The collision-induced dissociation (CID), electron capture (EC), and dissociative capture (DC) processes in ${{\mathrm{H}}_2}^{+} + \mathrm{He}$ collision are investigated by the quantum-mechanical molecular-orbital close-coupling method in the energy range of 0.02–10 keV/u. The studies are based on the ab initio calculation of the molecular structure data for different ${{\mathrm{H}}_2}^{+}$ alignments with respect to the incident beam direction. The cross sections are compared with the available experimental and theoretical results. The CID process dominates the EC and DC processes. The CID cross sections sensitively depend on the molecular orientations at the time of dissociation in the energy region considered. Dissociation is found to be more likely to occur when the molecular axis is aligned perpendicular to the collision trajectory. The rotational couplings play an important role in dissociation processes.

We calculate the dynamical current and kinetic-energy correlation functions for the first Mott lobe of the one-dimensional Bose-Hubbard model. We employ the strong-coupling expansion up to sixth order in $x=t/U$ and the dynamical density-matrix renormalization group method on rings with 64 sites. The correlation functions are finite above the single-particle gap with a square-root onset, as also found from field theory close to the Mott transition. The correlation functions display a featureless superposition of the primary and tertiary Hubbard bands. We find very good agreement between all methods in the interaction and frequency regimes where they are applicable.

Lack of both spatial and temporal coherence at present x-ray light sources has hindered the application of coherent diffractive imaging techniques, as the computational algorithms which solve the phase problem to invert coherent speckle patterns have traditionally required fully coherent illumination. For this reason, new algorithms which incorporate knowledge of the illumination's coherence properties have been developed and shown to improve the reconstructed image of the scatterer in relatively simple test systems. In this paper we examine the performance of a reconstruction algorithm which incorporates knowledge of the spatial coherence of the illumination in the case of samples with complicated phase structure, and find that even perfect knowledge of the illumination's coherence does not improve the reconstructed image. We speculate that this failure to improve the reconstruction results from an underdetermination of the phase problem in the case of partially coherent illumination.

The optical force acting on a chiral particle is qualitatively different from that acting on an achiral particle due to chirality-dependent forces which couple mechanical linear momentum with optical spin angular momentum. We show that such chirality-induced coupling can serve as a mechanism to realize optical pulling forces that can be predicted analytically and are also observed in full wave simulations for chiral structures.

We investigate experimentally and theoretically the onset of parametric oscillation in a single-pass atomic Raman scattering process without the optical feedback. We find that this Raman oscillation effect is due to the coherent buildup of the atomic spin waves, and therefore the threshold of the Raman pump power for the onset of the oscillation depends on the decoherence time of the atomic spin waves and the Raman coupling constant but is not sensitive to the loss of the Stokes field, unlike the effect of amplified spontaneous emission. A simple theory of atomic Raman process fits the experimental results very well. The long decoherence time of the atomic spin waves in the atomic Raman process leads to a threshold power as low as a few hundred microwatts.

Coherent processing of quantum information and attosecond science have had so far little in common. We here show that recent data in high harmonic emission reveal a realization of a qubit and its coherent manipulation at the attosecond time scale. By observing the interference pattern created by the spatiotemporal overlap of photons emitted by two interfering electron paths we generate a photon Hadamard gate and thus erase the electron-trajectory information. This allows the measurement of the relative phase in electron-trajectory quantum superpositions which realize the qubit, opening the possibility for more elaborate schemes of coherent information processing within high-field physics.

We study the long-time evolution of the bipartite entanglement in translationally invariant one-dimensional harmonic lattice systems. We show that for Gaussian states, in quadratic interactions with periodic boundary conditions, there exists a lower bound for the von Neumann entropy which increases linearly in time. This implies that the dynamics of harmonic lattice systems can in general not efficiently be simulated by algorithms based on matrix-product decompositions of the quantum state, and interactions are needed to suppress the entanglement growth with time.

Quantum state transfer along a one-dimensional spin chain has become a fundamental ingredient for quantum communication between distant nodes in a quantum network. We study the average fidelity of quantum state transfer (QST) along an $XY$ spin chain by adjusting the basis identification between the first spin and the last spin. In a proper choice of the basis identification, we find that the QST fidelity depends only on the average parity of the initial state linearly. We propose a simple scheme to adjust the basis identification to optimize the average fidelity such that it depends linearly on the absolute value of the average parity. In the case where the absolute value of the average parity is 1 we prove that the fidelity takes the maximum at any time over arbitrary initial state and basis identification.

The exchange of orbital angular momentum between a Laguerre-Gaussian beam of light and the center-of-mass motion of an atom or molecule is well known. We show that the orbital angular momentum of light can also be transferred to the internal electronic or rotational motion of an atom or a molecule provided the internal and center-of-mass motions are coupled. However, this transfer does not happen directly to the internal motion, but via center-of-mass motion. If atoms or molecules are cooled down to the recoil limit, then an exchange of angular momentum between the quantized center-of-mass motion and the internal motion is possible during the interaction of cold atoms or molecules with the Laguerre-Gaussian beam. The orientation of the exchanged angular momentum is determined by the sign of the winding number of the Laguerre-Gaussian beam. We present selective results of numerical calculations for the quadrupole transition rates in the interaction of the Laguerre-Gaussian beam with an atomic Bose-Einstein condensate to illustrate the underlying mechanism of light orbital angular momentum transfer. We discuss how the alignment of diatomic molecules will facilitate exploration of the effects of light orbital angular momentum on the electronic motion of molecules.

Localized defects, unavoidable in real solids, may be simulated in (generically defect-free) cold-atom systems, e.g., via modifications of the optical lattice. We study the Hubbard model on a square lattice with single impurities, pairs of nearby impurities, or lines of impurities using numerically exact determinantal quantum Monte Carlo simulations. In all cases, correlations on the “impurity” sites are enhanced either by larger on-site interactions or by a reduced coupling to the environment. We find highly nontrivial magnetic correlations, which persist at elevated temperatures and should be accessible in cold-atom systems, with current experimental techniques. With improved cooling techniques, these features could be followed towards generic quantum antiferromagnetism in the homogeneous limit. More generally, tunable crossing points between different correlation functions could be used, in a quantum steelyard balance setup, as robust thermometers.

We study the microwave absorption of a driven three-level quantum system, which is realized by a superconducting flux quantum circuit (SFQC), with a magnetic driving field applied to the two upper levels. The interaction between the three-level system and its environment is studied within the Born-Markov approximation, and we take into account the effects of the driving field on the damping rates of the three-level system. We study the linear response of the driven three-level SFQC to a weak probe field. The linear magnetic susceptibility of the SFQC can be changed by both the driving field and the bias magnetic flux. When the bias magnetic flux is at the optimal point, the transition from the ground state to the second-excited state is forbidden and the three-level SFQC has a ladder-type transition. Thus, the SFQC responds to the probe field like natural atoms with ladder-type transitions. However, when the bias magnetic flux deviates from the optimal point, the three-level SFQC has a cyclic transition, thus it responds to the probe field like a combination of natural atoms with ladder-type transitions and natural atoms with $\Lambda$-type transitions. In particular, we provide detailed discussions on the conditions for realizing electromagnetically induced transparency and Autler-Townes splitting in three-level SFQCs.

We investigate a method for generating nonlinear phase shifts on superpositions of photon number states. The light is stored in a Bose-Einstein condensate via electromagnetically induced transparency memory techniques. The atomic collisions are exploited to generate a nonlinear phase shift of the stored state. The stored light is then revived with the nonlinear phase shift imprinted upon it. We show that this method can be used as a nonlinear-sign gate in the regime where the Thomas-Fermi and mean-field approximations are valid. We test these approximations using realistic parameters and find that these approximations pass the standard tests for validity in a single-component condensate. However, for the two-component condensates considered here, we find that these conditions are insufficiently strict. We find a stronger set of conditions and show for the same set of parameters that the approximations are invalid.

We show that an analog of the physics at the Planck scale can be found in the propagation of tightly focused laser beams. Various equations that occur in generalized quantum mechanics are formally identical to those describing the nonlinear nonlocal propagation of nonparaxial laser beams. The analysis includes a generalized uncertainty principle and shows that the nonlinear focusing of a light beam with dimensions comparable to the wavelength corresponds to the spontaneous excitation of the so-called maximally localized states. The approach, driven by the ideas of the quantum gravity physics, allows one to predict the existence of self-trapped subwavelength solitary waves for both focusing and defocusing nonlinearities, and opens the way to laboratory simulations of phenomena that have been considered to be inaccessible.

We report absolute frequency measurement of the $6s6p{}^1{P}_1–6s7s{}^1{S}_0$ transition at 1077.3 nm of ${}^{174}\mathrm{Yb}$ in a hollow-cathode lamp (HCL). The frequency of coupling laser was stabilized to the $6s^2{}^1{S}_0–6s6p{}^1{P}_1$ transition at 398.9 nm using an Yb atomic beam, and the coupling laser beam was then overlapped with the counterpropagating probe laser beam at 1077.3 nm in the Yb HCL to obtain a Doppler-free signal of laser-induced birefringence for the frequency stabilization of the probe laser. Systematic frequency shifts of both transitions as a function of the discharge current of the Yb HCL were analyzed using an Yb-doped fiber laser frequency comb. The absolute frequency of the $6s6p{}^1{P}_1–6s7s{}^1{S}_0$ transition was measured to be 278 281 521.4(7.2) MHz, and from the published frequency of the transition, the absolute frequency of the $6s^2{}^1{S}_0–6s7s{}^1{S}_0$ two-photon transition at 291.1 nm was determined to be 1 029 807 509.2(7.2) MHz with the relative standard uncertainty of $7.0\times {10}^{-9}$.

The scattering of single free electrons by free electrons has been measured directly. The methods of measurement included changing the incident relative velocities of the electrons by changing either the angle between the beams or the beam energies and then identifying uniquely the outgoing electron pairs with energy and coincidence time detection. The measured energy and angular differential scattering cross sections over the relative interaction velocity range from $0.3\times 10{}^7$ to $3.0\times 10{}^7$ m/s for forward and backward scattering angles which were calibrated absolutely to within ±2% and with a statistical uncertainty within ±2% at an angle of 30° between the beams and an observational angle of 60°. Subsequent relative data showed a dependence on (relative velocity)${}^{-4}$ and ${\cos }^2$(beam intersection angle) scattering parameters as expected for an exact quantum mechanical description for pure Coulomb field Møller scattering. While the precision and reproducibility of all data are at the percent level, four of the 48 data points vary significantly from the expected quantum values and are not included in our overall experimental uncertainty.

The time-dependent restricted-active-space self-consistent-field (TD-RASSCF) method is formulated based on the TD variational principle. The SCF based TD orbitals contributing to the expansion of the wave function are classified into three groups, between which orbital excitations are considered with the RAS scheme. In analogy with the configuration-interaction singles (CIS), singles-and-doubles (CISD), and singles-doubles-and-triples (CISDT) methods in quantum chemistry, the TD-RASSCF-S, -SD, and -SDT methods are introduced as extensions of the TD-RASSCF-doubles (-D) method [Phys. Rev. A 87, 062511 (2013)]. Based on an analysis of the numerical cost and test calculations for one-dimensional (1D) models of atomic helium, beryllium, and carbon, it is shown that the TD-RASSCF-S and -D methods are computationally feasible for systems with many electrons and more accurate than the TD Hartree-Fock (TDHF) and TDCIS methods. In addition to the discussion of methodology, an analysis of electron dynamics in the high-order harmonic generation (HHG) process is presented. For the 1D beryllium atom, a state-resolved analysis of the HHG spectrum based on the time-independent HF orbitals shows that while only single-orbital excitations are needed in the region below the cutoff, single- and double-orbital excitations are essential beyond, where accordingly the single-active-electron (SAE) approximation and the TDCIS method break down. On the other hand, the TD-RASSCF-S and -D methods accurately describe the multiorbital excitation processes throughout the entire region of the HHG spectrum. For the 1D carbon atom, our calculations show that multiorbital excitations are essential in the HHG process even below the cutoff. Hence, in this test system a very accurate treatment of electron correlation is required. The TD-RASSCF-S and -D approaches meet this demand, while the SAE approximation and the TDCIS method are inadequate.

We explore the possibility of retrieving Auger spectra with FEL radiation. Using a laser pulse of 260 eV photon energy, we study the interplay of photoionization and Auger processes following the initial formation of a $2p$ inner-shell hole in Ar. Accounting for the fine structure of the ion states we demonstrate how to retrieve the Auger spectrum of ${\mathrm{Ar}}^{+}\to {\mathrm{Ar}}^{2+}$. Moreover, considering two electrons in coincidence we also demonstrate how to retrieve the Auger spectrum of ${\mathrm{Ar}}^{2+}\to {\mathrm{Ar}}^{3+}$.

In this paper, we provide a comprehensive study of the quantum magnetism in the Mott insulating phases of the one-dimensional (1D) Bose-Hubbard model with Abelian or non-Abelian synthetic gauge fields, using the density-matrix renormalization-group (DMRG) method. We focus on the interplay between the synthetic gauge field and the asymmetry of the interactions, which give rise to a very general effective magnetic model: an XYZ model with various Dzyaloshinskii-Moriya (DM) interactions. The properties of the different quantum magnetic phases and phases transitions of this model are investigated.

We consider the decoherence of a pseudospin ensemble under collective random rotations and study, both theoretically and experimentally, how a nondestructive measurement combined with real-time feedback correction can protect the state against such a decoherence process. We theoretically characterize the feedback efficiency with different parameters—coherence, entropy, fidelity—and show that a maximum efficiency is reached in the weak measurement regime, when the projection of the state induced by the measurement is negligible. This article presents in detail the experimental results published previously [T. Vanderbruggen et al., Phys. Rev. Lett. 110, 210503 (2013)], where the feedback scheme stabilizes coherent spin states of trapped ultracold atoms and nondestructively probes them with dispersive optical detection. In addition, we study the influence of several parameters, such as atom number and rotation angle, on the performance of the method. We analyze the various decoherence sources limiting the feedback efficiency and propose a way to mitigate their effect. The results demonstrate the potential of the method for the real-time coherent control of atom interferometers.