The intrinsic lifetime of the upper level in the bound-bound $3d^{9}4s^2$ ${}^2{D}_{3/2} \to 3d^{9}4s^2$ ${}^2{D}_{5/2}$ radiative transition in ${\text{Ni}}^{-}$ was measured to be $15.1\pm 0.4\mathrm{s}$. The experiment was performed at cryogenic temperatures in one of the ion-beam storage rings of the Double ElectroStatic Ion Ring ExpEriment facility at Stockholm University. The storage lifetime of the ${\text{Ni}}^{-}$ ion beam was measured to be close to 5 min at a ring temperature of 13 K.

Inner-shell vacancy production for the $\mathrm{O}^{8+}$-Li collision system at 1.5 MeV/amu is studied theoretically. The theory combines single-electron amplitudes for each electron in the system to extract multielectron information about the collision process. Doubly differential cross sections obtained in this way are then compared with the recent experimental data by A. C. LaForge et al. [J. Phys. B 46, 031001 (2013)], yielding good resemblance, especially for low outgoing electron energy. A careful analysis of the processes that contribute to inner-shell vacancy production shows that the improvement of the results as compared to single-active-electron calculations can be attributed to the leading role of two-electron excitation-ionization processes.

Electron-impact single ionization of $\mathrm{W}^{19+}$ ions has been studied both experimentally and theoretically. By employing the crossed-beams method, absolute cross sections have been measured in the energy range from the ionization onset up to 1000 eV. Theoretical calculations of the ionization-cross-section function were performed using the configuration-averaged distorted-wave method for electron-ion collision energies up to 150 keV. Contributions of excited long-lived primary-ion-beam admixtures were taken into account. The role of different competing ionization mechanisms contributing to the studied cross section is discussed. Plasma rate coefficients have been inferred and are presented in parametrized form.

The two-center convergent close-coupling method is used to calculate antihydrogen $\left(\overline{\mathrm{H}}\right)$ formation via positronium (Ps) scattering on antiprotons $\left(\overline{p}\right)$ at near threshold energies. For excited Ps of energy $\varepsilon$, the $1/\varepsilon$ behavior of the $\overline{\mathrm{H}}$ formation cross sections is valid strictly only at the respective threshold, as is the $1/\sqrt{\varepsilon }$ behavior for Ps in the ground state. Simple equations are given for the $\overline{\mathrm{H}}(n\le 4)$ formation cross sections from $\text{Ps}(n\le 3)$ from zero to around 0.1 eV above threshold. Some of the implications of using $\overline{p}$-Ps collisions to form antihydrogen in beams, and held in traps, are discussed.

With the classical ensemble model, we investigate nonsequential double ionization of aligned molecules by few-cycle laser pulses at low intensity, where the two electrons finally are ionized through a transition doubly excited state induced by recollision. The correlated electron momentum distribution of parallel molecules exhibits the line-shaped structure parallel to the diagonal. Our analysis indicates that besides the ionization time difference of two electrons from the doubly excited state, the final-state e-e Coulomb repulsion plays a vital role in the formation of the line-shaped structural momentum distribution. For perpendicular molecules, due to the prominent near half-cycle ionization time difference between the two electrons from the doubly excited state, the momentum distribution shows clear anticorrelation behavior.

The disintegration of nonicosahedral rare-gas clusters in ultraintense extreme ultraviolet pulses is studied. The clusters quickly form a nanoplasma and evolve only according to the nanoplasma's dynamics, which are determined predominately by the cluster's initial shape. It is found that the cluster's disintegration follows a simple model that is well predicted using only the initial structure. The main finding is that the ions disintegrate tangentially from the surface of the cluster's overall shape. In ellipsoidal clusters, the work done on the ions near the semiminor axis by the other particles (ions and electrons) is larger than the work done on the ions near the semimajor axis. This leads to an inversion of the ellipsoidal axes due to the different axes expanding at different rates.

By numerically solving the time-dependent Schrödinger equation, we investigate electron–nuclear-energy sharing in strong-field fragmentation of the ${\mathrm{H}}_2{}^{+}$ molecule. We find a counterintuitive energy shift in the joint electron–nuclear-energy spectrum. This energy shift becomes larger for lower nuclear energies. Through tracing the time evolution of the electron wave packet of bound states, we identify that the energy shift originates from the Stark effect due to the coupling of the ground state and the first exited state of the ${\mathrm{H}}_2{}^{+}$ molecule in strong laser fields. We achieve a good agreement between the ab initio result and the analytic method that includes the Stark effect of molecules.

We study the interaction and dynamics of two half-quantized vortices in two-component Bose-Einstein condensates. Using the Padé approximation for the vortex core profile, we calculate the intervortex potential, whose asymptotic form for a large distance has been derived by Eto et al. [Phys. Rev. A 83, 063603 (2011)]. Through numerical simulations of the two-dimensional Gross-Pitaevskii equations, we reveal different kinds of dynamical trajectories of the vortices depending on the combinations of signs of circulations and the intercomponent density coupling. Under the adiabatic limit, we derive the equations of motion for the vortex coordinates, in which the motion is caused by the balance between Magnus force and the intervortex forces. The initial velocity of the vortex motion can be explained quantitatively by this point vortex approximation, but understanding the long-time behavior of the dynamics needs more consideration beyond our model.

We have investigated the dynamics of atom-molecule conversion using a periodically driven spin-boson model both quantum mechanically and semiclassically. The semiclassical dynamics is fully chaotic for small driving frequencies, so that the regular atom-molecule conversion gradually vanishes. In quantum mechanics, however, the periodic conversion takes place at small frequencies since the quantum adiabatic condition is fulfilled. Moreover, it survives for rather larger frequencies that breaks the adiabatic condition, which is understood by considering the so-called dynamical localization. It implies that the periodic atom-molecule conversion is more robust than we expect from the semiclassical approximation. We also show that for much larger frequencies delocalization takes place so that the dynamics becomes diffusive.

We present a general form of the effective spin-chain model for strongly interacting atomic gases with an arbitrary spin in one-dimensional (1D) traps. In particular, for high-spin systems the atoms can collide in multiple scattering channels, and we find that the resulting form of spin-chain model generically follows the same structure as that of the interaction potentials. This is a unified form working for any spin, statistics (Bose or Fermi), and confinement potentials. We adopt the spin-chain model to reveal both the ferromagnetic (FM) and antiferromagnetic (AFM) magnetic orders for strongly interacting spin-1 bosons in 1D traps. We further show that by adding the spin-orbit coupling, the FM and AFM orders can be gradually destroyed and eventually the ground state exhibits universal spin structure and contacts that are independent of the strength of spin-orbit coupling.

High-finesse optical resonators found in ultrasensitive laser spectrometers utilize supermirrors ideally consisting of isotropic high-reflectivity coatings. Strictly speaking, however, the optical coatings are often nonuniformly stressed during the deposition process and therefore do possess some small amount of birefringence. When physically mounted the cavity mirrors can be additionally stressed in such a way that large optical birefringence is induced. Here we report a direct measurement of optical birefringence in a two-mirror Fabry-Pérot cavity with $R=99.99\%$ by observing ${\mathrm{TEM}}_{00}$ mode beating during cavity decays. Experiments were performed at a wavelength of 4.53 $\mu \mathrm{m}$, with precision limited by both quantum and technical noise sources. We report a splitting of ${\delta }_{\nu }=618\left(1\right)$ Hz, significantly less than the intrinsic cavity line width of ${\delta }_{\text{cav}}\approx 3$ kHz. With a cavity free spectral range of 96.9 MHz, the equivalent fractional change in mirror refractive index due to birefringence is therefore $\mathrm{\Delta }n/n=6.38\left(1\right)\times {10}^{-6}$.

Electromagnetically induced transparency (EIT) has been proposed as a way to greatly enhance cross-phase modulation, with the possibility of leading to few-photon-level optical nonlinearities [Schmidt and Imamoglu, Opt. Lett. 21, 1936 (1996)]. This enhancement grows as the transparency window width, ${\mathrm{\Delta }}_{\mathrm{EIT}}$, is narrowed. Decreasing ${\mathrm{\Delta }}_{\mathrm{EIT}}$, however, has been shown to increase the response time of the nonlinear medium. This suggests that, for a given pulse duration, the nonlinearity would diminish once the window width became narrower than this pulse bandwidth. We show that this is not the case: the peak phase shift saturates but does not decrease. We show that in the regimes of most practical interest—narrow EIT windows perturbed by short signal pulses—the enhancement offered by EIT is not only in the magnitude of the nonlinear phase shift but also in its increased duration. That is, for the case of signal pulses much shorter (temporally) than the inverse EIT bandwidth, the narrow window serves to prolong the effect of the passing signal pulse, leading to an integrated phase shift that grows linearly with $1/{\mathrm{\Delta }}_{\mathrm{EIT}}$; this continued growth of the integrated phase shift improves the detectability of the phase shift, in principle, without bound. For many purposes, it is this detectability which is of more interest than the absolute magnitude of the peak phase shift. We present analytical expressions based on a linear time-invariant model that accounts for the temporal behavior of the cross-phase modulation for several parameter ranges of interest. We conclude that in order to optimize the detectability of the EIT-based cross-phase shift, one should use the narrowest possible EIT window and a signal pulse that is as broadband as the excited-state linewidth and detuned by half a linewidth.

We investigate a one-dimensional modified Jaynes-Cummings-Hubbard chain of $N$ identical QED cavities with nearest-neighbor photon tunneling and periodic boundary conditions. Each cavity contains an embedded three-level atom which is coupled to a cavity mode and an external classical control field. In the case of two excitations and common large detuning of two Raman-resonant fields, we show the emergence of two different species of dark-polariton bound pairs (DPBPs) that are mutually localized in their relative spatial coordinates. Due to the high degree of controllability, we show the appearance of either one or two DPBPs, having the energies within the energy gaps between three bands of mutually delocalized eigenstates. Interestingly, in a different parameter regime with negatively detuned Raman fields, we find that the ground state of the system is a DPBP which can be utilized for the photon storage, retrieval, and controllable state preparation. Moreover, we propose an experimental realization of our model system.

We present measurements of silica optomechanical resonators, known as bottle resonators, passively cooled in a cryogenic environment. These devices possess a suite of properties that make them advantageous for preparation and measurement in the mechanical ground state, including high mechanical frequency, high optical and mechanical quality factors, and optomechanical sideband resolution. Performing thermometry of the mechanical motion, we find that the optical and mechanical modes demonstrate quantitatively similar laser-induced heating, limiting the lowest average phonon occupation observed to just $\sim 1500$. Thermalization to the 9 mK thermal bath would facilitate quantum measurements on these promising nanogram-scale mechanical resonators.

Motivated by the recent experiment [H. Kadau et al., arXiv:1508.05007], we study roton instability and droplet formation in a Bose-Einstein condensate of ${}^{164}\mathrm{Dy}$ atoms with strong magnetic dipole-dipole interaction. We numerically solve the cubic-quintic Gross-Pitaevskii equation with dipole-dipole interaction, and show that the three-body interaction plays a significant role in the formation of droplet patterns. We numerically demonstrate the formation of droplet patterns and crystalline structures, decay of droplets, and hysteresis behavior, which are in good agreement with the experiment. Our numerical simulations provide the first prediction on the values of the three-body interaction in a ${}^{164}\mathrm{Dy}$ Bose-Einstein condensate. We also predict that the droplets remain stable during the time-of-flight expansion. From our results, further experiments investigating the three-body interaction in dipolar quantum gases are required.

Using random matrices, we study the reduced dynamics of a two-level system interacting with a generic environment. In the weak-coupling limit, the result can be obtained directly from known results for purity decay, and result in Markovian dynamics. We then focus on the case of strong coupling, when the dynamics is known to be non-Markovian. In this regime, the coupling dominates over the local parts of the Hamiltonian, and thus we treat the latter as a perturbation of the former. With the help of a linear response approximation, this allows us to obtain an analytical description of the reduced dynamics. Finally, we find a transition from non-Markovian to Markovian dynamics at a point where the coupling and the local Hamiltonian are comparable in size.

We present a protocol based on continuous-variable quantum teleportation and Gaussian postselection that can be used to correct errors introduced by a lossy channel. We first show that the global transformation enacted by the protocol is equivalent to an effective system composed of a noiseless amplification (or attenuation), and an effective quantum channel, which can in theory have no loss and an amount of thermal noise arbitrarily small, hence tending to an identity channel. An application of our protocol is the probabilistic purification of quantum non-Gaussian states using only Gaussian operations.

Monogamy is a defining feature of entanglement, having far-reaching applications. Recently, Regula et al. [Phys. Rev. Lett. 113, 110501 (2014)] proposed a stronger version of monogamy relation for concurrence. We have extended the strong monogamy inequality for another entanglement measure, viz., negativity. In particular, we have concentrated on the four-qubit system and provided a detailed study on the status of strong monogamy on pure states. Further, we have analytically provided some classes of states for which negativity and squared negativity satisfy strong monogamy. Numerical evidences have also been shown in proper places. Our analysis also provides cases where strong monogamy is violated.

We show that the Dirac quantum cellular automaton [A. Bisio, G. M. D'Ariano, and A. Tosini, Ann. Phys. (N.Y.) 354, 244 (2015)] shares many properties in common with the discrete-time quantum walk. These similarities can be exploited to study the automaton as a unitary process that takes place at regular time steps on a one-dimensional lattice, in the spirit of general quantum cellular automata. In this way, it becomes an alternative to the quantum walk, with a dispersion relation that can be controlled by a parameter that plays a similar role to the coin angle in the quantum walk. The Dirac Hamiltonian is recovered under a suitable limit. We provide two independent analytical approximations to the long-term probability distribution. It is shown that, starting from localized conditions, the asymptotic value of the entropy of entanglement between internal and motional degrees of freedom overcomes the known limit that is approached by the quantum walk for the same initial conditions and is similar to the ones achieved by highly localized states of the Dirac equation.