We propose a potentially practical scheme for creating entanglement between two distant nitrogen-vacancy-center (N-$V$) spin ensembles using a current-biased Josephson junction superconducting qubit and transmission line resonators. The idea provides a scalable way to an N-$V$ ensemble-based quantum network, which is close to the reach with currently available technology.

For potential wells with long-range attractive tails proportional to $-1/r^3$, as occur in the resonant dipole-dipole interaction in homonuclear alkali-metal dimers, we present a highly accurate analytical expression for the tail contribution to the quantization function $F(E)$. This quantization function determines the near-threshold bound-state energies via the quantization rule $n_{\mathrm{th}}-n=F(E_n)$. The performance of the quantization function derived in this paper is demonstrated by applying it to a model Lennard-Jones potential and to vibrational bound-state spectra of sodium dimers (Na${}_2$). These results are compared with those obtained via the semiclassical LeRoy-Bernstein formula which neglects quantum effects that are important in the near-threshold regime.

We theoretically investigate high-order harmonic generation and attosecond pulses by numerically solving the three-dimensional time-dependent Schrödinger equation from a helium ion in a two-color laser field, which is synthesized by adding a 1600-nm laser pulse to a multicycle 800-nm laser pulse. The numerical results show that the short quantum path selection and broadband continuum spectra are achieved by adjusting the relative phase between two laser pulses, and isolated attosecond pulses can be generated successfully. Compared with the case of He${}^{+}$ ions from the 1$s$ ground state, the emission efficiency of the continuous harmonics and the intensity of the isolated attosecond pulse are enhanced approximately thirteen orders of magnitude by preparing He${}^{+}$ ions in a coherent superposition of the states 1$s$ and 2$s$. Furthermore, the bandwidth of the continuum spectrum is further broadened by increasing the intensity of the 1600-nm laser pulse, and an intense 38-as isolated pulse with a bandwidth of 109 eV is straightforwardly obtained.

We study theoretically some effects produced by a propagation of the charged particles in dilute gases of alkali-metal atoms in the state with Bose-Einstein condensates. The energy change of the high-speed (relativistic) particle that corresponds to the Cherenkov effect in the condensate is investigated. We show that in the studied cases the particle can both lose and receive the energy from a gas. We find the necessary conditions for the particle acceleration in the multicomponent condensate. It is shown that the Cherenkov effect in Bose-Einstein condensates can be used also for defining the spectral characteristics of atoms.

We show that current in a two-dimensional electron gas (2DEG) can trap ultracold atoms $<$1 $\mu$m away with orders of magnitude less spatial noise than a metal trapping wire. This enables the creation of hybrid systems, which integrate ultracold atoms with quantum electronic devices to give extreme sensitivity and control: For example, activating a single quantized conductance channel in the 2DEG can split a Bose-Einstein condensate (BEC) for atom interferometry. In turn, the BEC offers unique structural and functional imaging of quantum devices and transport in heterostructures and graphene.

We demonstrate experimentally the nonlocal reversal of a partial-collapse quantum measurement of a two-photon entangled state. Both the partial-collapse measurement and the reversal operation are implemented in linear optics with two displaced Sagnac interferometers, characterized by single-qubit quantum-process tomography. The recovered state is measured by quantum-state tomography, and its nonlocality is characterized by testing the Bell inequality. Our result will be helpful in quantum communication and quantum error correction.

Recently, Freilich et al. [Science 329, 1182 (2010)] experimentally discovered stationary states of vortex dipoles, pairs of vortices of opposite circulation, in dilute Bose-Einstein condensates. To explain their observations, we perform simulations based on the Gross-Pitaevskii equation and obtain excellent quantitative agreement on the size of the stationary dipole. We also investigate how their imaging method, in which atoms are repeatedly extracted from a single condensate, affects the vortex dynamics. We find that it mainly induces isotropic size oscillations of the condensate without otherwise disturbing the vortex trajectories. Thus, the imaging technique appears to be a promising tool for studying real-time superfluid dynamics.

In the context of the paraxial regime, usually valid for optical frequencies and also in the microwave spectrum of guided waves, the propagation of electromagnetic fields can be analyzed through a paraxial wave equation, which is analogous to the nonrelativistic Schrödinger equation of quantum mechanics but replacing time $t$ with spatial coordinate $z$. Considering that, here it is shown that for lossless media in optical frequencies it is possible to construct a Lagrangian operator with an one-to-one correspondence with nonrelativistic quantum mechanics, which allows someone to use the same mathematical methods and techniques for solving problems. To demonstrate that, we explore a few applications in optics with increasing levels of complexity. In the spirit of a Hamiltonian formulation, the ray-tracing trajectories of geometric optics in paraxial regime are obtained in a clear manner. Following that, the gauge symmetries of the optical-field Lagrangian density is discussed in a detailed way, leading to the general form of the interaction Hamiltonian. Through the use of perturbation theory, we discuss a classical analog for a quantum not gate, making use of mode coupling in an isotropic chiral medium. At last, we explore the optical spin Hall effect and its possible applications using an effective geometric optics equation derived from an interaction Hamiltonian for the optical fields. We also predict within the framework of paraxial optics a spin Hall effect of light induced by gravitational fields.

Tripartite entanglement is examined when one of the three parties moves with a uniform acceleration with respect to other parties. As the Unruh effect indicates, tripartite entanglement exhibits a decreasing behavior with increasing acceleration. Unlike bipartite entanglement, however, tripartite entanglement does not completely vanish in the infinite acceleration limit. If the three parties, for example, share the Greenberger-Horne-Zeilinger or $W$ state initially, the corresponding $\pi$-tangle, one of the measures of tripartite entanglement, is shown to be $\pi /6~0.524$ or $0.176$ in this limit, respectively. This fact indicates that tripartite quantum-information processing may be possible even if one of the parties approaches the Rindler horizon. The physical implications of this striking result are discussed in the context of black-hole physics.

We consider two recently proposed measures of non-Markovianity applied to a particular quantum process describing the dynamics of a driven qubit in a structured reservoir. The motivation for this study is twofold: on one hand, we study the differences and analogies of the non-Markovianity measures, and on the other hand, we investigate the effect of the driving force on the dissipative dynamics of the qubit. In particular we ask if the driving force introduces new channels for energy and/or information transfer between the system and the environment or if it amplifies existing ones. We show under which conditions the presence of the driving force slows down the inevitable loss of quantum properties of the qubit.

We present a general criterion for entanglement of $N$ indistinguishable particles decomposed into arbitrary $s$ subsystems based on the unambiguous measurability of correlation. Our argument provides a unified viewpoint on the entanglement of indistinguishable particles, which is still unsettled despite various proposals made mainly for the $s=2$ case. Even though entanglement is defined only with reference to the measurement setup, we find that the so-called independently and identically distributed states form a special class of bosonic states which are universally separable.

Spin chains have been proposed as a reliable and convenient way of transferring information and entanglement in a quantum computational context. Nonetheless, it has to be expected that any physical implementation of these systems will be subject to several perturbative factors which could potentially diminish the transfer quality. In this paper, we investigate a number of possible fabrication defects in the spin chains themselves as well as the effect of nonsynchronous or imperfect input operations, with a focus on the case of multiple excitation and qubit transfer. We consider both entangled and unentangled states and, in particular, the transfer of an entangled pair of adjacent spins at one end of a chain under the mirroring rule and also the creation of entanglement resulting from injection at both end spins.

The optimal control of unitary transformations is a fundamental problem in quantum control theory and quantum information processing. The feasibility of performing such optimizations is determined by the computational and control resources required, particularly for systems with large Hilbert spaces. Prior work on unitary transformation control indicates that (i) for controllable systems, local extrema in the search landscape for optimal control of quantum gates have null measure, facilitating the convergence of local search algorithms, but (ii) the required time for convergence to optimal controls can scale exponentially with the Hilbert space dimension. Depending on the control-system Hamiltonian, the landscape structure and scaling may vary. This work introduces methods for quantifying Hamiltonian-dependent and kinematic effects on control optimization dynamics in order to classify quantum systems according to the search effort and control resources required to implement arbitrary unitary transformations.

We analyze the optimal measurements to access classical correlations in arbitrary two-qubit states. Two-qubit states can be transformed into the canonical forms via local unitary operations. For the canonical forms, we investigate the probability distribution of the optimal measurements. The probability distribution of the optimal measurements is found to be centralized in the vicinity of a specific von Neumann measurement, which we call the maximal-correlation-direction measurement (MCDM). We prove that, for the states with zero discord and maximally mixed marginals, the MCDM is the optimal measurement. Furthermore, we give an upper bound of quantum discord based on the MCDM, and investigate its performance for approximating the quantum discord.

Changes in the mean square nuclear charge radii along the lithium isotopic chain were determined using a combination of precise isotope shift measurements and theoretical atomic structure calculations. Nuclear charge radii of light elements are of high interest due to the appearance of the nuclear halo phenomenon in this region of the nuclear chart. During the past years we have developed a laser spectroscopic approach to determine the charge radii of lithium isotopes which combines high sensitivity, speed, and accuracy to measure the extremely small field shift of an 8-ms-lifetime isotope with production rates on the order of only 10 000 atoms/s. The method was applied to all bound isotopes of lithium including the two-neutron halo isotope ${}^{11}\mathrm{Li}$ at the on-line isotope separators at GSI, Darmstadt, Germany, and at TRIUMF, Vancouver, Canada. We describe the laser spectroscopic method in detail, present updated and improved values from theory and experiment, and discuss the results.

We study the polarity of the Casimir force between two different parallel slabs separated by vacuum when the saturated ferrite materials under the influence of an external magnetic field are taken into consideration. Between the ordinary dielectric slab and the ferrite slab, repulsive Casimir force may be observed by adjusting the applied magnetic field. For the ferrite material, we consider the frequency dependence of the permeability modified by the external magnetic field to analyze the formation of the repulsive Casimir force. The restoring force, which means the transition of the force polarity from repulsion to attraction with the increasing slab separation, can also be obtained between two different ferrite slabs.

We address a model for atomic population kinetics in fluctuating plasmas. An analytical expression for the ensemble-average populations is obtained in terms of two statistical functions that can be retrieved experimentally, namely the probability density function and the autocorrelation of the plasma fluid fields. This expression, allowing for very fast calculations, is of great interest to thoroughly analyze the radiative properties of fluctuating plasmas. Two limits, where the fluctuations are either faster or slower than the atomic relaxation time scales, are discussed in detail. Finally, an application to atomic hydrogen is presented.

Dielectronic recombination (DR) of xenonlike ${\mathrm{W}}^{20+}$ forming ${\mathrm{W}}^{19+}$ has been studied experimentally at a heavy-ion storage ring. A merged-beams method has been employed for obtaining absolute rate coefficients for electron-ion recombination in the collision-energy range 0–140 eV. The measured rate coefficient is dominated by strong DR resonances even at the lowest experimental energies. At plasma temperatures where the fractional abundance of ${\mathrm{W}}^{20+}$ is expected to peak in a fusion plasma, the experimentally derived plasma recombination rate coefficient is over a factor of 4 larger than the theoretically calculated rate coefficient which is currently used in fusion plasma modeling. The largest part of this discrepancy stems most probably from the neglect in the theoretical calculations of DR associated with fine-structure excitations of the ${\mathrm{W}}^{20+}$([Kr]$4d^{10}\hspace{0.5em}4f^8$) ion core.

Polarization spectroscopy of He${}^{+}$($\mathit{nl}$) ion produced in He${}^{2+}$-H(1$s$) collisions in a Debye plasma is studied by the two-center atomic orbital close-coupling (TC-AOCC) method in the energy range 1–200 keV/u. The atomic orbitals and electron binding energies of atomic states are calculated within the Debye-Hückel approximation of the screened Coulomb potential and used in the AOCC dynamics formalism to calculate the magnetic substate-selective electron-capture cross sections. It is demonstrated that the screening of Coulomb interactions affects the entire collision dynamics and the magnitude and energy behavior of $m$-state-selective cross sections. The changes in magnetic substate-selective electron-capture cross sections when the interaction screening varies introduce dramatic changes in the polarization spectroscopy with respect to the unscreened interaction case. The magnetic substate-selective electron-capture cross sections, as well as the polarization degrees for the $2p\hspace{0.5em}{}^2{P}_{3/2}$–$1s\hspace{0.5em}{}^2{S}_{1/2},3p\hspace{0.5em}{}^2{P}_{3/2}$–$2s\hspace{0.5em}{}^2{S}_{1/2},3d\hspace{0.5em}{}^2{D}_{5/2}$–$2p\hspace{0.5em}{}^2{P}_{3/2}$, and $3d\hspace{0.5em}{}^2{D}_{3/2}$–$2p\hspace{0.5em}{}^2{P}_{1/2}$ transitions in the He${}^{+}$ ion produced in this collision system, are presented and discussed for a number of representative screening parameter values.

The collisional magnetic reorientation rate constant $g_R$ is measured for magnetically trapped atomic dysprosium (Dy), an atom with large magnetic dipole moments. Using buffer gas cooling with cold helium, large numbers ($>{10}^{11}$) of Dy are loaded into a magnetic trap and the buffer gas is subsequently removed. The decay of the trapped sample is governed by collisional reorientation of the atomic magnetic moments. We find $g_R=1.9\pm 0.5\times {10}^{-11}$ cm${}^3$ s${}^{-1}$ at 390 mK. We also measure the magnetic reorientation rate constant of holmium (Ho), another highly magnetic atom, and find $g_R=5\pm 2\times {10}^{-12}$ cm${}^3$ ${\mathrm{s}}^{-1}$ at 690 mK. The Zeeman relaxation rates of these atoms are greater than expected for the magnetic dipole-dipole interaction, suggesting that another mechanism, such as an anisotropic electrostatic interaction, is responsible. Comparison with estimated elastic collision rates suggests that Dy is a poor candidate for evaporative cooling in a magnetic trap.