Large-scale relativistic configuration-interaction method combined with many-body perturbation theory is consistently applied to calculations of the energy levels of the ground and inner-$L$-shell excited states of berylliumlike ions in the range $10\le Z\le 92$. The quantum electrodynamics, nuclear recoil, and frequency-dependent Breit corrections are taken into account. The obtained results are supplemented with the systematical estimation of the uncertainties.

Dissociative electron attachment to molecules in vibrationally excited states is associated with lower threshold energies and higher cross sections than for cold targets. To date, fewer studies have been performed for polyatomic molecules than for diatomic molecules, due to the experimental challenges and theoretical difficulties in describing the complicated dynamics. Here we report an observation of the dissociative electron attachment to $\mathrm{C}{\mathrm{O}}_2$ in the vibrationally excited states, where the vibrationally excited $\mathrm{C}{\mathrm{O}}_2$ targets are prepared by the electron-impact excitation at energies close to the ${}^2{\mathrm{\Pi }}_u$ resonant state of $\mathrm{C}{{\mathrm{O}}_2}^{-}$. The high-resolution anion momentum images of the ${\mathrm{O}}^{-}$ yield indicate that the rovibrational state distributions of the coproduct CO are dependent on the vibrationally excited states of symmetric bond stretching and bending modes of $\mathrm{C}{\mathrm{O}}_2$.

We investigate the time-optimal control of the purification of a qubit interacting with a structured environment, consisting of a strongly coupled two-level defect in interaction with a thermal bath. On the basis of a geometric analysis, we show for weak and strong interaction strengths that the optimal control strategy corresponds to a qubit in resonance with the reservoir mode. We investigate under which conditions qubit coherence and correlation between the qubit and the environment can speed up the control process.

Producing a substantial and stable resonant Bose-Einstein condensate (BEC) has proven to be a challenging experimental task due to heating and three-body losses that may occur even before the gas comes to thermal equilibrium. In this paper, by considering two-body correlations, we note that a sudden quench from small to large scattering lengths is not an efficient way to prepare a resonant BEC. As an alternative, we propose a two-step scheme that involves an intermediate scattering length, between 0 and $\infty$, which serves to maximize the transfer probability of $N$ bosons of mass $m$ in a harmonic trap with frequency $\omega$ to the resonant state. We find that the intermediate scattering length should be $a\approx 3.16N^{-2/3}\sqrt{\hslash /\left(m\omega \right)}$, and that it produces an optimum transition probability of $1.03N^{-1/6}$.

The dynamics of quantized vortices in two-dimensional two-component miscible Bose-Einstein condensates trapped by a box potential has been numerically studied using the Gross-Pitaevskii model. We have discovered a phenomenon where the vortices of the two components spatially separate from each other, which we call the phase separation of the distribution of vortices. This phase separation occurs when the intercomponent coupling is strong. Onsager vortices, on the other hand, are formed in both components when the intercomponent coupling is weak. We distinguish between Onsager vortices and phase-separated vortices by two types of effective distances between vortices. The dependence of the transition between the Onsager vortices and phase-separated vortices on the intercomponent interaction is also studied.

When seeking a numerical representation of a quantum-mechanical multiparticle problem it is tempting to replace a singular short-range interaction by a smooth finite-range pseudopotential. Finite basis set expansions, e.g., in Fock space, are then guaranteed to converge exponentially. The need to faithfully represent the artificial length scale of the pseudopotential, however, places a costly burden on the basis set. Here we discuss scaling relations for the required size of the basis set and demonstrate the basis set convergence on the example of a two-dimensional system of few fermions with short-range $s$-wave interactions in a harmonic trapping potential. In particular we show that the number of harmonic oscillator basis functions needed to reach a regime of exponential convergence for a Gaussian pseudopotential scales with the fourth power of the pseudopotential length scale, which can be improved to quadratic scaling when the basis functions are rescaled appropriately. Numerical examples for three fermions with up to a few hundred single-particle basis functions are presented and implications for the feasibility of accurate numerical multiparticle simulations of interacting ultracold-atom systems are discussed.

Quantum synchronization and its connection with other quantum correlations have attracted considerable attention. Here we present a theoretical scheme to simultaneously represent and significantly enhance the level of quantum synchronization and entanglement between two indirectly coupled mechanical membranes, which are coupled to a common optical field within a cavity. By applying a two-tone driving laser with weighted amplitudes and specific frequencies, both synchronization gauged by Mari's criterion and entanglement estimated by logarithmic negativity can be greatly enhanced. We then clarify the relationship between quantum synchronization and entanglement in detail. Numerical simulation results show that the influence of the coupling asymmetry $G_2/G_1$ on quantum complete synchronization behaves similarly to that on the purity while the influence of the coupling asymmetry $G_2/G_1$ on quantum phase synchronization is more similar to that on quantum entanglement. Besides, we demonstrate that although quantum synchronization and quantum entanglement are not directly related, both of them are sensitive to the squeezing parameter and the cooling effect. Furthermore, it is also shown that detuning the frequencies of two mechanical oscillators can actually help quantum synchronization and entanglement, which is somewhat similar to the case of quantum synchronization blockade.

Frequency-resolved quantum correlation of resonance fluorescence is investigated in a two-atom radiating system. In this quantum radiating system, only one atom is driven by a laser field, and the spontaneous transition of the undriven atom resonates with one of the Rabi sidebands of the driven atom. A single-mode empty cavity is applied to serve as a Lorentzian filter to output the superbunched fluorescent photon pairs when its frequency is tuned to halfway between the central peak and one of the side peaks. In the case of large filter width, two-photon correlation signal and its physical correspondence can be bridged analytically in our approach. It reveals that this superbunching effect turns out to be the constructive quantum interference between a pair of coupled two-photon cascaded transitions. Ulteriorly, it is the consequence of the modulations of the unfiltered dressed-state transition amplitudes by the filter. Our analytical formalism also shows that, although the dipole-dipole interaction is usually weak, the interatomic coherence caused by this weak perturbation can also play a crucial role in breaking through the superbunching limit obtained from a single two-level atom in the same parameter regime. In addition to being a treasurable quantum pump to probe into the target quantum system, it is also found in our investigation that this superbunched fluorescence can serve as a promising quantum response in detecting this weak perturbation in the interior of the quantum source. A general case is also considered when the two-atom radiating system is monitored by two filter-detector monitoring systems. It is found that this filtered strong quantum correlation can be maintained even though the two photons are spatially separated.

We demonstrate a magnetically controllable nonreciprocal Goos-Hänchen (GH) shift enabled by the magnetic plasmonic gradient metasurfaces (GMSs) consisting of an array of ferrite rods with a rotation gradient introduced to the rod dimers in the unit cell. The incident Gaussian beam is observed to exhibit an evident GH shift about $5\lambda$ when it is incident from one side, whereas for the Gaussian beam incident from the geometrically symmetric direction with respect to surface normal only a very small GH shift comes into appearance. The phenomenon arises from the unidirectional coupling of the incident beam with the magnetic plasmonic GMSs due to the time-reversal-symmetry-breaking nature of the magnetic system as evidenced from the photonic band diagrams of the edge states. The unidirectional GH shift can also be implemented at different incident angles by either engineering the gradient of the GMSs or tuning the bias magnetic field. By designing the magnetic plasmonic GMSs with more exquisite configurations we can expect a lot more nonreciprocal properties, adding additional freedom in manipulating electromagnetic waves.

Topological insulators are studied via tight-binding approximations of longitudinally driven photonic lattices with three lattice sites per unit cell. Two cases are considered in detail: Lieb and kagome lattices. The lattice is decomposed into three sublattices, each of which is allowed to move independently of the others. Emphasis is placed on periodic driving induced by laser-etched helical coils along the direction of propagation. The linear Floquet bands are constructed for various intersublattice rotation patterns such as different radii, different frequency, phase offset, and quasi-one-dimensional motion. Depending on the nature of the band structure, bulk spectral bands with nonzero Chern number are found to support topologically protected edge states which can move unidirectionally. In this case, the modes move scatter-free around defects due to underlying topological protection. Intriguing mode dynamics are found including bidirectional topological modes and bulk-edge leakage, i.e., excitation of bulk modes at a defect for edge modes with dispersion frequencies near the bulk bands. Finally, certain nonlinear edge modes are also found to propagate unidirectionally and scatter-free around lattice defects.

We propose and analyze an efficient scheme for realizing high-sensitive mass sensor in a quadratically coupled optomechanical system via nonlinear second-order sideband process. This is achieved by exploiting a well-established optomechanical circumstance, where a degenerate parametric amplifier (DPA) is embedded into a membrane-in-the-middle cavity driven by a strong control field and a weak probe pulse. Beyond the conventional linearized approximation, we derive analytical expressions for the efficiency of a second-order sideband and the sensitivity of a mass sensor by using a perturbation method. In this scheme, an added mass deposited on the dielectric membrane can be measured by monitoring the efficiency variation of second-order sideband generation. Using experimentally achievable parameters, we identify the conditions under which nonlinear gain of DPA allows us to enhance the efficiency of a second-order sideband and improve the sensitivity of a mass sensor beyond what is achievable in the linearly coupled optomechanical system based on the detection of mechanical frequency shift. More importantly, we also find that the maximum efficiency of a second-order sideband and the optimum sensitivity of a mass sensor simultaneously serve as an efficient detection for the added mass of a membrane when a control field and nonlinear gain of DPA become strong. The present proposal offers a practical opportunity to design an all-optical nonlinear mass sensor at the picogram level.

We study a circuit QED setup involving a single cavity mode and a cyclic qutrit whose parameters are time modulated externally. It is shown that in the dispersive regime this system behaves as a versatile platform to implement effective $n$-photon Jaynes-Cummings (JC) and anti-Jaynes-Cummings (AJC) models by suitably setting the modulation frequency. The atomic levels and the cavity Fock states involved in the effective Hamiltonians can be controlled through adjustment of the system parameters, and different JC and AJC interactions can be implemented simultaneously using multitone modulations. Moreover, one can implement some models that go beyond simple JC and AJC-like interaction, such as multiphoton coupling between certain entangled states. We estimate analytically the associated transition rates and study numerically the dynamics in the presence of Markovian dissipation, demonstrating that lower-order effective Hamiltonians can be implemented with current technology.

We study the entanglement-breaking (EB) space, such that the entanglement of formation (EOF) of a bipartite quantum state is additive when its range is an EB subspace. We systematically construct the EB spaces in the Hilbert space ${\mathbb{C}}^m\otimes {\mathbb{C}}^3$, and the two-dimensional EB space in ${\mathbb{C}}^2\otimes {\mathbb{C}}^n$. We characterize the expression of two-qubit states of rank two with nonadditive EOF, if they exist. We further apply our results to construct EB spaces of arbitrarily given dimensions. We show that the example in Phys. Rev. Lett. 89, 027901 (2002) is a special case of our results. We further work out the entanglement cost of a qubit-qutrit state in terms of the two-atom system of the Tavis-Cummings model.

We present a theoretical study of magneto-optical trapping (MOT) force exerted on magnesium monofluoride (MgF) with three-dimensional rate equations, in which we have considered the complex vibrational and rotational levels and the effects of small internal splittings and degeneracies, including fine and hyperfine structures and the magnetic quantum numbers. We investigate the feasibility of MOT for MgF with a very small excited-state $g$ factor $(g_e=-0.0002)$ and a large radiative decay rate $(\mathrm{\Gamma }=2\pi \times 22\mathrm{MHz})$ for the electronic transition of $X{}^2{\mathrm{\Sigma }}^{+}$ to $A{}^2{\mathrm{\Pi }}_{1/2}$ states. We also optimize the MOT with reference to the three-, four-, and more-frequency component models with various polarization configurations and detunings. By applying the dual-frequency arrangement to more than one hyperfine level, we suggest a configuration of the $\left(3+1\right)$-frequency components for achieving the MOT of MgF.

An alkali-metal–noble-gas NMR gyroscope is widely used for precision rotation measurement in fundamental and applied physics. By numerically simulating the alkali-metal–nuclear–nuclear trispin dynamics, we investigate the dependence of gyroscope response on alkali-metal spin relaxation time and nuclear spin magnetization. We found additional resonance peaks appear due to a new source of instability, namely, the inherent multistability of trispin dynamics. The numerical simulation results agree well with the recent experiment, enabling a better understanding and exploitation of the gyroscope signal.

Highly efficient, optical-degenerate wave-mixing involving bosonic quantum gas is studied using a crossed-beam geometry. We show a very large enhancement due to nonlinear gain from a matter-optical wave-mixing process involving four photons and two matter-wave excitations. Under suitable conditions with a red-detuned pump field this optically degenerate, four-photon process can be stronger than the traditional two-photon inelastic light-scattering mechanism, leading to strong nonlinear growth of the observed matter-wave scattering. This optically degenerate four-wave mixing is independent of other enhancements arising from bosonic stimulations. More importantly, this highly efficient, backward degenerate wave generation and growth process does not have a correspondence in thermal gases in nonlinear optics.

Quantum heat engines based on electromagnetically induced transparency first proposed by Harris [S. E. Harris, Phys. Rev. A 94, 053859 (2016)] have been recently demonstrated in experiment. Here we present an improved version of such a heat engine by adding a microwave field in the $\mathrm{\Lambda }$-type atomic system to modify the cross sections for absorption and emission. We find that this microwave-assisted atomic system behaves as a heat engine only when (i) all three laser or microwave fields resonate with their corresponding transitions and (ii) the relative phase between these fields is $n\pi$ with $n=0,\pm 1,\pm 2,\cdots$. In particular, below the threshold of lasing without inversion, the engine's output field exhibits a brightness increasing with the microwave's intensity, and the brightness in the presence of a weak microwave field can be tens of (or even larger) that in the absence of a microwave field. We also note that the microwave field has the equivalent effect of increasing (decreasing) the hot (cold) reservoir's temperature.

Light-induced control of ions within small Coulomb crystals is investigated. By intense intracavity optical standing-wave fields, subwavelength localization of individual ions is achieved for one-, two-, and three-dimensional crystals. Based on these findings, we illustrate numerically how the application of such optical potentials can be used to tailor the normal-mode spectra and patterns of multidimensional Coulomb crystals. The results represent, among others, important steps towards controlling the crystalline structure of Coulomb crystals, investigating heat-transfer processes at the quantum limit, and quantum simulations of many-body systems.

Reference-frame-independent quantum key distribution (RFI-QKD) has been proposed as a method to avoid the requirement of real-time alignment of reference frames. However, the source flaw has not been considered in most of the previously reported works. In this paper, based on the analysis of Wang et al. [Phys. Rev. A 92, 042319 (2015)], we perform a proof-of-concept experiment to demonstrate the loss-tolerant RFI-QKD by constructing a time-bin phase-encoded system and quantifying its source flaws. Moreover, the finite-key analysis is taken into account while estimating the secure key rate. Our experimental demonstration makes a positive contribution to the realization of quantum cryptography in a complex and imperfect real-world environment.

In this work we study the vicinage effects that arise in the interaction of molecular projectiles with solids, considering, in particular, the effects produced by the excitation of inner shells. For this purpose, we use two different approaches. On one side we extend the use of the semiclassical impact-parameter model for the excitation of atomic shells, considering quantum corrections and the role of target screening in the vicinage effects. On the other hand, we adapt our extended wave-packet model, developed in a previous work to the calculation of stopping ratios and ionization cross sections for correlated ions. This model introduces modifications to the wave-packet method originally proposed by Kaneko, using the Levine and Louie technique to take into account the energy gaps corresponding to the different atomic levels of the target. Finally, we add the contribution of valence electrons calculated with the Lindhard free-electron-gas model and compare with experimental results of vicinage effects in the energy-loss and ionization cross sections for hydrogen molecules interacting with C, Al, Si, ${\text{Al}}_2{\text{O}}_3$, and ${\text{SiO}}_2$ targets.