We show that controllable inhomogeneous coupling between two-level systems and a common data bus provides a fast mechanism to produce multipartite entanglement. Our proposal combines resonant interactions and engineering of coupling strengths—between the qubits and the single mode—leading to well-defined entangled states. Furthermore, we show that, if the two-level systems interact dispersively with the quantized mode, engineering of coupling strengths allows the controlled access of the symmetric Hilbert space of qubits.

In this paper, we study electron correlation and Gaunt interaction effects in ionization potentials (IPs) and hyperfine constants A of $2p{}^2{P}_{1/2}$ and $2p{}^2{P}_{3/2}$ states along with fine structure splitting (FSS) between them for the boron isoelectronic sequence using the relativistic coupled-cluster method. The range of atomic number $z$ is taken from 8 to 21. The Gaunt contributions can be calculated at both the Dirac-Fock and the coupled-cluster levels from our presentation. Calculated IPs and FSS are compared with the results of the National Institute of Standards and Technology. Important correlation contributions like the core correlation, core polarization, and pair correlation effects are studied for hyperfine constants A. Many distinct features of the correlation and relativistic effects are observed in these studies.

The copper $K\alpha$ spectrum contains asymmetries and satellite features due to secondary electron emission during ionization. Theoretical attempts at determining the intensities of these features are highly discrepant from experimental results. This discrepancy has been the subject of much discussion. In the present work we show that widely applied fitting procedures produce satellite intensities which depend strongly on assumptions regarding parametrization. We also show that recent high-accuracy satellite calculations can provide a copper $K\alpha$ spectrum in good agreement with experiment, thus resolving the discrepancy. This represents a major step toward a complete ab initio x-ray spectrum, a goal with significant implications for astrophysics, plasma physics, and tests of quantum electrodynamics.

A quantum study of the bound states and infrared predissociation spectra of H${}_5{}^{+}$ is done using a recently proposed global and accurate potential-energy surface [Aguado et al., J. Chem. Phys. 133, 024306 (2010)]. The bound states are calculated for seven degrees of freedom using an iterative Lanczos method, yielding a dissociation energy in very good agreement with the available experimental data. The predissociation states are described by a wave-packet treatment considering a shared-proton model, in which the three-dimensional motion of the central atom is described using non-Jacobi bond coordinates. The justification of this model is that the change in the electric dipole moment is larger as a function of the motion of the central atom, responsible for the proton transfer in the H${}_3{}^{+}+{\mathrm{H}}_2\to {\mathrm{H}}_2+{\mathrm{H}}_3{}^{+}$ reaction. The electric dipole moment is calculated at the level of coupled-cluster theory with single and double excitations and fitted in nine dimensions to an analytical function. With it, the infrared predissociation spectrum is simulated, yielding a reasonable agreement with recent measurements.

We present a time-dependent method to investigate the mass polarization effect on the resonant energies of $\overline{p}-{\text{He}}^{+}$ and the state-specified protonium formation in low-energy antiproton–hydrogen-atom collisions in Jacobi coordinates. Comparing with our previous calculations in the $\mathsf{V}$-shaped coordinates by neglecting the mass polarization, we confirm that the mass polarization effect on the protonium formation is negligibly small. We also calculated the resonant energies of $\overline{p}-{\text{He}}^{+}$ in the $\mathsf{V}$ coordinates and Jacobi coordinates. The differences of the resonant energies between the two calculations are less than 4 meV. This also confirms that the mass polarization effect is negligibly small even in the resonant region.

Nonlinear magneto-optical resonances on the hyperfine transitions belonging to the $D_2$ line of rubidium were changed from bright to dark resonances by changing the laser power density of the single exciting laser field or by changing the vapor temperature in the cell. In one set of experiments atoms were excited by linearly polarized light from an extended cavity diode laser with polarization vector perpendicular to the light's propagation direction and magnetic field, and laser-induced fluorescence was observed along the direction of the magnetic field, which was scanned. A low-contrast bright resonance was observed at low laser power densities when the laser was tuned to the $F_g=2\to F_e=3$ transition of ${}^{87}$Rb and near to the $F_g=3\to F_e=4$ transition of ${}^{85}$Rb. The bright resonance became dark as the laser power density was increased above 0.6 mW/cm${}^2$ or 0.8 mW/cm${}^2$, respectively. When the $F_g=2\to F_e=3$ transition of ${}^{87}$Rb was excited with circularly polarized light in a second set of experiments, a bright resonance was observed, which became dark when the temperature was increased to around 50${}^{\circ }$C. The experimental observations at room temperature could be reproduced with good agreement by calculations based on a theoretical model, although the theoretical model was not able to describe measurements at elevated temperatures, where reabsorption was thought to play a decisive role. The model was derived from the optical Bloch equations and included all nearby hyperfine components, averaging over the Doppler profile, mixing of magnetic sublevels in the external magnetic field, and a treatment of the coherence properties of the exciting radiation field.

We investigate the dynamics of molecular photoexcitation by unchirped femtosecond laser pulses using RbCs as a model system. This study is motivated by a goal of optimizing a two-color scheme of transferring vibrationally excited ultracold molecules to their absolute ground state. In this scheme the molecules are initially produced by photoassociation or magnetoassociation in bound vibrational levels close to the first dissociation threshold. We analyze here the first step of the two-color path as a function of pulse intensity from the low-field to the high-field regime. We use two different approaches, a global one, the “wave-packet” method, and a restricted one, the “level-by-level” method, where the number of vibrational levels is limited to a small subset. The comparison between the results of the two approaches allows one to gain qualitative insights into the complex dynamics of the high-field regime. In particular, we emphasize the nontrivial and important role of far-from-resonance levels which are adiabatically excited through “vertical” transitions with a large Franck-Condon factor. We also point out the spectacular excitation blockade due to the presence of a quasidegenerate level in the lower electronic state. We conclude that selective transfer with femtosecond pulses is possible in the low-field regime only. Finally, we extend our single-pulse analysis and examine population transfer induced by coherent trains of low-intensity femtosecond pulses.

The loading dynamics of an alkali-metal-atom magneto-optical trap can be used as a reliable measure of vacuum pressure, with loading time $\tau$ indicating a pressure less than or equal to ($2\times {10}^{-8}$ Torr s)/$\tau$. This relation is accurate to approximately a factor of 2 over wide variations in trap parameters, background gas composition, or trapped alkali-metal species. The low-pressure limit of the method does depend on the trap parameters, but typically extends to below $1\times {10}^{-9}$ Torr.

We investigate theoretically the prospect of realizing a topological superfluid in one-dimensional spin-orbit-coupled atomic Fermi gases under a Zeeman field in harmonic traps. In the absence of spin-orbit coupling, it is well known that the system is either a Bardeen-Cooper-Schrieffer superfluid or an inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluid. Here we show that with spin-orbit coupling it could be driven into a topological superfluid, which supports zero-energy Majorana modes. However, in the weakly interacting regime the topological superfluid prefers to stay at the trap edge, in contrast to a FFLO superfluid, which occurs near the trap center. As a result, it is unlikely to experimentally observe an inhomogeneous FFLO superfluid with topo-logical order without specifically tailoring the geometry or other parameters of the Fermi cloud.

Breaking the diffraction limit in microscopy by utilizing the quantum properties of light has been the goal of intense research in recent years. We propose a super-resolution technique based on nonclassical emission statistics of fluorescent markers, routinely used as contrast labels for bioimaging. The technique can be readily implemented with current technology.

The physics of photons and electrons carrying orbital angular momentum (OAM) is an exciting field of research in quantum optics and electron microscopy. Usually, one considers propagation of these vortex beams in a medium or an external field and their absorption or scattering on fixed targets. Here we consider instead a beam-beam collision. We show that elastic scattering of a Bessel vortex beam with a counterpropagating plane wave naturally leads to two vortex-entangled outgoing beams. The vortex entanglement implies that the two final particles are entangled not only in their orbital helicities but also in the opening angles of their momentum cones. Our results are driven by kinematics of vortex-beam scattering and apply to particle pairs of any nature: $e\gamma$, $e^{+}{e}^{-}$, $ep$, etc. This collisional vortex entanglement can be used to create pairs of OAM-entangled particles of different nature and to transfer a phase vortex, for example, from low-energy electrons to high-energy protons.

The effects arising from the inherent continuous-mode nature of photonic pulses were poorly understood but significantly influence the performance of quantum devices employing photonic pulse interaction in nonlinear media. Such effects include the entanglement between the continuous wave-vector modes due to pulse interaction as well as the consequence of a finite system bandwidth. We present an analysis on these effects for interactions between single-photon pulses, demonstrating their impact on the performance of quantum phase gates based on such process. Our study clarifies a realistic picture of this type of quantum devices.

We demonstrate the generation of slow and fast light (SFL) in a linear and passive three-beam interferometer. Such propagation regimes occur for narrowband pulses with center frequency close to the transmission minima. A model that fully describes SFL effects in this system is developed and an analytical approximate expression for the group delay at the minima is derived. We demonstrate that slow light is not possible if the length difference between adjacent branches of the interferometer is a constant. If a small length detuning ($\xi$) in one of the branches is introduced, slow light at one of the two minima can be obtained as long as $\xi$ exceeds a critical value. Simultaneously, tunneling, superluminal, or normal regime is sustained at the other minimum, depending on the system's length. A proof-of-model experiment is performed in the radiofrequency range using coaxial cables and $1\times 3$ power splitters. The possible realization and performance of such a system in the optical range is also discussed. This system is proposed as a simple alternative to active systems and photonic band-gap structures for sustaining both slow and fast light.

The interaction of a single-photon wave packet with an initially excited two-level atom in free space is studied in semiclassical and quantum approaches. It is shown that the final state of the field does not contain doubly occupied modes. The process of the atom's transition to the ground state may be accelerated, decelerated, or even reversed by the incoming photon, depending on parameters. The spectrum of emitted radiation is close to the sum of the spectrum of the incoming single-photon wave packet and the natural line shape, with small and complicated deviations.

Principles of monitoring non-Markovian open quantum systems are analyzed. We use a field representation of the environment [C. W. Gardiner and M. J. Collett, Phys. Rev. A 31, 3761 (1985)] for the separation of its memory and detector part, respectively. We claim that the system-plus-memory compound becomes Markovian; the detector part is tractable by standard Markovian monitoring. Because of non-Markovianity, only the mixed state of the system can be predicted; the pure state of the system can be retrodicted. We present the corresponding non-Markovian stochastic Schrödinger equation.

We present a route to Bose-Einstein condensation devised for two-electron atoms, which do not admit practicable cooling techniques based upon narrow intercombination lines. A dipole trap for ${}^{40}$Ca atoms in the singlet ground state is loaded from a moderately cold source of metastable triplet atoms via spatially and energetically selective optical pumping permitting 4 orders of magnitude increase of the phase-space density. Further cooling to quantum degeneracy is achieved by forced evaporation optimized to minimize three-body losses. In a combined loading and evaporation cycle of less than 3 s we are able to condense 3000 atoms.

In order to quantify the information content of quantum states in the presence of conserved quantities, Wigner and Yanase introduced the notion of skew information [Proc. Natl. Acad. Sci. USA 49, 910, (1963)], which was later identified as a paradigmatic version of quantum Fisher information. The skew information is quite different from, yet deeply connected with, the ubiquitous quantum entropies and has important applications in quantum theory. In this paper, pursuing further the original idea of Wigner and Yanase, we propose a measure for correlations in terms of the skew information, investigate its fundamental properties, and elucidate its characteristics. An appealing feature of this measure for correlations is that its evaluation does not involve any optimization, in sharp contrast to the entanglement and discord measures, and can be straightforwardly calculated. In particular, the algorithm and explicit formulas for general bipartite states are prescribed, and simple analytical expressions for some special states, including arbitrary bipartite pure states, the Bell-diagonal states, and some highly symmetric states such as the Werner states and the isotropic states, are obtained.

Some chemical reactions are described by electron transfer (ET) processes. The underlying mechanism could be modeled as a polaron motion in the molecular crystal—the Holstein model. By taking spin degrees of freedom into consideration, we generalize the Holstein model (molecular crystal model) to microscopically describe an ET chemical reaction. In our model, the electron spins in the radical pair simultaneously interact with a magnetic field and their nuclear-spin environments. By virtue of the perturbation approach, we obtain the chemical reaction rates for different initial states. It is discovered that the chemical reaction rate of the triplet state demonstrates its dependence on the direction of the magnetic field while the counterpart of the singlet state does not. This difference is attributed to the explicit dependence of the triplet state on the direction when the axis is rotated. Our model may provide a possible candidate for the microscopic origin of the avian compass.

We present triple-differential cross sections for electron-impact ionization of a $3p$ electron in Ar. Results from a fully nonperturbative close-coupling formalism using our $B$-spline $R$-matrix approach are compared with those from a hybrid distorted-wave plus $R$-matrix (close-coupling) expansion as well as recent experimental data of Ren et al. [Phys. Rev. A 83, 052714 (2011)]. We find overall good agreement between the two sets of entirely independent theoretical predictions but serious discrepancies with the published experimental data regarding the drop of the cross section with increasing detection angle of “scattered projectile,” i.e., the faster of the two outgoing electrons. A detailed investigation of the dependence of the results on this angle suggests that obtaining reliable data, both experimentally and theoretically, is highly challenging in the regime where the largest discrepancies occur.

We consider the optimal control problem of transferring population between states of a quantum system where the coupling proceeds only via intermediate states that are subject to decay. We pose the question whether it is generally possible to carry out this transfer. For a single intermediate decaying state, we recover the stimulated Raman adiabatic passage process, which we identify as the global optimum in the limit of infinite control time. We also present analytical solutions for the case of transfer that has to proceed via two consecutive intermediate decaying states. We show that in this case, for finite power the optimal control does not approach perfect state transfer even in the infinite time limit. We generalize our findings to characterize the topologies of paths that can be achieved by coherent control under the assumption of finite power. If two or more consecutive states in an $N$-level chain are subject to decay, complete population transfer with finite-power controls is not possible.