We present a comprehensive study of maximally entangled symmetric states of arbitrary numbers of qubits in the sense of the maximal mixedness of the one-qubit reduced density operator. A general criterion is provided to easily identify whether given symmetric states are maximally entangled in that respect or not. We show that these maximally entangled symmetric (MES) states are the only symmetric states for which the expectation value of the associated collective spin of the system vanishes, as well as in corollary the dipole moment of the Husimi function. We establish the link between this kind of maximal entanglement, the anticoherence properties of spin states, and the degree of polarization of light fields. We analyze the relationship between the MES states and the classes of states equivalent through stochastic local operations with classical communication (SLOCC). We provide a nonexistence criterion of MES states within SLOCC classes of qubit states and show in particular that the symmetric Dicke state SLOCC classes never contain such MES states, with the only exception of the balanced Dicke state class for even numbers of qubits. The 4-qubit system is analyzed exhaustively and all MES states of this system are identified and characterized. Finally the entanglement content of MES states is analyzed with respect to the geometric and barycentric measures of entanglement, as well as to the generalized $N$-tangle. We show that the geometric entanglement of MES states is ensured to be larger than or equal to $1/2$, but also that MES states are not in general the symmetric states that maximize the investigated entanglement measures.

We report the experimental realization of a single-species atomic four-wave mixing process with Bose-Einstein-condensate collisions for which the angular distribution of scattered atom pairs is not isotropic, despite the collisions being in the $s$-wave regime. Theoretical analysis indicates that this anomalous behavior can be explained by the anisotropic nature of the gain in the medium. There are two competing anisotropic processes: classical trajectory deflections due to the mean-field potential and Bose-enhanced scattering which bears similarity to superradiance. We analyze the relative importance of these processes in the dynamical buildup of the anisotropic density distribution of scattered atoms and compare the Bose enhancement effects to those in optically pumped superradiance.

We investigate the dynamics of a mechanical resonator in which is embedded an ensemble of two-level systems interacting with an optical cavity field. We show that this hybrid approach to optomechanics allows for enhanced effective interactions between the mechanics and the cavity field, leading, for instance, to ground-state cooling of the mechanics, even in regimes, like the unresolved sideband regime, in which standard radiation pressure cooling would be inefficient.

The dynamics of a quantum anharmonic oscillator whose position is monitored by a Bose-Einstein condensate trapped in a symmetric double well potential is studied. The (nonexponential) decoherence induced on the oscillator by the measuring device is analyzed. A detailed quasiclassical and quantum analysis is presented. In the first case, for an arbitrary initial coherent state, two different decoherence regimes are observed: an initial Gaussian decay followed by a power law decay for longer times. The characteristic time scales of both regimes are reported. Analytical approximated expressions are obtained in the full quantum case where algebraic time decay of decoherence is observed.

We present a method of developing analytical measures of $k\text{−}\mathrm{partite}$ delocalization in arbitrary $n\text{−}\mathrm{body}$ $W$-like states, otherwise known as mixed states in the single-excitation subspace. These measures calculate the distance of a state to its closest reference state with $k-1$ entanglement. We find that the reference state is determined by the purity of the state undergoing measurement. Measures with up to 6-body delocalization for a 6-body system are derived in full, while an algorithm for general $k\text{−}\mathrm{partite}$ measures is given.

Clusters usually exhibit unique structures as an intermediate form of matter. However, their structures are still difficult to be determined with available experimental techniques. There are many structure distinct isomers for carbon-dioxide clusters. Here we report an experimental and theoretical joint study to determine the structures of the lowest-energy isomers. In the experiment, we exploded carbon-dioxide clusters by laser-driven multiple ionization and obtained precise momentum vectors of explosion fragments for each explosion channel. In the theory, we calculated low-energy structures of carbon-dioxide clusters and simulated their explosion dynamics. In comparison with the momentum vectors of explosion fragments, the lowest-energy isomers were identified for carbon-dioxide dimers, trimers, and tetramers.

We demonstrate the capabilities of time-dependent density functional theory (TDDFT) for strong-field, short-wavelength (soft x-ray) physics, as compared to a formalism based on rate equations. We find that TDDFT provides a very good description of the total and individual ionization yields for Ne and Ar atoms exposed to strong laser pulses. We assess the reliability of different adiabatic density functionals and conclude that an accurate description of long-range interactions by the exchange and correlation potential is crucial for obtaining the correct ionization yield over a wide range of intensities (from ${10}^{13}$ to $5\times {10}^{15}{\mathrm{W}/\mathrm{cm}}^2$). Our TDDFT calculations disentangle the contribution from each ionization channel based on the Kohn-Sham wave functions.

We experimentally study an optomechanical cavity consisting of an oscillating mechanical resonator embedded in a superconducting microwave transmission line cavity. Tunable optomechanical coupling between the mechanical resonator and the microwave cavity is introduced by positioning a niobium-coated single-mode optical fiber above the mechanical resonator. The capacitance between the mechanical resonator and the coated fiber gives rise to optomechanical coupling, which can be controlled by varying the fiber-resonator distance. We study radiation-pressure-induced self-excited oscillation as a function of microwave driving parameters (frequency and power). Intermittency between limit-cycle and steady-state behaviors is observed with blue-detuned driving frequency. The experimental results are accounted for by a model that takes into account the Duffing-like nonlinearity of the microwave cavity. A stability analysis reveals a subcritical Hopf bifurcation near the region where intermittency is observed.

We prepare heralded single photons from a photon pair source based on nondegenerate four-wave mixing in a cold atomic ensemble via a cascade decay scheme. Their statistics shows strong antibunching with a value for the second order correlation at zero delay less than 0.03, indicating a near single photon character. In an optical homodyne experiment, we directly measure the temporal envelope of these photons and find, depending on the heralding scheme, an exponentially decaying or rising profile. Such a rising envelope is required for the efficient interaction between a single photon and a two level system. At the same time, their observation illustrates the breakdown of a realistic interpretation of the heralding process in terms of defining an initial condition of a physical system.

We propose energy-resolved ultrafast electron diffraction as a means of directly imaging target electronic motions whose space, time, and energy information can be simultaneously retrieved from time-resolved diffraction measurements. The energy-resolved diffraction images are simulated for breathing, wiggling, and hybrid modes of electronic motion in the H atom. The simulations demonstrate the capabilities of ultrafast electron diffraction to image and distinguish different kinds of electronic motion. The theoretical analysis of the scattering process identifies the requirements for time- and state-resolved imaging of electronic motion and provides interpretations of the results.

Multichannel quantum defect theory (MQDT) has been widely applied to resonant and nonresonant scattering in a variety of atomic collision processes. In recent years, the method has been applied to cold collisions with considerable success, and it has proven to be a computationally viable alternative to full close-coupling (CC) calculations when spin, hyperfine, and external field effects are included. In this paper, we describe a hybrid approach for molecule-molecule scattering that includes the simplicity of MQDT while treating the short-range interaction explicitly using CC calculations. This hybrid approach, demonstrated for ${\mathrm{H}}_2\text{−}{\mathrm{H}}_2$ collisions in full dimensionality, is shown to adequately reproduce cross sections for quasiresonant rotational and vibrational transitions in the ultracold $\left(1\mu \mathrm{K}\right)$ and $\sim 1-10\mathrm{K}$ regime spanning seven orders of magnitude. It is further shown that an energy-independent short-range $K$ matrix evaluated in the ultracold regime $\left(1 \mu \mathrm{K}\right)$ can adequately characterize cross sections in the mK-K regime when no shape resonances are present. The hybrid CC-MQDT formalism provides an alternative approach to full CC calculations at considerably less computational expense for cold and ultracold molecular scattering.

Positron scattering and annihilation on noble-gas atoms is studied ab initio using many-body theory methods for positron energies below the positronium formation threshold. We show that in this energy range, the many-body theory yields accurate numerical results and provides a near-complete understanding of the positron–noble-gas atom system. It accounts for positron-atom and electron-positron correlations, including the polarization of the atom by the positron and the nonperturbative effect of virtual positronium formation. These correlations have a large influence on the scattering dynamics and result in a strong enhancement of the annihilation rates compared to the independent-particle mean-field description. Computed elastic scattering cross sections are found to be in good agreement with recent experimental results and Kohn variational and convergent close-coupling calculations. The calculated values of the annihilation rate parameter $Z_{\mathrm{eff}}$ (effective number of electrons participating in annihilation) rise steeply along the sequence of noble-gas atoms due to the increasing strength of the correlation effects, and agree well with experimental data.

Tunneling ionization is one of the fundamental processes for molecules in intense laser fields. Depending on the ionizing molecular orbitals, molecular ions are in ground or excited electronic states. Here, we report an experimental study of tunneling ionization of nitrogen molecules using spectroscopic methods. The molecular ions in the excited electronic state were detected through the fluorescence spectra. The molecular ions in the ground electronic state were detected through the laser-induced-fluorescence (LIF) spectra. The gas-pressure dependences of fluorescence and LIF intensities demonstrated the collision-induced population redistribution in these electronic states.

This paper begins with the premise that stimulated Raman adiabatic passage (STIRAP) from one state to another by coupling to an intermediate state necessarily requires some population of that intermediate state. The usual view of the process depends on having no population in the intermediate state, but a simple argument from the time-dependent Schrödinger equation shows that such adiabatic transfer cannot occur. From another perspective, removing the time dependence of the Hamiltonian by making the rotating frame transformation cancels the optical frequency oscillations but does not account for the envelopes of the pulses commonly used for STIRAP. By assuming a small but finite intermediate state amplitude ${\varepsilon }_1$ in the wave function, the propagator for the evolution between initial and final states is derived. Applying the propagator to the initial state produces a single compact formula for the final intermediate-state population that is readily evaluated for an arbitrary choice of laser pulse parameters. Several plots for various experimental conditions show that under the conditions normally used for STIRAP experiments the effects are typically a few percent or less, but in the end this effect puts an ultimate limit on total efficiency.

A drastic modification of the conventional streaking patterns of double differential cross section for attosecond near-threshold ionization of atoms in a strong infrared (IR) field is theoretically investigated. Rescattering of slow electrons from the ionic core leads to appearance of the interference fringes and strong diffraction maxima in the spectra of photoelectrons at small energies. The computations based on solving the time-dependent Schrödinger equation show that both these spectral features strongly depend on the IR intensity and frequency. Investigations of low-energy streaking can provide unique information on slow photoelectron dynamics in combined fields of ionic core and strong IR laser.

We experimentally demonstrate fast separation of a two-ion crystal in a microstructured segmented Paul trap. By the use of spectroscopic calibration routines for the electrostatic trap potentials, we achieve the required precise control of the ion trajectories near the critical point, where the harmonic confinement by the external potential vanishes. The separation procedure can be controlled by three parameters: a static potential tilt, a voltage offset at the critical point, and the total duration of the process. We show how to optimize the control parameters by measurements of ion distances, trap frequencies, and the final motional excitation. We extend the standard measurement technique for motional excitation to allow for discriminating thermal and oscillatory states, and to cover a dynamic range covering more than 4 orders of magnitude in energy. It is shown that for fast separation times, oscillatory motion is excited, while a predominantly thermal state is obtained for long times. At a separation duration of $80\mu \mathrm{s}$, a minimum mean excitation of $\overline{n}=4.16\left(0.16\right)$ vibrational quanta per ion is achieved, which is consistent with the adiabatic limit given by our particular trap. The presented technique does not rely on specific trap geometry parameters and can therefore be adopted for different segmented traps.

We introduce a technique to simulate the dynamics of a thermal dilute Bose gas. With the method of the nonequilibrium statistical operator, a correction to the Gross-Pitaevskii equation (GPE) is derived. This modification of the GPE describes spontaneous scattering processes. It regularizes the ultraviolet catastrophe and captures the dynamics of the particles at all energies, including the tail of the momentum distribution. We apply our model to the simulation of an evaporative cooling quench of a two-dimensional Bose gas.

We study a dissipative Bose-Hubbard chain subject to an engineered bath using a superoperator approach based on matrix product operators. The dissipation is engineered to stabilize a Bose-Einstein condensate wave function in its steady state. We then characterize the steady state emerging from the interplay between incompatible Hamiltonian and dissipative dynamics. While it is expected that interactions lead to this competition, even the kinetic energy in an open boundary condition setup competes with the dissipation, leading to a nontrivial steady state. We also present results for the transient dynamics and probe the relaxation time revealing the closing of the dissipative gap in the thermodynamic limit.

We demonstrate a tool for filtering technical and electronic noises from pulses of light, especially relevant for signal processing methods in quantum optics experiments as a means to achieve the shot-noise level and reduce strong technical noise by means of a pattern function. We provide the theory of this pattern-function filtering based on balance detection. Moreover, we implement an experimental demonstration where 10 dB of technical noise is filtered after balance detection. Such filter can readily be used for probing magnetic atomic ensembles in environments with strong technical noise.

We consider the propagation of two-photon light in a random medium. We show that the Wigner transform of the two-photon amplitude obeys an equation that is analogous to the radiative transport equation for classical light. Using this result, we investigate the propagation of an entangled photon pair.