In the present work we study the applicability of the binary-encounter-dipole (BED) model and its simpler version the binary-encounter-Bethe model for positron-impact direct ionization. We show that ignoring the exchange and interference effects in the theory of Kim and Rudd [Phys. Rev. A 50, 3954 (1994)] yields simple analytical formulas with no fitting parameters that can be used to estimate cross sections over a wide energy range except near-threshold region. To correct this deficiency we combine BED theory with the Wannier-type threshold law derived by Klar [J. Phys. B 14, 4165 (1981)] for positron-impact ionization. We show that such combination is necessary in order to predict cross sections at low positron energies where strong polarization-correlation effects are present during collision and the positronium formation is a dominant scattering process. The present theory is tested for a wide range of targets including helium (He), atomic hydrogen (H), neon (Ne), argon (Ar), molecular hydrogen (${\mathrm{H}}_2$), nitrogen (${\mathrm{N}}_2$), oxygen (${\mathrm{O}}_2$), carbon monoxide (CO), carbon dioxide (${\mathrm{CO}}_2$), and methane (${\mathrm{CH}}_4$). An extensive comparison with available experiments and theories is done.

Distinct oscillations of the purity of the single-particle density matrix for many-body open quantum systems have been shown to exist [D. Dast, D. Haag, H. Cartarius, and G. Wunner, Phys. Rev. A 93, 033617 (2016)]. They are found in $\mathcal{PT}$-symmetric Bose-Einstein condensates, in which the coherence of the condensate drops and is almost completely restored periodically. For this effect the presence of a gain and loss of particles turned out to be essential. We demonstrate that it can also be found in closed quantum systems whose subsystems experience a gain and loss of particles. This is shown with two different lattice setups for cold atoms, viz., a ring of six lattice sites with periodic boundary conditions and a linear chain of four lattice wells. In both cases pronounced purity oscillations are found, and it is shown that they can be made experimentally accessible via the average contrast in interference experiments. This shows that it is possible to identify this characteristic effect in closed quantum systems which have been proposed to realize $\mathcal{PT}$-symmetric quantum mechanics via separation into subsystems.

Superstripe phases in Bose-Einstein condensates (BECs), possessing both crystalline structure and superfluidity, open a new avenue for exploring exotic quantum matters—supersolids. However, conclusive detection and further exploration of a superstripe is still challenging in experiments because of its short period, low visibility, fragility against magnetic-field fluctuation, or short lifetime. Here we propose a scheme in a spin-orbit coupled BEC which overcomes these obstacles and generates a robust magnetic superstripe phase, with only spin-density (no total density) modulation due to the magnetic translational symmetry, ready for direct real-space observation. In the scheme, two hyperfine spin states are individually Raman coupled with a largely detuned third state, which induce a momentum-space separation between two lower band dispersions, yielding an effective spin-1/2 system with tunable spin-orbit coupling and Zeeman fields. Without effective Zeeman fields, spin-dependent interaction dominates, yielding a magnetic superstripe phase with a long tunable period and high visibility. Our scheme provides a platform for observing and exploring exotic properties of superstripe phases as well as novel physics with tunable spin-orbit coupling.

We studied the scattering problem of two distinguishable atoms with unequal mass, where one atom (atom $\alpha$) is trapped in a quasi-one-dimensional (quasi-1D) tube and the other one (atom $\beta$) is localized by a 3D harmonic trap. We show that in such a system if atom $\alpha$ is much heavier than $\beta$, confinement-induced resonance (CIR) can appear when the 3D $s$-wave scattering length $a_s$ of these two atoms is much smaller than the characteristic lengths (CLs) of the confinements, for either $a_s>0$ or $a_s<0$. This is quite different from the usual CIRs which occur only when $a_s$ is comparable with the CL of confinement. Moreover, the CIRs we find are broad enough to serve as a tool for the control of effective interatomic interaction. We further show the mechanism of these CIRs via the Born-Oppenheimer approximation. Our results can be used for the realization of strongly interacting systems with ultracold atoms with weak 3D background interaction (i.e., small $a_s$), e.g., the realization of ultracold gases with strong spin-dependent interaction at zero magnetic fields.

We study Tan's contact of a one-dimensional quantum gas of $N$ repulsive identical bosons confined in a harmonic trap at finite temperature. This canonical ensemble framework corresponds to the experimental conditions, with the number of particles being fixed for each experimental sequence. We show that in the strongly interacting regime, the contact rescaled by the contact at the Tonks-Girardeau limit is a universal function of two parameters: the rescaled interaction strength and temperature. This means that all pair and correlation effects in Tan's contact are embedded in Tan's contact in the Tonks-Girardeau limit.

We study the properties of atom-photon bound states and single-photon cooperative dynamics in a waveguide system which consists of a finite-bandwidth channel with model dispersion and an ensemble of two-level atoms whose size is ignorable when compared with the lattice constant. The bound states are formed by all atoms and a localized photonic excitation. We find that the effect of atomic collection is equivalent to the case of one atom by rescaling the coupling strength with the square root of the atom number, as far as the eigenenergy equation is concerned. Besides, it is found there is a quantum phase transition when more than one type of atom are present. The characteristic lengths and wave functions are analyzed near the phase transition point. The exact analytical results for the cooperative dynamics at the single excitation level are obtained and we point out the dark state in this system leads to a universal population trapping in the time evolution process. This type of trapping obeys a simple law that is only associated with the atom number. A direct conclusion that results from the trapping law is that the single-photon cooperative emission is suppressed when the number of atoms is large enough.

Single-photon sources are subjected to a fundamental limitation in the speed of operation dictated by the spontaneous emission rate of quantum emitters (QEs). The current paradigm of the rate acceleration suggests coupling of a QE to a metal nanostructure, in particular, a metal nanoparticle (MNP). Here, we demonstrate that, in contrast to this approach, a MNP itself being excited via a strong coupling with a QE can emit light with quantum properties. We determine both the first- and second-order correlation functions of light spontaneously emitted by a MNP in such a compound and show that this light should exhibit sub-Poissonian photon statistics and perfect photon antibunching. This discovery opens a prospect to single-photon sources with generation rates up to 100 THz.

In a numerical study, we investigate the steady-state generation of nonclassical states of light from a coherently driven two-level atom in a one-dimensional waveguide. Specifically, we look for states with a negative Wigner function, since such nonclassical states are a resource for quantum information processing applications, including quantum computing. We find that a waveguide terminated by a mirror at the position of the atom can provide Wigner-negative states, while an infinite waveguide yields strictly positive Wigner functions. Moreover, our paper reveals a connection between the purity of a quantum state and its Wigner negativity. We also analyze the effects of decoherence on the negativity of a state.

We present a thorough computational study of the existence, stability, and comb properties of cnoidal waves—dissipative periodic patterns—in Kerr microresonators. We show that cnoidal waves comprise a large set with multiple periods. Optimal comb power efficiency and bandwidth are obtained for highly red-detuned, intermediate-strength pump, and short-period waves, that are similar to a train of cavity solitons. We demonstrate a deterministic access path for optimal waves that yields combs of soliton-class bandwidth with a much higher power efficiency.

A generalized multiparameter Hong-Ou-Mandel interferometer is presented which extends the conventional “Mandel dip” configuration to the case where a symmetric biphoton source is used to monitor the contemporary absence of $k$ independent time delays. Our construction results in a two-input/two-output setup, obtained by concatenating 50:50 beam splitters with a collection of adjustable achromatic wave plates. For $k=1,2$ and $k=4$ explicit examples can be exhibited that prove the possibility of uniquely linking the zero value of the coincidence counts registered at the output of the interferometer with the contemporary absence of all the time delays. Interestingly enough the same result cannot be extended to $k=3$. Besides, the sensitivity of the interferometer is analyzed when the time delays are affected by strong fluctuations, i.e., the fluctuations over timescales that are larger than the inverse of the frequency of the pump used to generate the biphoton state.

We examine the fermionic entanglement in the ground state of the fermionic Lipkin model and its relation with bipartite entanglement. It is first shown that the one-body entanglement entropy, which quantifies the minimum distance to a fermionic Gaussian state, behaves similarly to the mean-field order parameter and is essentially proportional to the total bipartite entanglement between the upper and lower modes, a quantity meaningful only in the fermionic realization of the model. We also analyze the entanglement of the reduced state of four single-particle modes (two up-down pairs), showing that its fermionic concurrence is strongly peaked at the phase transition and behaves differently from the corresponding up-down entanglement. We finally show that the first measures and the up-down reduced entanglement can be correctly described through a basic mean-field approach supplemented with symmetry restoration, whereas the concurrence requires at least the inclusion of random-phase-approximation–type correlations for a proper prediction. Fermionic separability is also discussed.

We are at the verge of a new era, which will be dominated by noisy intermediate-scale quantum devices. Prototypical examples for these new technologies are present-day quantum annealers. In the present work, we investigate to what extent static disorder generated by an external source of noise does not have to be detrimental, but can actually assist quantum annealers in achieving better performance. In particular, we analyze the graph coloring problem that can be solved on a sparse topology (i.e., chimera graph) via suitable embedding. We show that specifically tailored disorder can enhance the fidelity of the annealing process and thus increase the overall performance of the annealer.

Pseudopotentials are an essential ingredient in diffusion quantum Monte Carlo (DMC) calculations to increase efficiency substantially. A new generation of effective core potentials (ccECP) has been recently developed for DMC calculations. In this paper, performance of DMC using ccECP potentials on total energies, ionization potentials (IPs) and electron affinities (EAs) of some second- and third-row atoms and molecules is investigated systematically with different types of trial wave functions. DMC results are compared with those of high-level coupled-cluster methods extrapolated to complete basis set limit (CC-CBS). Error of ccECP potentials on IPs and EAs is also evaluated through a comparison with those from all-electron calculations. Our results show that mean errors in DMC energies with the ccECP potentials are smaller than those with the pseudopotentials developed by Burkatzki, Filippi, and Dolg (BFD), when the same type of trial wave functions is adopted. Mean absolute deviations (MADs) on IPs of DMC compared with those of CC-CBS are about 1.6 kcal/mol with single-determinant-Jastrow trial wave functions, and 1 kcal/mol with multideterminant-Jastrow trial wave functions using either ccECP or BFD potentials. MADs on EAs with DMC using the ccECP potentials are about 1 kcal/mol and slightly larger than those with the BFD potentials. Our results show that ccECP potentials are able to provide reliable IPs and EAs in DMC calculations. Accuracy of IPs and EAs from DMC calculations using ccECP potentials is similar to that with the BFD potentials, although mean error in total DMC energies with ccECP potentials is smaller. Furthermore, error of ccECP potentials in DMC calculations on IPs and EAs is smaller than that of BFD potentials compared with all-electron results.

As a microscopic theory to express the electromagnetic response of the system to the external fields, we present the time-dependent current-density-functional theory (td-CDFT), taking into account the effect of the energy dissipation from the system. In order to express such an effect, the non-Hermitian term which corresponds to the lifetime of the higher-energy component of the many-electron state is added to the Hamiltonian of the system. Specifically, the occupation number of the Bloch states, the energies of which are higher than the Fermi energy, is chosen as a basic variable in addition to the electron density and paramagnetic current density. In order to confirm its validity, the present td-CDFT is applied to the two-level system. We successfully revisit the electric susceptibility of the Drude-Lorentz model via the present theory.

Electron-impact single-ionization cross sections for the ${\mathrm{W}}^{5+}$ ion have been studied experimentally and theoretically. Measurements of a detailed ionization spectrum and of absolute cross sections were performed employing the crossed-beams method in the energy range from the ionization onset up to 1000 eV. The experimental data show a prominent contribution of ${\mathrm{W}}^{5+}$ ions in metastable states. The theoretical analysis includes level-to-level calculations from the $4f^{14}5s^{2}5p^{6}5d$ ground configuration and long-lived levels of the $6s$, $5p^{5}5d^2$, $4f^{13}5d^2$, and $4f^{13}5d6s$ configurations. Direct-ionization and excitation-autoionization contributions to the total single-ionization cross sections were calculated employing a distorted-wave approximation. Radiative damping was taken into account. It is shown that correlation effects play an important role and lead to substantial reduction of cross sections. Theory and experiment are in quite good agreement when, within a statistical model, a ($85\pm 9$)% fraction of parent ions in the ground configuration is assumed. The strongest contributions of metastable parent ions arise from the $5p^{5}5d^2$ and $4f^{13}5d^2$ configurations.

We probe electric-field noise in a surface ion trap for ion-surface distances $d$ between 50 and 300 µm in the normal and planar directions. We find the noise distance dependence to scale as $d^{-2.6}$ in our trap and a frequency dependence which is consistent with $1/f$ noise. Simulations of the electric-field noise specific to our trap geometry provide evidence that we are not limited by technical noise sources. Our distance scaling data is consistent with a noise correlation length of about 100 µm at the trap surface, and we discuss how patch potentials of this size would be modified by the electrode geometry.

We investigate confinement-induced resonances in atom-ion quantum mixtures confined in hybrid traps for small atom-ion mass ratios. Specifically, we consider an ion confined in a time-dependent radio-frequency Paul trap with linear geometry, while the atom is constrained to move into a quasi-one-dimensional optical waveguide within the ion trap. We evaluate the impact of the ion intrinsic micromotion on the resonance position. Thus, we solve the atom-ion dynamics semiclassically, namely, the atom dynamics is governed by the three-dimensional time-dependent Schrödinger equation, whereas the ion motion is described by the classical Hamilton equations. We find that the energy of the ion provided by the oscillating radio-frequency fields can affect the resonance position substantially. Notwithstanding, the peculiar phenomenology of those resonances regarding perfect transmission and reflection is still observable. These findings indicate that the intrinsic micromotion of the ion is not detrimental for the occurrence of the resonance and that its position can be controlled by the radio-frequency fields. This provides an additional means for tuning atom-ion interactions in low spatial dimensions. The study represents an important advancement in the scattering physics of compound atomic quantum systems in time-dependent traps.

In this paper we explore the transport properties of three-component Fermi gases confined to one spatial dimension, interacting via a three-body interaction, in the high temperature limit. At the classical level, the three-body interaction is scale invariant in one dimension. However, upon quantization, an anomaly appears which breaks the scale invariance. This is very similar to the physics of two-component fermions in two spatial dimensions, where the two-body interaction is also anomalous. Previous studies have already hinted that the physics of these two systems are intimately related. Here we expand upon those studies by examining the thermodynamic properties of this anomalous one-dimensional system in the high temperature limit. We show there is an exact mapping between the traditional two-body anomalous interaction in two dimensions, to that of three-body interaction in one dimension. This result is valid in the high temperature limit, where the thermodynamics can be understood in terms of few-body correlations.

We introduce a type of self-bound droplet which carries an emergent color charge. We consider a system of classical particles hopping on a lattice and interacting via a commensurately sign-changing potential which is attractive at a short range. The droplet formation is heralded by spontaneous crystallization into topologically distinct domains. This endows each droplet with an emergent color charge governing their mutual interactions: attractive for equal colors and repulsive otherwise. The number of allowed colors is fixed only by the discrete spatial symmetries of the sign-changing part of the interaction potential. With an increasing interaction range, the droplets become progressively more mobile, with their color charge still being energetically protected, allowing for nontrivial viscous dynamics of the interacting droplet plasmas formed during cooling. Sign-changing potentials with a short-range attraction appear quite naturally for light-mediated interactions and we concretely propose a realization in state-of-the-art experiments with cold atoms in a multimode optical cavity.

From an operational criterion of physical reality, a quantifier of realism-based nonlocality was recently introduced for two-part quantum states. This measure has been shown to capture aspects that are rather different from Bell nonlocality. Here we take a step further and introduce a tripartite realism-based nonlocality quantifier. We show that this measure reduces to genuine tripartite entanglement for a certain class of pure tripartite states and manifests itself in correlated mixed states even in the absence of quantum correlations. A case study for noisy Greenberger-Horne-Zeilinger (GHZ) and $W$ states points out the existence of scenarios where the realism-based nonlocality is monogamous.