We investigate dipolar Bose-Einstein condensates in a complex external double-well potential that features combined parity and time-reversal symmetry. On the basis of the Gross-Pitaevskii equation we study the effects of the long-ranged anisotropic dipole-dipole interaction on ground and excited states by the use of a time-dependent variational approach. We show that the property of a similar nondipolar condensate of possessing real energy eigenvalues in certain parameter ranges is preserved despite the inclusion of this nonlinear interaction. Furthermore, we present states that break the $\mathcal{PT}$ symmetry and investigate the stability of the distinct stationary solutions. In our dynamical simulations we reveal a complex stabilization mechanism for $\mathcal{PT}$-symmetric as well as for $\mathcal{PT}$-broken states which are, in principle, unstable with respect to small perturbations.

It is common knowledge that coupling to a heat bath, in general, tends to reduce the entanglement in a quantum system. In recent years, increasing interest has been devoted to the opposite situation where thermal or specifically tailored environments may stabilize or even generate entanglement. We prove the universality of this phenomenon for multipartite entanglement in the frame of an exactly solvable dephasing model. We show by evaluating analytical lower bounds for the appropriate entanglement measures that a common thermal environment can dynamically generate all entanglement classes of three qubits for almost any initial product state. For sufficiently weak dissipation this entanglement may persist up to arbitrarily large times.

We measure ion heating following transport throughout a $\mathsf{Y}$-junction surface-electrode ion trap. By carefully selecting the trap voltage update rate during adiabatic transport along a trap arm, we observe minimal heating relative to the anomalous heating background. Transport through the junction results in an induced heating between 37 and 150 quanta in the axial direction per traverse. To reliably measure heating in this range, we compare the experimental sideband envelope, including up to fourth-order sidebands, to a theoretical model. The sideband envelope method allows us to cover the intermediate heating range inaccessible to the first-order sideband and Doppler recooling methods. We conclude that quantum information processing in this ion trap will likely require sympathetic cooling in order to support high fidelity gates after junction transport.

Charge-exchange cross sections are presented for collisions of positrons with hydrogen and neutral and singly ionized helium targets using a variant of the classical trajectory Monte Carlo approach. As a check on the method a comparison is made with the corresponding proton results. An extended error analysis is presented. Reasonable agreement with available experimental data is found, and the charge-exchange cross section for positrons on ${\text{He}}^{+}$ is predicted.

We study the creation and entanglement of quasiparticle pairs due to a periodic variation of the mode frequencies of a homogeneous quantum system. Depending on the values of the parameters describing the periodic modulation, the number of created pairs either oscillates or, in a narrow resonant frequency interval, grows exponentially in time. For a system initially in a thermal state, we determine in which cases the final state is quantum mechanically entangled, i.e., where the bipartite state is nonseparable. We include some weak dissipation, expected to be found in any experimental setup, and study the corresponding reduction of the quantum entanglement. Our findings are used to interpret the results of two recent experiments.

We consider an ultracold bosonic binary mixture confined in a quasi-one-dimensional double-well trap. The two bosonic components are assumed to be two hyperfine internal states of the same atom. We suppose that these two components are spin-orbit coupled to each other. We employ the two-mode approximation starting from two coupled Gross-Pitaevskii equations and derive a system of ordinary differential equations governing the temporal evolution of the interwell population imbalance of each component and of the polarization, which is the imbalance of the total populations of the two species. From this set of equations we disentangle the different macroscopic quantum tunneling and self-trapping scenarios occurring for both population imbalances and the polarization in terms of the interplay between the interatomic interactions and the other relevant energies in the problem, like the spin-orbit coupling or the conventional tunneling term. We find a rich dynamics in all three variables and discuss the experimental feasibility of such a system.

We show that two interacting physical systems may admit entangled pure or nonseparable mixed states evolving in time as if the mutual interaction Hamiltonian were absent. In this paper we define these interaction-free evolving (IFE) states and characterize their existence for a generic binary system described by a time-independent Hamiltonian. A comparison between IFE subspace and the decoherence-free subspace is reported. The set of all pure IFE states is explicitly constructed for a nonhomogeneous spin-star-system model

We show that propagation with the best possible adiabatic approximation in time-dependent density-functional theory fails to properly transfer charge in an asymmetric two-site Hubbard model when beginning in the ground state. The approximation is adiabatic but exact otherwise, constructed from the exact ground-state exchange-correlation functional that we compute via constrained search. The model shares the essential features of charge-transfer dynamics in a real-space long-range molecule, so the results imply that the best possible adiabatic approximation, despite being able to capture nonlocal ground-state step features relevant to dissociation and charge-transfer excitations, cannot capture fully time-resolved charge-transfer dynamics out of the ground state.

We use selective laser excitation to an autoionizing state to observe the microwave transitions of Ba from the $6sng$ Rydberg states to the $6snh$, $6sni$, and $6snk$ states for $15\le n\le 18$. We extract the dipole and quadrupole polarizabilities of Ba${}^{+}$ from the measured $\Delta \ell$ intervals of the Ba $6sn\ell$ states of $\ell \ge 5$ using a nonadiabatic core polarization model. The values we determine for the dipole and quadrupole polarizabilities are ${\alpha }_d=124.81\left(25\right)a_0^3$ and ${\alpha }_q=2478\left(50\right)a_0^5$, respectively.

We present a nonadiabatic treatment of the hydrogen-antihydrogen system. The technique used to describe $\mathrm{H}\text{−}\overline{\mathrm{H}}$ collisions is based on the coupled-rearrangement-channel method. Within this approach the total, nonadiabatic wave function of the system is divided into two parts: an inner and an outer one. To describe the inner part a set of square-integrable four-body functions is used. These functions are obtained by a diagonalization of the total Hamiltonian projected on a chosen $L^2$ subspace; they explicitly contain components of various arrangement channels expressed in terms of corresponding Jacobi coordinates. The outer part of the total wave function reflects its asymptotic character. Our procedure leads to a system of nonlocal integrodifferential equations that are solved iteratively and simultaneously determine the outer part of the solution and the coefficients in the four-body expansion of the inner part. To solve these equations the compact fine difference method was applied. Using this formalism we perform a one-channel calculation of the elastic scattering to obtain the $S$ matrix, the nonadiabatic scattering length, and the cross section for the low-energy elastic scattering in the $\mathrm{H}\text{−}\overline{\mathrm{H}}$ channel.

Experimental observations of nonstationary coherent optical phenomena, i.e., optical nutation, free induction, and photon echo, in the acetylene (${}^{12}$C${}_2$H${}_2$) filled hollow-core photonic-crystal fiber (PCF) are reported. The presented results were obtained for the acetylene vibration-rotational transition $P$9 at wavelength 1530.37 nm at room temperature under a gas pressure of <0.5 Torr. An all-fiber pumped-through cell based on the commercial 2.6-m-long PCF with a 10-μm hollow-core diameter was used. The characteristic relaxation time $T_2$ during which the optical coherent effects were typically observed in our experiments was estimated to be ≈8 ns. This time is governed by the limited time of the acetylene molecules’ presence inside the effective PCF modal area and by intermolecule collisions. An accelerated attenuation of the optical nutation oscillations is explained by a random orientation of acetylene molecules.

We investigate the ionization dynamics of a crystal structure driven by ultrafast coherent x-ray pulses of moderate to high intensities for excitations where dipole-allowed, single-photon ionization dominates. Using a simple model of the crystal, we demonstrate that quantum coherences may already play an important role at moderate pulse intensities, leading to qualitatively novel features which cannot be described by rate equations. In particular, the ionization may exhibit a minimum as a function of the pulse duration, where the ionization drops to almost zero, although during the pulse a noticeable fraction of the electrons is promoted to unbound states. For higher intensities, the qualitative deviations between the coherent quantum-mechanical treatment and the rate description is even more pronounced. In particular, due to the presence of quantum-mechanical coherences, the full theory predicts, even for the single-photon transitions to a continuum of free-electron states, a Rabi-type behavior similar to what is known for two-level systems.

We investigate the effect of the down-conversion angle between the signal and idler beams in spontaneous parametric down-conversion on the bandwidth of the modal spectrum (Schmidt number) of the down-converted quantum state. For this purpose, we consider both the full Schmidt number and the azimuthal Schmidt number in the Laguerre-Gaussian basis. For the full Schmidt number, we show that the approximation of the phase-matching function with a Gaussian function gives results that disagree significantly from those obtained with the more physical sinc-type phase-matching function. We found a drastic increase in both the Schmidt number and the azimuthal Schmidt number for small down-conversion angles, in agreement with recent experimental results.

We propose an experiment to test the influence of Lorentz contraction on the interference pattern of a beam of electrons. The electron beam is split and recombined by two pairs of bichromatic laser pulses, using a variation of the Kapitza-Dirac effect. Between the pairs, the electrons are accelerated to relativistic speed. We show that Lorentz contraction of the distance between two partial beams will then lead to a reduction of fringe visibility. The connection of the proposal to Bell's spaceship paradox is discussed.

We present a protocol for quantum fingerprinting that is ready to be implemented with current technology and is robust to experimental errors. The basis of our scheme is an implementation of the signal states in terms of a coherent state in a superposition of time-bin modes. Experimentally, this requires only the ability to prepare coherent states of low amplitude and to interfere them in a balanced beam splitter. The states used in the protocol are arbitrarily close in trace distance to states of $O\left({log}_{2}n\right)$ qubits, thus exhibiting an exponential separation in abstract communication complexity compared to the classical case. The protocol uses a number of optical modes that is proportional to the size $n$ of the input bit strings but a total mean photon number that is constant and independent of $n$. Given the expended resources, our protocol achieves a task that is provably impossible using classical communication only. In fact, even in the presence of realistic experimental errors and loss, we show that there exist a large range of input sizes for which our quantum protocol transmits an amount of information that can be more than two orders of magnitude smaller than a classical fingerprinting protocol.

We propose a scheme to control the coupling between the charge qubit and the nitrogen-vacancy center ensemble (NVCE). The coupling is mediated by a large Josephson junction (JJ) and a flux qubit. The large JJ can be treated classically and behaves like an effective inductance to switch the coupling between the charge and flux qubits. The flux qubit serves as a bridge to couple the NVCE and the charge qubit, and quantum state transfer can be implemented, with the NVCE acting as a memory unit. The hybrid circuit can reinforce various advantages: controllability of the charge qubit and robust spin coherence of the NVCE.

We study excitation and ionization in the 1.5-MeV/amu ${\mathrm{O}}^{8+}$-Li collision system, which was the subject of a recent reaction-microscope-type experiment [D. Fischer et al., Phys. Rev. Lett. 109, 113202 (2012)]. Starting from an independent-electron model based on determinantal wave functions and using a single-electron basis generator method calculation and a single-electron continuum distorted-wave with eikonal initial-state calculation, we show that pure single ionization of a lithium $K$-shell electron is too weak a process to explain the measured electron-energy-differential cross section. Rather, our analysis suggests that two-electron excitation-ionization processes occur and have to be taken into account when comparing with the data. Good agreement is obtained only if we replace the independent-electron calculation by an independent-event model for one of the excitation-ionization processes and also take a shake-off process into account.

Using fitted model potential curves of the ground and lowest three excited states yielded by the relativistic Kramers-restricted multireference configuration interaction method with 19 electrons correlated, we theoretically investigate the rovibrational properties including the number of vibrational state and diagonally distributed Franck-Condon factors for a ${}^{87}\text{Rb}{}^{84}\text{Sr}$ molecule. Benefiting from a turning point at about $v^{\prime }=20$ for the Franck-Condon factors between the ground state and spin-orbit $2(\Omega =1/2)$ excited state, we choose $|2(\Omega =1/2),v^{\prime }=21,J^{\prime }=1\rangle$ as the intermediate state in the three-level model to theoretically analyze the possibility of performing stimulated Raman adiabatic passage to transfer weakly bound RbSr molecules to the rovibrational ground state. With 1550 nm pump laser (2 W/${\text{cm}}^2$) and 1342 nm dump laser (10 mW/${\text{cm}}^2$) employed and appropriate settings of pulse time length (about 300 $\mu$s), we have formalistically achieved a round-trip transfer efficiency of 60%, namely 77% for one-way transfer. The results demonstrate the possibility of producing polar ${}^{87}\text{Rb}{}^{84}\text{Sr}$ molecules efficiently in a submicrokelvin regime, and further provide promising directions for future theoretical and experimental studies on alkali-alkaline(rare)-earth dimers.

Quantum magnetism is a fundamental phenomenon of nature. As of late, it has garnered a lot of interest because experiments with ultracold atomic gases in optical lattices could be used as a simulator for phenomena of magnetic systems. A paradigmatic example is the time evolution of a domain-wall state of a spin-$1/2$ Heisenberg chain, the so-called domain-wall melting. The model can be implemented by having two species of bosonic atoms with unity filling and strong on-site repulsion $U$ in an optical lattice. In this paper, we study the domain-wall melting in such a setup on the basis of the time-dependent density matrix renormalization group (tDMRG). We are particularly interested in the effects of defects that originate from an imperfect preparation of the initial state. Typical defects are holes (empty sites) and flipped spins. We show that the dominating effects of holes on observables like the spatially resolved magnetization can be taken account of by a linear combination of spatially shifted observables from the clean case. For sufficiently large $U$, further effects due to holes become negligible. In contrast, the effects of spin flips are more severe as their dynamics occur on the same time scale as that of the domain-wall melting itself. It is hence advisable to avoid preparation schemes that are based on spin flips.

We study the dephasing-assisted parameter estimation precision (PEP) enhancement in an atom interferometer with dynamical decoupling (DD) pulses. Through calculating the spin squeezing (SS) and the quantum Fisher information, we find that dephasing noise can improve PEP by inducing SS, and the DD pulses can maximize the improvement. It is indicated that when using the DD pulse the dephasing-induced SS can reach the limit of “one-axis twisting” model ${\xi }^2\simeq N^{-2/3}$ with ${\xi }^2$ being the SS parameter and $N$ the number of atoms. In particular, we find that the DD pulses can amplify the dephasing-induced quantum Fisher information by a factor of $\simeq N/2$ compared with the noise-free case, which means that under the control of DD pulses, the dephasing noise can enhance the precision of parameter estimation to the scale of $\sqrt{2}/N$, the same scale of the Heisenberg limit ($1/N$).