The electron emission processes in charged and neutral helium projectiles colliding with water molecules are theoretically investigated. The individual reactions, namely, target ionization and projectile electron loss, are studied within the continuum distorted wave–eikonal initial state model. Comparison of theoretical predictions with experimental data will help to improve the accuracy of models that describe such collisions. This can have implications for a wide range of fields, including plasma physics, astrophysics, and radiation biology. The calculations presented here are shown in the form of differential cross sections, in the energy range from 300 keV/u to 10 MeV/u, which describe the energy and angular spectra of the electrons emitted. Also, the computed total cross sections are compared with available theoretical and experimental results in the energy range from 40 keV/u up to 50 MeV/u.

Ultralong-range Rydberg molecules, composed of an excited Rydberg atom and a ground-state atom, are characterized by large bond lengths, dipole moments, sensitivity to external fields, and an unusual binding mechanism based on low-energy elastic electron scattering. Although Rydberg molecules formed between alkali atoms have received the most attention, the additional complexity found in atoms with more than a single valence electron poses new theoretical challenges as well as new possibilities for control and design of the molecular structure. In this paper, we extend the theory of Rydberg molecules to include the additional spin coupling of the Rydberg states of a multivalent atom. We employ this theory to describe the properties of Rydberg molecules composed of mercury atoms. We calculate the potential energy curves of both heteronuclear (${\mathrm{Hg}}^{*}\mathrm{Rb}$) and homonuclear (${\mathrm{Hg}}^{*}\mathrm{Hg}$) molecules. In the former case, we propose the realization of long-range spin entanglement and remote spin flip. In the latter, we show how long-lived metastable molecular states of ${\mathrm{Hg}}^{*}\mathrm{Hg}$ exist as resonances above the dissociation threshold.

We investigate the electron-impact double ionization and subsequent fragmentation dynamics of acetonitrile using multiparticle coincident momentum spectroscopy. Through the measurements of the kinetic energy release for the two-body Coulomb explosion processes and the potential energy calculations in the reaction coordinate, we identify several fragmentation channels that are typically not accessible via direct Coulomb explosion, necessitating prior isomerization, which dominates over the direct Coulomb explosion channel. The potential energy calculations reveal that hydrogen migration reduces the energy barrier of C-C bond cleavage, thereby promoting the fragmentation of acetonitrile dications. Moreover, we find that, instead of transferring to the nitrogen atom yielding the double hydrogen migration channel, the roaming ${\mathrm{H}}_2$ captures a proton or transfers an electron to the remaining moiety, leading to the formation of ${\mathrm{H}}_3{}^{+}$ and ${\mathrm{H}}_2{}^{+}$ cations. This work provides insight into the competitive interplay between hydrogen migration and Coulomb explosion mechanisms in molecular fragmentation.

The visible spectrum of ${\mathrm{Mo}}^{15+}$ ions was measured using a room-temperature permanent magnet electron-beam ion trap at the Shanghai EBIT Laboratory, with an electron-beam energy $E_e=400\mathrm{eV}$, which is significantly below the ionization potential ($\text{IP}=544.0$ eV) of the ${\mathrm{Mo}}^{14+}$ ions in the ground configuration $3p^{6}3d^{10}$. To provide a detailed explanation of the experiment, we calculated the energy-level structure, radiative transition properties, electron-impact excitation, and electron-impact ionization cross sections for both the ground state and low-lying excited states of the ${\mathrm{Mo}}^{14+}$ ions using the Dirac-Fock-Slater method with a local central potential and the distorted-wave approximation. The results show good agreement with both experimental and theoretical data available in the literature. Through an analysis of the relevant atomic processes of ${\mathrm{Mo}}^{14+}$ ions, a scenario involving the stepwise ionization via the metastable configuration $3p^{6}3d^{9}4s$ was proposed to explain the presence of the ${\mathrm{Mo}}^{15+}$ ions for electron energies below the ionization threshold. The study confirms the importance of metastable states in ionization processes, and provides new insights into their ionization mechanisms, which will be valuable for future studies of EBIT and similar plasma environments.

Bosonic Josephson junctions provide a versatile platform for exploring quantum tunneling and coherence phenomena in ultracold atomic systems. While extensive research has examined the Josephson-junction dynamics in various double-well configurations, most studies have been limited to inertial reference frames. In the present work we pose the question how placing a Josephson junction in a noninertial reference frame would impact the quantum tunneling. Our findings demonstrate that accelerating a Josephson junction alters the tunneling dynamics. Conversely, tunneling behavior can be used to assess the acceleration of the system. By analyzing the changes in physical properties, we can assess the acceleration of the double well. We begin with the most simple noninertial frame: moving with constant acceleration. The tunneling time decreases exponentially as acceleration increases, making it effective for measuring larger accelerations. However, for smaller accelerations, accurate assessment requires accounting for many-body depletion, which decreases linearly as acceleration rises. Next we explore a more complex scenario where the acceleration is time dependent. In this case, the acceleration is mapped onto the tunneling time period and depletion, which again serve as predictors of acceleration. We also explore how the tunneling time and condensate depletion are affected by changes in the interparticle interaction. We go further by conducting a detailed analysis of the change in tunneling dynamics when the system deviates from constant or zero acceleration. The quantitative analysis shows that the depletion changes exponentially near constant acceleration, while around zero acceleration, the change follows a polynomial pattern. A comprehensive analysis of the effects of damped motion on the condensate depletion is performed as well. All in all, we quantify how the tunneling process and the mean-field and many-body properties evolve in a noninertial system of increasing complexity.

Precise control of topologically protected excitations, such as quantum vortices in atomtronic circuits, opens new possibilities for future quantum technologies. We theoretically investigate the dynamics of Josephson vortices (rotational fluxons) induced by coupled persistent currents in a system of coplanar double-ring atomic Bose-Einstein condensates. We study the Josephson effect in an atomic Josephson junction formed by coaxial ring-shaped condensates. Tunneling superflows, initiated by an imbalance in atomic populations between the rings, are significantly influenced by the persistent currents in the inner and outer rings. This results in pronounced Josephson oscillations in the population imbalance for both corotating and nonrotating states. If a linear acceleration is applied to the system, our analysis reveals peculiar azimuthal tunneling patterns and dynamics of Josephson vortices which leads to nonzero net tunneling current and shows sensitivity to the acceleration magnitude. When multiple Josephson vortices are present, asymmetric vortex displacements that correlate with both the magnitude and direction of acceleration can be measured, offering the potential for quantum sensing applications.

In a previous article, we studied stationary solutions to the dynamics of a Bose-Einstein condensate corresponding to acoustic (or Unruh) black or white holes, namely, configurations where the flow becomes supersonic, creating a horizon for phonons. In this paper, we consider again the Gross-Pitaevskii equation, but looking for stationary numerical solutions in the case where the couplings are position dependent in a prescribed manner. Initially, we consider a two-dimensional quantum gas in a funnel-like spatial metric. We then reinterpret this solution as a solution in a flat metric but with spatially dependent coupling and external potential. In these solutions, the local speed of sound and the magnitude of flow velocity cross, indicating the existence of a supersonic region and therefore of sonic analogs of black and white holes and wormholes. We discuss the numerical techniques used. We also study phase (and density) fluctuations in these solutions and derive approximate acoustic metric tensors. For certain external potentials, we find uniform-density acoustic-black-hole configurations and obtain their Hawking temperature.

Parity describing the symmetry of quantum-mechanical wave functions under space inversion transformation not only plays an essential role in solving quantum systems but also can be used to manipulate and measure the motional quantum states of hybrid quantum systems such as the quantum Rabi model (QRM) and/or its variants through parity measurements. Here we address an exotic parity behavior of the QRM in its superradiant phase by numerical exact diagonalization, namely, the parities of eigenstates of the QRM behave irregularly in the strong-coupling regime but the sum of parities for each pair of neighboring eigenstates beginning from the ground state remains vanishing. It is found that this exotic behavior originates from the comparability of the photon distribution in the odd and even components of Fock basis when the eigenenergies of each pair of neighboring eigenstates approach close enough to each other, and physically this is due to the emergent double-well potential induced by the strong coupling between the single-mode photon field and the two-level atom. The result not only uncovers the physics not known previously in the QRM but also makes an intrinsic limitation on the measurement precision of motional quantum states through parity measurements in modern quantum science and technology.

A major challenge in near-term quantum computing is its application to large real-world datasets due to scarce quantum hardware resources. One approach to enabling tractable quantum models for such datasets involves finding low-dimensional representations that preserve essential information for downstream analysis. In classical machine learning, variational autoencoders (VAEs) facilitate efficient data compression, representation learning for subsequent tasks, and novel data generation. However, no quantum model has been proposed that exactly captures all of these features for direct application to quantum data on quantum computers. Some existing quantum models for data compression lack regularization of latent representations, thus preventing direct use for generation and control of generalization. Others are hybrid models with only some internal quantum components, impeding direct training on quantum data. To address this, we present a fully quantum framework, $\zeta$-QVAE, which encompasses all the capabilities of classical VAEs and can be directly applied to map both classical and quantum data to a lower-dimensional space, while effectively reconstructing much of the original state from it. Our model utilizes regularized mixed states to attain optimal latent representations. It accommodates various divergences for reconstruction and regularization. Furthermore, by accommodating mixed states at every stage, it can utilize the full data density matrix and allow for a training objective defined on probabilistic mixtures of input data. Doing so, in turn, makes efficient optimization possible and has potential implications for private and federated learning. In addition to exploring the theoretical properties of $\zeta$-QVAE, we demonstrate its performance on representative genomics and synthetic data. Our results indicate that $\zeta$-QVAE consistently learns representations that better utilize the capacity of the latent space and exhibits similar or better performance compared with matched classical models.

Certain unitary invariants, known as Bargmann invariants or multivariate traces of quantum states, have recently gained attention due to their applications in quantum information theory. However, determining the boundaries of sets of Bargmann invariants remains a theoretical challenge. In this study, we address the problem by developing a unified, dimension-independent formulation that characterizes the sets of the third and fourth Bargmann invariants. In particular, our result for the set of fourth Bargmann invariants confirms the conjecture given by Fernandes et al. [Phys. Rev. Lett. 133, 190201 (2024)]. Based on the obtained results, we conjecture that the unified, dimension-independent formulation of the boundaries for sets of third- and fourth-order Bargmann invariants may extend to the general case of the $n\mathrm{th}$-order Bargmann invariants. These results deepen our understanding of the fundamental physical limits within quantum mechanics and pave the way for novel applications of Bargmann invariants in quantum information processing and related fields.

The equivalence between absolutely separable states and absolutely positive partial transposed (PPT) states in general remains an open problem in quantum entanglement theory. In this work, we study an analogous question for symmetric multiqubit states. We show that symmetric absolutely PPT (SAPPT) states (symmetric states that remain PPT after any symmetry-preserving unitary evolution) are not always symmetric absolutely separable by providing explicit counterexamples. More precisely, we construct a family of entangled five-qubit SAPPT states. Similar counterexamples for larger odd numbers of qubits are identified.

Ramsey spectroscopy serves as a key tool for frequency measurement. Although entanglement can enhance the measurement precision, the atom-atom interaction required for entanglement generation may lead to a frequency shift. In addition, the spectra also suffer from light shift caused by the interaction between light and atoms. Thus, it is difficult to achieve high precision without inducing additional systematic errors. Here, we propose to realize high-precision entanglement-enhanced Ramsey spectroscopy with composite pulses. First, we utilize spin-squeezed states generated by one-axis twisting (OAT) dynamics to enhance the measurement precision. We find that by applying a suitable pulse after OAT, the resonant frequency can be inferred and the light shift can be reduced by using composite pulses. To increase the signal slope, we use anti-OAT dynamics to perform interaction-based readout after interrogation. By designing suitable composite pulses, it can significantly increase the signal response and further improve the measurement precision. Although both schemes can mitigate the light shift, it cannot be entirely eliminated. Thus, we develop the autobalanced entanglement-enhanced Ramsey spectroscopy. By using combination of different sequences with short and long interrogation times and feedback control, the light shift of both our protocols can be completely suppressed, which increases the measurement accuracy. Besides, we also investigate the effects of background atom-atom interaction and find that it only reduces the measurement precision without introducing additional frequency shift. Our study offers an alternative method for achieving precise frequency measurement, which has the potential to be of significant application in quantum sensors such as atomic clocks.

The adiabatic theorem in quantum mechanics implies that if a system is in a discrete eigenstate of a Hamiltonian and the Hamiltonian evolves in time arbitrarily slowly, the system will remain in the corresponding eigenstate of the evolved Hamiltonian. Understanding corrections to the adiabatic result that arise when the evolution of the Hamiltonian is slow, but not arbitrarily slow, has become increasingly important, especially since adiabatic evolution has been proposed as a method of state preparation in quantum computing. This paper identifies two regimes, an adiabatic regime in which corrections are generically small and can depend on details of the evolution throughout the path and a hyperadiabatic regime in which the error is given by a form similar to an asymptotic expansion in the inverse of the evolution time with the coefficients depending principally on the behavior at the endpoints. However, the error in this hyperadiabatic regime is neither given by a true asymptotic series nor solely dependent on the endpoints: The coefficients combine the contributions from both endpoints, with relative phase factors that depend on the average spectral gaps along the trajectory, multiplied by the evolution time. The central result of this paper is to identify a quantity, referred to as the typical error, which is obtained by appropriately averaging the error over evolution times that are small compared to the evolution time itself. This typical error is characterized by an asymptotic series and depends solely on the endpoints of the evolution, remaining independent of the details of the intermediate evolution.

Precise evaluation of the isotope shift (IS) factors for seven low-lying potassium states is achieved using relativistic coupled-cluster theory. The energies of these states are compared with the experimental data to confirm the accuracy of wave functions calculated at different approximations and to highlight the significance of many-body and relativistic effects. Various methods are used to compute the IS factors, with the finite-field (FF) approach yielding results that align with observed and semiempirical data. This is attributed to orbital relaxation effects that are present naturally in the FF method but emerge only through perturbation in other methods. Using results from the FF approach, we review the mean-square-radius difference between ${}^{38m}\mathrm{K}$ and ${}^{39}\mathrm{K}$, which is combined with muonic atom x-ray spectroscopy and updated calculation of the nuclear polarizability effect to deduce the absolute radius of ${}^{38m}\mathrm{K}$. Finally, we evaluate the isospin symmetry breaking (ISB) in the isotriplet by integrating the radius of ${}^{38m}\mathrm{K}$ with an updated radius of ${}^{38}\mathrm{Ca}$, concluding that the ISB is compatible with zero. This finding offers a stringent benchmark for nuclear models of ISB corrections in nuclear beta decay, which play a key role in determining the $V_{ud}$ matrix element.

Alignment-transfer rate coefficients $C_0^{K{K}^{\prime }}$, with nonzero even values of $K$ and $K^{\prime }$, enter in the population-alignment collisional-radiative model and are useful in anisotropic plasma diagnostics. Here, these coefficients are calculated for electron impact excitation of the Be-like ion O v from its metastable $2s2p{}^3{P}_{1,2}$ levels in a nonthermal plasma with an anisotropic two-temperature Maxwellian electron velocity distribution. The calculations take into account coherences between magnetic sublevels within initial and final levels of the collisional transfer of alignment, via integration of linear combinations of alignment-transfer ${\sigma }_{Q=0}^{K{K}^{\prime }}$ and coherence-transfer ${\sigma }_{Q\ne 0}^{K{K}^{\prime }}$ cross sections. Computations of these cross sections are performed up to 1.8 keV of the scattered electron energy in the relativistic distorted-wave approximation using an extended version of the flexible atomic code. Selected results of $C_0^{22}$ and $C_0^{44}$ for $2s2p{}^3{P}_{1,2}\to 2s3p{}^3{P}_{1,2}$ and $2s2p{}^3{P}_{1,2}\to 2s3d{}^3{D}_{2,3}$ excitations are given to highlight the importance of coherences. Ignoring the coherence-transfer cross sections ${\sigma }_{Q\ne 0}^{K{K}^{\prime }}$ in the calculations may lead to underestimating the alignment-transfer rate coefficients by more than 80%. The present results could shed light on the discrepancies reported between theoretical predictions ignoring ${\sigma }_{Q\ne 0}^{K{K}^{\prime }}$ and measurements of the polarization of O v lines emitted from WT-3 tokamak plasma.

We investigate the multipeak structure observed in resonant two-photon ionization of hydrogen, where Rabi oscillations occur. Our results show that the weak peaks appear both between and outside the Autler-Townes (AT) doublets for a rectangular pulse, in contrast to the sin-squared pulse case, where the peaks only appear between the AT doublets. Utilizing a three-level model, we demonstrate that the multipeak structure for the rectangular pulse is attributed to single-slit diffraction in the time domain, rather than to dynamic interference as in the sin-squared pulse. Furthermore, we show that the photoelectron yield of these peaks oscillates with laser intensity for both pulse shapes, with the yields from resonant and nonresonant paths oscillating in opposite directions. This oscillatory behavior is attributed to interference between split energy levels in the energy domain. Our work completes the understanding of the multipeak structure in Rabi oscillations for different laser pulse shapes and provides a way to manipulate the yield of these peaks through laser intensity.

Studies of high harmonic generation (HHG) have entered a new stage by using the quantum light instead of the classical field. Recent theoretical progress has demonstrated promising features, such as the significantly higher maximum harmonic photon energy in a squeezed or thermal light field, which is not observed under classical field conditions with the same number of fundamental mode photons. However, there are many approximations in the theoretical treatments. More accurate methods are required to check the appealing results. By employing the generalized von Neumann lattice basis, here we develop a general and precise method to describe the HHG process, taking into account the coherent superposition of the diagonal part components and the nondiagonal part components from the overlap of the coherent states. It can be extended to calculate the multimode quantum light, such as two-mode squeezed vacuum. Our study provides a powerful tool for investigating the full properties of quantum light in HHG.

Impurities immersed in a Bose-Einstein condensate (BEC) can interact indirectly through the exchange of Bogoliubov excitations. These impurities, which form dressed quasiparticles known as Bose polarons due to their interaction with the BEC, can pair up to form a bound state called a bipolaron, via an induced interaction. Previous studies on induced interactions have focused primarily on cases with an isotropic excitation spectrum. In this work, we investigate the properties of induced interactions and bipolarons mediated by anisotropic Bogoliubov excitations using field theory. Taking a BEC with spin-orbit coupling as an example, we show that the induced interaction becomes anisotropic. Notably, a double-minima feature appears in the induced interaction in momentum space due to the exchange of roton excitations. Additionally, we calculate the binding energy and wave functions of these bipolarons induced by anisotropic interactions. Unlike previously studied bipolarons formed through the exchange of isotropic phonon excitations, we identify an alternative type of bipolaron whose wave functions feature a double-peak structure under strong impurity-boson interactions. Our work extends the theory of induced interactions from isotropic to anisotropic systems, and it reveals the novel features in both the induced interactions and bipolarons arising from BEC with an unconventional excitation spectrum.

Self-evolving vector Bessel beams exhibit a spatially varying polarization profile that gradually evolves as the beam field propagates. Such beams can also carry both spin and orbital angular momentum. We show that in our recently introduced metasurface platform it is possible to gain control over the entire range of possible values for both types of angular momentum by tuning the incident field polarization. We also propose a geometric phase related to the polarization changes due to the propagation of the output field. We find a hitherto unrecognized conservation of the sum of the normalized geometric phase and orbital and spin angular momentum for all incident polarization states.

Continuous-variable entanglement, especially in the non-Gaussian states exhibiting Wigner negativity, is essential for realizing quantum advantages in quantum information processing tasks. Here, we propose a method to efficiently generate multimode non-Gaussian entangled states using a single Rb vapor cell integrated with a cyclic optical system. By employing multiple pump light pulses and recycling the output signal light as a seed light, our system is equivalent to implementing the multiple cascaded four-wave mixing processes while maintaining resource efficiency. Through the analysis of the properties of multipartite steering and entanglement in the cyclic system, we find a hierarchical Gaussian steering structure, which enables the photon number projection on superior signal mode to prepare a multimode entangled non-Gaussian state in the idler modes. Conversely, the projection on each subordinate idler mode can induce more negativity in the signal mode. More importantly, our approach significantly improves the success probability of photon number projections, thereby augmenting the yield of non-Gaussian states and enhancing their potential for delivering robust quantum advantages.