We investigate the interaction between three rubidium atoms in highly excited ($58p$) Rydberg states lying along a common axis and calculate the potential energy surfaces between the three atoms. We find that three-body long-range potential wells exist in some of these surfaces, indicating the existence of very extended bound states that we label macrotrimers. We calculate the lowest vibrational eigenmodes and the resulting energy levels and show that the corresponding vibrational periods are rapid enough to be detected spectroscopically.

We present a systematic experimental and theoretical study of angular differential cross sections of single-electron transfer in collisions of N${}^{5+}$, O${}^{6+}$, and Ne${}^{8+}$ with ground-state Na($3s$) in the collision energy range from 1 to 8 keV/amu. Experiments were performed using recoil-ion momentum spectroscopy in combination with a magneto-optically cooled Na atom target. The results are compared with three-body classical-trajectory Monte Carlo theory. Experimental and theoretical angular differential cross sections for capture into highly excited states show an oscillatory structure which is linked to the number of times the active electron crosses the potential energy saddle, i.e., oscillates between the two nuclear centers during the collision process.

Orders of magnitude increases of the cross sections are predicted for laser-assisted low-energy electron-atom scattering (accompanied by absorption of laser photons) as the laser ellipticity is increased. These ellipticity-controlled enhancements are manifestations of the field-free electron-atom scattering dynamics, such as the Ramsauer-Townsend effect in low-energy elastic electron-atom scattering. The strong sensitivity of laser-assisted scattering cross sections to this dynamics and the laser ellipticity is illustrated for $e$-Ne and $e$-Ar scattering in both midinfrared ($\lambda =3.5\mu$m) and CO${}_2$ ($\lambda =10.6\mu$m) laser fields of moderate intensities.

We find a fraction of atoms remain unionized after the laser pulse when the tunneled electrons are released in a certain window of initial field phase and transverse velocity. The survival window shifts with laser polarization ellipticity and its width varies with respect to laser intensity and atomic ionization potential. Neutral atom yield can be calculated by summing up tunneling probabilities in the window. Our theory can quantitatively reproduce the distribution of the survival yields vs laser ellipticity observed for helium in experiment. For other atom species with smaller ionization potential such as magnesium, our theory predicts a wider distribution than the strong-field approximation model while closer to the three-dimensional semiclassical electron ensemble simulations, indicating the important role of the Coulomb effects.

The availability of long-distance underground fiber-optic links opens a perspective of implementing interferometric fiber-optic gyroscopes embracing very large areas. We discuss the potential sensitivity, as well as some disturbances and approaches to overcome them.

We implement a complete randomized benchmarking protocol on a system of two superconducting qubits. The protocol consists of randomizing over gates in the Clifford group, which experimentally are generated via an improved two-qubit cross-resonance gate implementation and single-qubit unitaries. From this we extract an optimal average error per Clifford operation of $0.0936$. We also perform an interleaved experiment, alternating our optimal two-qubit gate with random two-qubit Clifford gates, to obtain a two-qubit gate error of $0.0653$. We compare these values with a two-qubit gate error of $\sim 0.12$ obtained from quantum process tomography, which is likely limited by state preparation and measurement errors.

Fluctuations of the thermal or classical component of the van der Waals force between two dielectric slabs, modeled as an ensemble of polarizable dipoles which interact via the usual electrostatic dipole-dipole interaction, are evaluated. In the model the instantaneous force is a deterministic function of the dipole configurations in the slabs, and its fluctuations are purely due to dipole fluctuations (no background thermal fluctuations of the electromagnetic field are considered). The average of the force and its variance are computed. The fluctuations of the force exhibit normal thermodynamic scaling in that they are proportional to the area of the two plates and, even more importantly, do not depend on any microscopic cutoff in the theory. The average and the variance of the thermal van der Waals forces give a unique fingerprint of these fluctuation interactions.

We experimentally demonstrate a dynamic fashion of quantum Zeno effect in nuclear magnetic resonance systems. The frequent measurements are implemented through quantum entanglement between the target qubit(s) and the measuring qubit, which dynamically results from the unitary evolution of duration ${\tau }_m$ due to dispersive coupling. Experimental results testify to the presence of “the critical measurement time effect,” that is, the quantum Zeno effect does not occur when ${\tau }_m$ takes some critical values, even if the measurements are frequent enough. Moreover, we provide an experimental demonstration of an entanglement preservation mechanism based on such a dynamic quantum Zeno effect.

We study the Kitaev-Ising model, where ferromagnetic Ising interactions are added to the Kitaev model on a lattice. This model has two phases which are characterized by topological and ferromagnetic order. Transitions between these two kinds of order are then studied on a quasi-one-dimensional system, on a ladder, and on a two-dimensional periodic lattice, a torus. By exactly mapping the quasi-one-dimensional case to an anisotropic $XY$ chain we show that the transition occurs at zero $\lambda$, where $\lambda$ is the strength of the ferromagnetic coupling. In the two-dimensional case the model is mapped to a two-dimensional Ising model in transverse field, where it shows a transition at a finite value of $\lambda$. A mean-field treatment reveals the qualitative character of the transition and an approximate value for the transition point. Furthermore with perturbative calculation, we show that the expectation value of Wilson loops behaves as expected in the topological and ferromagnetic phases.

An approach to generating the entangled photonic states $|{\Psi }_1,{\Psi }_2\rangle \pm |{\Psi }_2,{\Psi }_1\rangle$ from two arbitrary states $|{\Psi }_1\rangle$ and $|{\Psi }_2\rangle$ is proposed. The protocol is implemented by the conditionally induced beam-splitter coupling, which leads to the selective swapping between two photonic modes. Such coupling occurs in a quantum system prepared in the superposition of two ground states with only one of them being involved in the swapping. All the entangled states in the category, such as entangled pairs of coherent states or Fock states (NOON states), can be efficiently produced in the same way by this method.

This paper presents fully ab initio calculations of the broadenings and spectral shapes of O${}_2$ infrared absorption lines in a free gas and when confined in nanoporous media. These calculations are performed, without use of any adjusted parameter, using a recently proposed approach [Phys. Rev. A 87, 013403 (2013)] that is based on requantized classical molecular-dynamics simulations. This involves studying the time evolutions of the translational and rotational motions of large numbers of molecules taking molecule-molecule and molecule-surface collisions into account through realistic interaction potentials. These simulations enable predictions of dipole autocorrelation functions whose Fourier-Laplace transforms yield the associated spectra. Comparisons are then made with broadening coefficients and line shapes provided by new and previous experiments. The good agreement between calculated and measured results confirms the veracity of the proposed model for a free gas and shows that the effects of confinement, which induce significant modifications to the line shapes, are correctly predicted. The need for improved characterization of the shape and size of pores in random nanoporous media is highlighted.

We show that multichannel quantum defect theory (MQDT) can be applied successfully as an efficient computational method for cold molecular collisions in $\text{Li}+\text{NH}$, which has a deep and strongly anisotropic interaction potential. In this strongly coupled system, closed-channel poles restrict the range over which the MQDT $\mathit{Y}$ can be interpolated. We present an improved procedure to transform the MQDT reference functions so that the poles are removed from the energy range of interest. Effects due to very-long-range spin dipolar couplings are outside the scope of MQDT, but can be added perturbatively. This procedure makes it possible to calculate the elastic and inelastic cross sections, over the entire range of energies and fields needed to evaluate the feasibility of sympathetic cooling of NH by Li, using coupled-channel calculations at only five combinations of energy and field.

The time-dependent density functional theory is applied to investigate the charge transfer and electron-loss processes during He${}^{2+}$-Ar collisions in the energy range of 4–300 keV/amu. A coordinate space translation technique is employed to focus our investigation on some certain space of interest such as the regions around the projectile or target. It is shown that both charge transfer and electron-loss processes are important in the considered energy range. One-electron-capture processes dominate charge transfer with the cross sections one to two orders of magnitude larger than two-electron-capture cross sections. The ionization cross sections decrease with increasing the number of ionized electrons. The calculated cross sections are in excellent agreement with available experimental and theoretical results. Evolution of electron density is also presented to explore interactions between the electrons as well as the correlation between the projectile and target during the collisions.

Angle-resolved photoelectron spectra, resulting from the strong-field ionization of atoms or molecules, carry a rich amount of information on ionization pathways, electron dynamics, and the target structure. We have investigated angle-resolved photoelectron spectra arising from the nonresonant ionization of xenon Rydberg atoms in the multiphoton regime, using intense midinfrared radiation from a free-electron laser. The experimental data reveal a rich oscillatory structure in the low-order above-threshold ionization region. By performing quantum-mechanical and semiclassical calculations, the observed oscillations could be well reproduced and explained by both a multiphoton absorption picture as by a model invoking electron wave-packet interferences. Furthermore, we demonstrate that the shape and orientation of the initial Rydberg state leaves its own fingerprint on the final angular distribution.

We investigate the highly polarized limit of a two-dimensional Fermi gas, where we effectively have a single spin-down impurity atom immersed in a spin-up Fermi sea. By constructing variational wave functions for the impurity, we map out the ground state phase diagram as a function of mass ratio $m_{\uparrow }/m_{\downarrow }$ and interaction strength. In particular, we determine when it is favorable for the dressed impurity (polaron) to bind particles from the Fermi sea to form a dimer, trimer, or even larger clusters. Similarly to three dimensions, we find that the Fermi sea favors the trimer state so that it exists for $m_{\uparrow }/m_{\downarrow }$ less than the critical mass ratio for trimer formation in the vacuum. We also find a region where dimers have finite momentum in the ground state, a scenario that corresponds to the Fulde-Ferrell-Larkin-Ovchinnikov superfluid state in the limit of large spin imbalance. For equal masses ($m_{\uparrow }=m_{\downarrow }$), we compute rigorous bounds on the polaron-dimer transition, and we show that the polaron energy and residue is well captured by the variational approach, with the former quantity being in good agreement with experiment. When there is a finite density of impurities, we find that this polaron-dimer transition is preempted by a first-order superfluid-normal transition at zero temperature, but it remains an open question what happens at finite temperature.

We provide a detailed description of our previously proposed scheme for topological interface engineering with constructed defects and textures perforating across coherent interfaces between different broken symmetries [Borgh and Ruostekoski, Phys. Rev. Lett. 109, 015302 (2012)]. We consider a spin-1 Bose-Einstein condensate, in which polar and ferromagnetic phases are prepared in spatially separated regions. We show that a stable coherent interface is established between the two phases, allowing defects of different topology to connect continuously across the boundary. We provide analytic constructions of interface-crossing defect solutions that could be experimentally phase imprinted using existing technology. By numerically minimizing the energy, we calculate the core structures of interface-crossing defect configurations. We demonstrate nontrivial core deformations to considerably more complex structures, such as the formation of an arch-shaped half-quantum line defect, an Alice arch, at the interface, with the topological charge of a point defect, whose emergence may be understood by the “hairy ball” theorem. Another example of an energetically stable object is the connection of a coreless vortex to a pair of half-quantum vortices. We show that rotation leads to spontaneous nucleation of defects in which a coreless vortex continuously transforms to a half-quantum vortex across the interface.

We consider bosonic atoms in an optical lattice at integer filling, tuned to the superfluid–Mott-insulator critical point, and coupled to a single, mobile impurity atom of a different species. This setup is inspired by current experiments with quantum gas microscopes, which enable tracking of the impurity motion. We describe the evolution of the impurity motion from the quantum wave packet spread at short times, to Brownian diffusion at long times. This dynamics is controlled by the interplay between dangerously irrelevant perturbations at the strongly interacting field theory describing the superfluid-insulator transition in two spatial dimensions.

Refractive index and absorption experienced by a probe field propagating through a three-level atomic medium can be effectively manipulated by the bistable behavior of a control field. The probe field couples the lower transition of the atom in a ladder configuration and experiences normal or anomalous dispersion depending on the control field being in the upper or lower bistable state, respectively. We also obtain nonlinear dynamical instability in the form of periodic self-pulsing as the lower bistable branch becomes unstable, quite unlike earlier demonstrations of an unstable regime in the upper branch. Consequently, the susceptibility experienced by the probe field varies periodically in time as dictated by the control field self-pulsing.

We theoretically investigate the phase-matching mechanism of supercontinuum generation with two-color Bessel-Gauss beams in a low-density rare-gas medium. The results show that the well-phase-matched supercontinua with tunable central frequencies can be generated efficiently in the plateau by adjusting the focusing half angle of the Bessel-Gauss beam. Furthermore, the obtained isolated attosecond pulses can achieve good beam quality with Gaussian-like spatial profiles and small divergence angles in the far field, which are beneficial for applications in ultrafast metrology with high spatial and temporal resolutions.

We predict the existence of the electromagnetically induced absorption (EIA) in the double-cavity configurations of the hybrid opto-electromechanical systems (OEMS). We discuss the origin of the EIA in OEMS which exhibit the existence of an absorption peak within the transparency window. We provide analytical results for the width and the height of the EIA peak. The combination of the electromagnetically induced transparency and EIA is especially useful for photoswitching applications. The EIA that we discuss is different from the one originally discovered by Lezama et al. [Phys. Rev. A 59, 4732 (1999)] in atomic systems and can be understood in terms of the dynamics of three coupled oscillators (rather than two) under different conditions on the relaxation parameters. The EIA we report can also be realized in metamaterials and plasmonic structures.