}_2$ dissociation in the atmospheres or exospheres of planets and moons.

This study aims to provide evidence regarding the emergence of a class of dissipative time crystals when $\mathcal{PT}$ symmetry of the systems is restored in collective spin systems with Lindblad dynamics. First, we show that a standard model of boundary time crystals (BTCs) satisfies the Liouvillian $\mathcal{PT}$ symmetry, and prove that BTC exists only when the stationary state is $\mathcal{PT}$ symmetric in the large-spin limit. Also, a similar statement is confirmed numerically for another BTC model. In addition, the mechanism of the appearance of BTCs is discussed through the development of a perturbation theory for a class of the one-spin models under weak dissipations. Consequently, we show that BTCs appear in the first-order correction when the total gain and loss are balanced. These results strongly suggest that BTCs are time crystals originating from $\mathcal{PT}$ symmetry.

We experimentally study the separation of broadband photon pairs that share the same properties in all degrees of freedom. We find that broadband photon pairs with a bandwidth of about 90 nm can be efficiently separated using time-reversed Hong-Ou-Mandel (HOM) interference. The frequency correlation of the separated photons is verified through joint spectral intensity measurements. The visibility of the HOM interference between the separated photons is shown to be higher than the classical limit of 50%, ensuring that the separated photons are indistinguishable. These results may contribute to improving the performance of quantum sensing and quantum communication.

Light-induced nonequilibrium spin-noise spectroscopy is theoretically and experimentally shown to be an efficient technique to reveal the structure and coherent effects in a probed transition. Indeed, using metastable helium, the spin-noise spectrum is shown to exhibit a dual-peak structure around the Larmor frequency. This feature is due to the light shifts of the involved levels and strongly depends on the probe power, detuning, and polarization orientation. Both numerical and analytical models reproduce very well the details of the split spin-noise spectra: This technique thus allows a simple and direct measurement of the light shifts, and its polarization dependence permits to reveal the level structure in a nonambiguous manner.

We study the multipartite quantum steering generated by the nonlinear process of three-photon parametric down-conversion in an optical cavity. We find that the genuine quadripartite quantum steering can be obtained in this nonlinear process, and the injected signal can improve the stability of quadripartite quantum steering. By using the criterion of multipartite quantum steering, we show that the quadripartite quantum steering can be produced in the case of three-photon spontaneous parametric down-conversion when the quantum property of the pump is considered. We also discuss the quantum steering properties in the case of single-seed, double-seed, and triple-seed configurations, respectively, in a wide range of parameters. The result may be helpful to better understand the quantum correlation among the optical fields generated by three-photon parametric down-conversion and has potential applications in quantum information processing.

The development of quantum computation and communication, recently shown to be supported by contextuality, arguably asks for a requisite supply of contextual sets. While that has been achieved in even-dimensional spaces, in odd-dimensional spaces only a dozen contextual critical Kochen-Specker (KS) sets have been found so far. In this Letter, we give three methods for the automated generation of arbitrarily many contextual KS and non-KS sets in any dimension for possible future application and implementation, and we employ them to obtain millions of KS and other contextual sets in dimensions 3, 5, 7, and 9 where previously only a handful of sets have been found. Also, we generate explicit vectors for the original Kochen-Specker set from 24 vector components.

We study quantized vortices in dipolar supersolids at the transition between the superfluid and the supersolid phase. We present an approach to the nucleation of vortices and their observation, based on the quenching of the $s$-wave scattering length across the phase transition. Starting from a slowly rotating, vortex-free configuration in the superfluid phase, we predict vortex nucleation as the system enters the supersolid phase, due to the strong reduction of the critical angular velocity in the supersolid. Once a vortex is created, we show that it is robustly preserved when the condensate is brought back to the superfluid phase, where it may be readily observed. These results may have a significant impact on ongoing experiments, given that the observation of quantized vortices would constitute a key probe of the superfluid character of dipolar supersolids.

We study the non-Hermitian skin effect in dissipative ultracold fermions with spin-orbit coupling, which has been implemented in a recent experiment (Z. Ren et al., Nat. Phys. 18, 385 (2022)). In this work we prove that the non-Hermitian skin effect in such a continuous system is robust to the variation of external parameters and trapping potentials. We further reveal a dynamic sticky effect in our system, which has a common physical origin with the non-Hermitian skin effect. Our work paves the way for studying novel physical responses of the non-Hermitian skin effect in quantum systems.

We investigate Klein tunneling through finite potential barriers with space-time-resolved solutions to relativistic quantum field equations. We find that no particle actually tunnels through a finite supercritical barrier, even in the case of resonant tunneling. The transmission is instead mediated by modulations in pair production rates, at each edge of the barrier, caused by the incoming electron. We further examine the effect of the barrier's width on the numbers of produced pairs in the fermionic case (characterized by saturation) and in the bosonic case (characterized by exponential superradiance). This work paves the way to precise studies of the radiating dynamics of supercritical barriers, and could be applied to certain analogs of Klein tunneling observed in systems modeled by relativistic wave equations.

A large ongoing research effort focuses on variational quantum algorithms (VQAs), representing leading candidates to achieve computational speed-ups on current quantum devices. The scalability of VQAs to a large number of qubits, beyond the simulation capabilities of classical computers, is still debated. Two major hurdles are the proliferation of low-quality variational local minima, and the exponential vanishing of gradients in the cost-function landscape, a phenomenon referred to as barren plateaus. In this work, we show that by employing iterative search schemes, one can effectively prepare the ground state of paradigmatic quantum many-body models, also circumventing the barren plateau phenomenon. This is accomplished by leveraging the transferability to larger system sizes of a class of iterative solutions, displaying an intrinsic smoothness of the variational parameters, a result that does not extend to other solutions found via random-start local optimization. Our scheme could be directly tested on near-term quantum devices, running a refinement optimization in a favorable local landscape with nonvanishing gradients.