We study the exchange and correlation hole of the valence shell of second-row atoms using variational Monte Carlo techniques, especially correlated estimates, and norm-conserving pseudopotentials. The well-known scaling of the valence shell provides a tool to probe the behavior of exchange and correlation as a functional of the density and thus test models of density functional theory. The exchange hole shows an interesting competition between two scaling forms—one caused by self-interaction and another that is approximately invariant under changes in the particle number, related to the known invariance of exchange under uniform scaling to high density with the particle number held constant. The correlation hole shows a scaling trend that is marked by the finite size of the atom relative to the radius of the hole. Both trends are well captured in the main by the Perdew-Burke-Ernzerhof generalized-gradient approximation model for the exchange-correlation hole and energy.

In this work, we present spectral data of helium atoms with the screened Coulomb interactions using the $B$-spline configuration interaction approach. One of the challenges that we face is the electron-electron interaction under the screening effect. Instead of using explicitly correlated exponential wave functions which depend on the interelectron distance, we expand the screened electron-electron potential utilizing the Gegenbauers addition theorem for the separation of the radial variables. Several energies of the ${}^{1,3}S$, ${}^{1,3}P$, and ${}^{1,3}D$ states varying with screening parameters are reported. It is noted that the screening effects lead to the level crossings for high-lying states. We demonstrate the influence of screening on the oscillator strengths for the transitions of ${}^{1}S$-${}^{1}P$, ${}^{3}S{-}^{3}P$, ${}^{1}P{-}^{1}D$, and ${}^{3}P{-}^{3}D$. The variation of the multipole polarizabilities, such as dipole, quadrupole, and octupole, with the screening effect is reported.

Nonadiabatic inelastic collision cross sections have been computed at low and ultralow energies, focusing in the present work on the Ca${}^{+}$$\left(4s{}^{2}S\right)$ + Rb$\left(5s{}^{2}S\right)$ $\to$ Ca$\left(4s4p{}^{3}P\right)$ + Rb${}^{+}{(}^{1}S)$ charge-exchange process at ultralow collision energies ($E$ $⩽$ 10${}^{-6}$ eV) induced by spin-orbit nonadiabatic coupling terms. The calculations reveal a wealth of sharp resonances which can substantially enhance charge-exchange probabilities over a specific energy window of around 10${}^{-6}$ to 10${}^{-8}$ eV. Such resonances are analyzed in detail, and the corresponding molecular mechanism is discussed for a specific example, where the nonadiabatic transition probabilities are seen to increase by nearly 2 orders of magnitude.

We consider the photoionization of the hydrogen molecular ion exposed to x-ray radiation for photon energies up to 500 eV in the fixed-nuclei approximation. The temporal development of the system is described by a fully ab initio time-dependent grid-based approach in prolate spheroidal coordinates. At sufficently high photon energies, the angular distributions of the electron ejected from the two-center diatomic target resemble the classical double-slit-like interference pattern more closely when the molecular axis and the linear laser polarization direction are perpendicular to each other than in the parallel geometry. Both the preferable emission modes and the confinement effect (in the parallel geometry) of the photoelectron also map out in the time evolution of the wave packets. The validity of a simple plane-wave model is analyzed by comparing its predictions with those from our accurate ab initio calculations.

We propose a theoretical scheme to realize a rotation-sensing interferometer with spin-orbit coupled atoms. The sensitivity of this interferometer is dependent on the atomic mass $m^{-\alpha }$ with a factor $\alpha ⩾1$. Thus, the sensitivity can be improved by about one order of magnitude when we choose Li instead of Rb. Furthermore, compared to the standard Raman interferometer, the response to high-frequency, time-dependent rotation can be improved through the continuous coupling between spin and orbital freedoms of the atoms.

We employ the concept of a dynamical, activity order parameter to study the Ising model in a transverse magnetic field coupled to a Markovian bath. For a certain range of values of the spin-spin coupling, magnetic field, and dissipation rate, we identify a first-order dynamical phase transition between active and inactive dynamical phases. We demonstrate that dynamical phase coexistence becomes manifest in an intermittent behavior of the bath quanta emission. Moreover, we establish the connection between the dynamical order parameter that quantifies the activity and the longitudinal magnetization that serves as static order parameter. The system that we consider can be implemented in current experiments with Rydberg atoms and trapped ions.

We consider qubit networks where adjacent qubits besides interacting via $XY$ coupling, also dissipate into the same environment. The steady states are computed exactly for all network sizes and topologies, showing that they are always symmetric under permutation of network sites, leading to a uniform distribution of the stationary entanglement across the network. The maximum entanglement between two arbitrary qubits is shown to depend only on the total number of qubits in the network, and scales linearly with it. A possible physical realization by means of an array of doped cavities is discussed for the case of a linear chain.

We consider a general procedure to evaluate the Bethe logarithm for a general few-body atomic or molecular system. As benchmarks we use calculations for the ground states of a helium atom and a H${}_2{}^{+}$ molecular ion. The obtained values are ${\beta }_{\mathrm{He}}=4.37016022306\left(2\right)$ for the helium atom and ${\beta }_{{\mathrm{H}}_2{}^{+}}=3.012230335\left(1\right)$ for the H${}_2{}^{+}$molecular ion. Both results substantially improve the best-known values for these quantities.

We report and propose a simple scheme to achieve efficient and fast coherent population transfer by utilizing either a nonlinearly chirped Gaussian-shaped few-cycle laser pulse or an unchirped sinc-shaped few-cycle laser pulse. The proposed scheme is shown to be fairly robust against the variation of the laser parameters such as temporal pulse width, chirp rates, carrier-envelope phases, and Rabi frequencies. We find that compared to the so-called stimulated Raman adiabatic passage technique, our scheme for complete population transfer with few-cycle Gaussian-shaped laser pulses requires less pulse area.

The electric dipole-forbidden, quadrupole $6S_{1/2}\leftrightarrow 5D_{3/2}$ transition in Ba${}^{+}$ near 2051 nm, with a natural linewidth of 13 mHz, is attractive for potential observation of parity nonconservation, and also as a clock transition for a barium ion optical frequency standard. This transition also offers a direct means of populating the metastable $5D_{3/2}$ state to measure the nuclear magnetic octupole moment in the odd barium isotopes. Light from a diode-pumped, solid-state Tm,Ho:YLF laser operating at 2051 nm is used to coherently drive this transition between resolved Zeeman levels in a single trapped ${}^{138}$Ba${}^{+}$ ion. The frequency of the laser is stabilized to a high-finesse Fabry-Pérot cavity at 1025 nm after being frequency doubled. Rabi oscillations on this transition indicate a laser-ion coherence time of 3 ms, most likely limited by ambient magnetic field fluctuations.

We show that in the sudden expansion of a spin-balanced, two-component Fermi gas into an empty optical lattice induced by releasing particles from a trap, over a wide parameter regime, the radius $R_n$ of the particle cloud grows linearly in time. This allow us to define the expansion velocity $V_{\mathrm{ex}}$ from $R_n=V_{\mathrm{ex}}t$. The goal of this work is to clarify the dependence of the expansion velocity on the initial conditions which we establish from time-dependent density matrix renormalization group simulations, both for a box trap and a harmonic trap. As a prominent result, the presence of a Mott-insulating region leaves clear fingerprints in the expansion velocity. Our predictions can be verified in experiments with ultracold atoms.

We theoretically verify that a coherent superposition of nearly degenerate Bessel beams characterizes the family of resonant geometric modes in circular billiards. With the ray-wave correspondence, we experimentally exploit a large-aperture cylindrical waveguide to generate the resonant geometric modes with large orbital angular momentum. Furthermore, the free-space propagation of the geometric modes emerging from the cylindrical waveguide is demonstrated to display the transient dynamics of the coherent states released from a circular quantum billiard.

Using the idealized integrable Maxwell-Bloch model, we describe random optical-pulse polarization switching along an active optical medium in the $\Lambda$ configuration with disordered occupation numbers of its lower-energy sublevel pair. The description combines complete integrability and stochastic dynamics. For the single-soliton pulse, we derive the statistics of the electric-field polarization ellipse at a given point along the medium in closed form. If the average initial population difference of the two lower sublevels vanishes, we show that the pulse polarization will switch intermittently between the two circular polarizations as it travels along the medium. If this difference does not vanish, the pulse will eventually forever remain in the circular polarization determined by which sublevel is more occupied on average. We also derive the exact expressions for the statistics of the polarization-switching dynamics, such as the probability distribution of the distance between two consecutive switches and the percentage of the distance along the medium the pulse spends in the elliptical polarization of a given orientation in the case of vanishing average initial population difference. We find that the latter distribution is given in terms of the well-known arc sine law.

We present exact solutions of the wave equation involving an arbitrary wave form with a phase closely similar to the general astigmatic phase of paraxial wave optics. Special choices of the wave form allow general astigmatic beamlike and pulselike waves with a Gaussian-type unrestricted localization in space and time. These solutions are generalizations of the known Bateman-type waves obtained from the connection existing between beamlike solutions of the paraxial parabolic equation and relatively undistorted wave solutions of the wave equation. As a technical tool, we present a full description of parametrizations of $2\times 2$ symmetric matrices with positive imaginary part, which arise in the theory of Gaussian beams.

The observation of nonperturbative high-order-harmonic generation in periodic solids has been interpreted in terms of a two step process comprising tunnel ionization and radiation from electrons undergoing frequency modulated Bloch oscillations in a strong laser field [Ghimire et al., Nat. Phys. 7, 138 (2011)]. Here we extend the model to include propagation effects. We show that the predicted efficiency is limited by phase velocity mismatch for emission below the band gap and by absorption above the band gap. The efficiency of harmonics scales with the square of the carrier density, suggesting that direct seeding could lead to higher yield. The emitted harmonics should have a temporal profile consisting of a train of subcycle pulses with duration on the order of 650 as depending on the crystal thickness.

Most quantum error-correcting codes are predicated on the assumption that there exists a reservoir of qubits in the state $|0\rangle$ which can be used as ancilla qubits to prepare multiqubit logical states. In this Brief Report, we examine the consequences of relaxing this assumption and propose a method to increase the fidelity produced by a given code when the ancilla qubits are initialized in mixed states, using the same number of qubits, at most doubling the number of gates when the recovery operation would already be implemented. The procedure implemented consists of altering the encoding operator to include the inverse of the unitary operation used to correct detected errors after decoding. This augmentation will be especially useful in quantum computing architectures that do not possess projective measurement, such as solid-state NMR quantum information processing.

In quantum measurement or control processes, there are often auxiliary modes coupling to the quantum system that we are interested in—they together form a bath or an environment for the system. The bath can have finite memory (non-Markovian), and simply ignoring its dynamics (i.e., adiabatically eliminating it) will prevent us from predicting the true quantum behavior of the system. We generalize the technique introduced by Strunz et al. [Phys. Rev. Lett. 82, 1801 (1999)], and develop a formalism that allows us to eliminate the bath nonadiabatically in continuous quantum measurements, and obtain a non-Markovian stochastic master equation for the system that we focus on. This formalism also illuminates how to design the bath—acting as a quantum filter—to effectively probe interesting system observables (e.g., the quantum-nondemolition observable).

Acetylene (C${}_2$H${}_2$), described by a one-dimensional model, is shown to be extremely efficiently ionized by an intense laser field when the two C-H distances are symmetrically stretched to twice the equilibrium distance. Up to six electrons are ejected from the three highest occupied molecular orbitals by tunneling ionization through a reduced potential barrier via temporal localization of the electron density at either of the two proton sites. The third highest occupied molecular orbital, which is initially localized in the area around the two C atoms, is found to act as an electron reservoir, through which the electron density is transferred to the proton sites. This electron transfer mechanism is considered to be universal and can be applied to interpret enhanced ionization found in a variety of hydrocarbon molecules.

By performing a molecular dynamics simulation, fragmentation and energy losses of neutral copper clusters Cu${}_n$ in grazing incidence on a single-crystal Al(111) surface under various azimuthal angles were studied. The interactions among copper atoms were modeled by tight-binding potential, and the position of each copper atom at each time step was calculated by integrating the Newton equations of motion. The percentage of unfragmented clusters as a function of incident energies, incident angles, and cluster sizes was found to be almost independent of the influence of surface structure. The regular-sized clusters without icosahedral structure appear to be less stable. The energy spectra of reflected particles for 30-keV copper clusters Cu${}_{147}$ grazing scattering were measured under three incident angles, and some additional peaks appearing on the left side of the first peak were observed for large grazing angles of incidence.