Observation of Auger electron emission from Al under the impact of He ions, at energies below the excitation threshold of Al-Al collisions, gives evidence for charge transfer in He-Al collisions. Excitation of $2p$-shell electrons in Al is produced by vacancy transfer, following double excitation of He by promotion of its $1s$ level. Studies of ion scattering from surfaces currently neglect the double excitation of He. We discuss implications on the debate about electronic excitations and energy deposition, which is relevant to many fields, since helium ions are applied in the characterization and imaging of materials.

The direct ionization of the helium atom by intense coherent high-frequency short laser pulses is investigated theoretically from first principles. To this end, we solve numerically the time-dependent Schrödinger equation for the two-electron wave packet and its interaction with the linearly polarized pulse by the efficient time-dependent restricted-active-space configuration-interaction method (TD-RASCI). In particular, we consider photon energies which are nearly resonant for the $1s\to 2p$ excitation in the ${\mathrm{He}}^{+}$ ion. Thereby, we investigate the dynamic interference of the photoelectrons of the same kinetic energy emitted at different times along the pulse in the two-electron system. In order to enable observation of the dynamic interference in the computed spectrum, the electron wave packets were propagated on large spatial grids over long times. The computed photoionization spectra of He exhibit pronounced interference patterns the complexity of which increases with the decrease of the photon energy detuning and with the increase of the pulse intensity. Our numerical results pave the way for experimental verification of the dynamic interference effect at presently available high-frequency laser pulse sources.

We investigate the composite adiabatic passage (CAP) reported by B. T. Torosov et al. [Phys. Rev. Lett. 106, 233001 (2011)] in a nonlinear two-level system in which the level energies depend on the occupation of the levels, representing a mean-field type of interaction between the particles. A high-fidelity, fast, and robust quantum manipulation is achieved in the system. We consider the effect of interparticle interaction and find that it tends to increase the number of the pulse sequences. The CAP technique can suppress the nonadiabatic oscillations below the quantum-information benchmark ${10}^{-4}$, as long as there exist sufficiently long composite sequences. We analyze the robustness against the variations in the field parameters. The difference between the nonlinear and linear systems on the CAP technique is also discussed.

We theoretically investigate two-particle and ground-state many-particle Anderson localizations of a spin-orbit coupled ultracold atomic Fermi gas trapped in a quasiperiodic potential and subjected to an out-of-plane Zeeman field. We solve exactly the two-particle problem in a finite length system by exact diagonalization and solve approximately the ground-state many-particle problem within the mean-field Bogoliubov-de Gennes approach. At a small Zeeman field, the localization properties of the system are similar to that of a Fermi gas with conventional $s$-wave interactions. As the disorder strength increases, the two-particle binding energy increases and the fermionic superfluidity of many particles disappears above a threshold. At a large Zeeman field, where the interatomic interaction behaves effectively like a $p$-wave interaction, the binding energy decreases with increasing disorder strength, and the resulting topological superfluidity shows a much more robust stability against disorder than the conventional $s$-wave superfluidity. We also analyze the localization properties of the emergent Majorana fermions in the topological phase. Our results could be experimentally examined in future cold-atom experiments, where the spin-orbit coupling can be induced artificially by using two Raman lasers, and the quasiperiodic potential can be created by using bichromatic optical lattices.

The dynamical instability of weakly interacting two-component Bose-Einstein condensates with coaxial quantized vortices is analytically investigated in a two-dimensional isotopic harmonic potential. We examine whether complex eigenvalues appear on the Bogoliubov–de Gennes equation, implying dynamical instability. Rather than solving the Bogoliubov–de Gennes equation numerically, we rely on a perturbative expansion with respect to the coupling constant which enables a simple, analytic approach. For each pair of winding numbers and for each magnetic quantum number, the ranges of intercomponent coupling constant where the system is dynamically unstable are exhaustively obtained. Corotating and counter-rotating systems show distinctive behaviors. The latter is much more complicated than the former with respect to dynamical instability, particularly because radial excitations contribute to complex eigenvalues in counter-rotating systems.

We analyze the correlation properties of light in nonlinear waveguides and transmission lines, predict the position-momentum realization of the Einstein-Podolsky-Rosen paradox for photon pairs in Kerr-type nonlinear photonic circuits, and we show how two-photon entangled states can be generated and detected.

In this work we develop a semianalytical variational Ansatz to study the properties of few-photon excitations interacting with a collection of quantum emitters in regimes that go beyond the rotating-wave approximation. This method can be used to approximate both the static and dynamical properties of a superconducting qubit in an open transmission line, including the spontaneous emission spectrum and the resonances in scattering experiments. The approximations are quantitatively accurate for rather strong couplings, as shown by a direct comparison to matrix-product-state numerical methods, and provide also a good qualitative description for stronger couplings well beyond the Markovian regime.

We report an efficient mechanism to generate a squeezed state of a mechanical mirror in an optomechanical system. We use an especially tuned parametric amplifier (PA) inside the cavity and the parametric photon phonon processes to transfer quantum squeezing from photons to phonons with almost 100% efficiency. We get 50% squeezing of the mechanical mirror which is limited by the PA. We present analytical results for the mechanical squeezing thus enabling one to understand the dependence of squeezing on system parameters like gain of PA, cooperativity, and temperature. As in cooling experiments the detrimental effects of mirror's Brownian and zero point noises are strongly suppressed by the pumping power. By judicious choice of the phases, the cavity output is squeezed only if the mirror is squeezed thus providing us a direct measure of the mirror's squeezing. Further considerable larger squeezing of the mirror can be obtained by adding the known feedback techniques.

The concept of directionally unbiased optical multiports is introduced, in which photons may reflect back out the input direction. A linear-optical implementation is described, and the simplest three-port version studied. Symmetry arguments demonstrate potential for unusual quantum information processing applications. The devices impose group structures on two-photon entangled Bell states and act as universal Bell-state processors to implement probabilistic quantum gates acting on state symmetries. These multiports allow optical scattering experiments to be carried out on arbitrary undirected graphs via linear optics and raise the possibility of linear-optical information processing using group structures formed by optical qudit states.

The optical isolation functionality is of key importance in many photonic systems. However, a robust optical isolation is rather challenging to obtain due to the weak nonreciprocal response of conventional materials. Here, we theoretically explore novel solutions to obtain one-way propagation of circularly polarized light using materials with a spontaneous nonreciprocal response combined with chiral metamaterials. Furthermore, we investigate the opportunity of using standard anisotropic nonreciprocal materials (e.g., ferrites or bismuth iron garnet) to obtain the optical isolation of circularly polarized light.

In one dimension, one can dissect a scattering potential $v\left(x\right)$ into pieces $v_i\left(x\right)$ and use the notion of the transfer matrix to determine the scattering content of $v\left(x\right)$ from that of $v_i\left(x\right)$. This observation has numerous practical applications in different areas of physics. The problem of finding an analogous procedure in dimensions larger than 1 has been an important open problem for decades. We provide a complete solution for this problem and discuss some of its applications. In particular, we derive an exact expression for the scattering amplitude of the $\delta$-function potential in two and three dimensions and a potential describing a slab laser with a surface line defect. We show that the presence of the defect makes the slab begin lasing for arbitrarily small gain coefficients.

Recently, a new type of Feshbach resonance, i.e., orbital Feshbach resonance (OFR), was proposed for the ultracold alkaline-earth-metal–like atoms and was experimentally observed in the ultracold gases of ${}^{173}\mathrm{Yb}$ atoms. Unlike most of the magnetic Feshbach resonances of ultracold alkali atoms, when the OFR of ${}^{173}\mathrm{Yb}$ atoms appears, the energy gap between the thresholds of the open channel (OC) and the closed channel (CC) is much smaller than the characteristic energy of the interatomic interaction, i.e., the van der Waals energy. In this paper we study the OFR in systems with a small CC-OC threshold gap. We show that in these systems the OFR can be induced by the coupling between the OC and either an isolated bound state of the CC or the scattering states of the CC. Moreover, we also show that in each case the two-channel Huang-Yang pesudopotential is always applicable for the approximate calculation of the low-energy scattering amplitude. Our results imply that in the two-channel theoretical calculations for these systems it is appropriate to take into account the contributions from the scattering states of the CC.

One-dimensional Bose-Hubbard models are well known to obey a transition from regular to quantum-chaotic spectral statistics. We are extending this concept to relatively simple two-dimensional many-body models. Also in two dimensions a transition from regular to chaotic spectral statistics is found and discussed. In particular, we analyze the dependence of the spectral properties on the bond number of the two-dimensional lattices and the applied boundary conditions. For maximal connectivity, the systems behave most regularly in agreement with the applicability of mean-field approaches in the limit of many nearest-neighbor couplings at each site.

We report on the temporal pulse characteristics of individual harmonics in an attosecond pulse train by means of photoelectron streaking in a strong low-frequency transient. The scheme allows one to retrieve the pulse durations and first-order chirp of individual harmonics without the need for temporal scanning. The measurements were performed using an intense THz field generated by a tilted phase front technique in ${\mathrm{LiNbO}}_3$. We compared the performance of Xe clusters and atomic He as a detection medium and retrieved the temporal properties for harmonics of order 19, 21, 23, 25, and 27. Our measurements confirm that the individual pulse durations and linear chirp decrease by harmonic order.

We consider the coupling of two optical waveguides in the few-cycle regime. The analysis is performed in the frame of a generalized Kadomtsev-Petviashvili model. A set of two coupled modified Korteweg–de Vries equations is derived, and it is shown that three types of coupling can occur, involving the linear index, the dispersion, or the nonlinearity. The linear nondispersive coupling is investigated numerically, showing the formation of vector solitons. Separate pulses may be trapped together if they have not initially the same location, size, or phase, and even if their initial frequencies differ.

We theoretically investigate the dipolar molecular system driven by monochromatic periodic, linear, parabolic, and ${\mathrm{sech}}^2$ forms external fields, respectively. The two-level Hamiltonian model with nonzero diagonal dipole matrix elements is adopted to describe the population dynamics of the driven dipolar molecule, and the corresponding exact solutions are presented in terms of the confluent Heun equations without the generalized rotating-wave approximation. The analytic solutions derived here are valid in the whole parameter space.

We demonstrate how to create localized $\mathcal{PT}$-symmetric directionally invisible scatterers directly from a governing wave equation. Moreover, we can construct general non-$\mathcal{PT}$-symmetric objects which exhibit directional invisibility, though such an effect is typically associated with $\mathcal{PT}$-symmetric objects. Whereas previously the determining condition for an optical $\mathcal{PT}$-symmetric device was a $\mathcal{PT}$-symmetric complex refractive index, we show that a broader condition, requiring only the scattering potential to be $\mathcal{PT}$-symmetric, leads to the same behavior. This enables the construction of $\mathcal{PT}$-symmetric objects without a $\mathcal{PT}$-symmetric complex refractive index, effectively doubling the number of possible invisible objects. Consequently, the set of gain-loss invisible objects is much broader than previously realized, and several examples are shown.