The exterior complex scaling (ECS) method is applied in the framework of time-dependent density-functional theory (TDDFT) to study multiphoton ionization (MPI) and high-order-harmonic generation (HHG) of multielectron atoms in intense laser fields. ECS allows one to impose correct (outgoing-wave) boundary conditions on the wave functions at large distances. In our implementation, ECS is combined with the time-dependent generalized pseudospectral method for accurate and efficient solution of the time-dependent Kohn-Sham equations. We make use of LB94 exchange-correlation potential which proved accurate in calculations of unperturbed electronic structure of Ar. Calculations of MPI and HHG are performed for the laser pulses with the wavelength of 800 nm and several peak intensities. The HHG spectrum exhibits an intensity-independent minimum corresponding to the photon energy of about 51 eV which is closely related to the Cooper minimum observed in the photoionization cross section of Ar. We found that MPI probabilities and HHG spectra calculated with the frozen-core potential (that is, not including dynamic response of the electron density) differ significantly from those obtained by TDDFT.

Blind quantum computation is a new secure quantum computing protocol which enables Alice (who does not have sufficient quantum technology) to delegate her quantum computation to Bob (who has a full-fledged quantum computer) in such a way that Bob cannot learn anything about Alice's input, output, and algorithm. In previous protocols, Alice needs to have a device which generates quantum states, such as single-photon states. Here we propose another type of blind computing protocol where Alice does only measurements, such as the polarization measurements with a threshold detector. In several experimental setups, such as optical systems, the measurement of a state is much easier than the generation of a single-qubit state. Therefore our protocols ease Alice's burden. Furthermore, the security of our protocol is based on the no-signaling principle, which is more fundamental than quantum physics. Finally, our protocols are device independent in the sense that Alice does not need to trust her measurement device in order to guarantee the security.

In Rydberg dressed ultracold gases, ground-state atoms inherit properties of a weakly admixed Rydberg state, such as sensitivity to long-range interactions. We show that through hyperfine-state-dependent interactions, a pair of atom clouds can evolve into a spin and subsequently into a spatial mesoscopic superposition state: The pair is in a coherent superposition of two configurations, with cloud locations separated by micrometers. The mesoscopic nature of the state can be proven with absorption imaging, while the coherence can be revealed though recombination and interference of the split wave packets.

We calculate the condensate fraction and the condensate and noncondensate spatial and momentum distribution of the Bose-Hubbard model in a trap. From our results, it is evident that using approximate distributions can lead to erroneous experimental estimates of the condensate. Strong interactions cause the condensate to develop pedestal-like structures around the central peak that can be mistaken as noncondensate atoms. Near the transition temperature, the peak itself can include a significant noncondensate component. Using distributions generated from quantum Monte Carlo simulations, experiments can map their measurements for higher accuracy in identifying phase transitions and temperature.

We propose a simple structure for stationary non-Markovian quantum chains in the framework of collisional dynamics of open quantum systems. To this end, we modify the microscopic Markovian system-reservoir model and consider multiple collisions with each of the molecules with an overlap between the collisional time intervals. We show how the equivalent Markovian quantum chain can be constructed with the addition of satellite quantum memory to the system. We distinguish quantum from classical non-Markovianity. Moreover, we define the counts of non-Markovianity by the required number of satellite qubits and bits, respectively. As the particular measure of quantum non-Markovianity, the discord of the satellite with respect to the system is suggested. The simplest qubit realizations are discussed and the significance for real system-environment dynamics is also pointed out.

Does the sum of correlations in subsystems constitute the correlation in the total system? Such a concept can be expressed by an additivity relationship of correlations. From a strong subadditivity condition of von Neumann entropy, four different additivity relations in total correlation are derived and quantified. Based upon the classification of the additivity in total correlation, we identify the corresponding additive relationships in entanglement. It is also discussed that similar relationships are satisfied for quantum discord of pure states, but it is not always true for mixed states.

In 1985, Leggett and Garg put forward the concept of macroscopic realism (macrorealism) and, in analogy to Bell's theorem, derived a necessary condition in terms of inequalities, which are now known as the Leggett-Garg inequalities. In this paper, we discuss another necessary condition called no-signaling in time. It solely bases on comparing the probability distribution for a macrovariable at some time for the cases where previously a measurement has or has not been performed. Although the concept is analogous to the no-signaling condition in the case of Bell tests, it can be violated according to quantum mechanical predictions even in situations where no violation of Leggett-Garg inequalities is possible.

Scattering of a quantum particle from an oscillating barrier or well does not generally conserve the particle energy owing to energy exchange with the photon field, and an incoming particle-free state is scattered into a set of outgoing (transmitted and reflected) free states according to Floquet scattering theory. Here we introduce two families of oscillating non-Hermitian potential wells in which Floquet scattering is fully suppressed for any energy of the incident particle. The scattering-free oscillating potentials are synthesized by application of the Darboux transformation to the time-dependent Schrödinger equation. For one of the two families of scattering-free potentials, the oscillating potential turns out to be fully invisible.

It is known that the $n$-qubit $W$-type state is determined by its bipartite reduced density matrices. In this paper, we show that the multipartite $W$-type state is uniquely determined by its reduced density matrices among $W$-type states in the sense of local unitary equivalence.

On the basis of the investigation of the asymptotic behavior of the correlation functions of the corresponding field currents with the necessary quantum numbers, the analytic method for the determination of the energy spectrum of the three-body Coulomb system is suggested. Within this framework the analytical approach to determine the energy spectrum of the molecular hydrogen ions in the ground and orbital excited state is used. In our case, relativistic corrections are taken into account by the constituent mass of the constituent particles, as well as by the interaction potential. Our results show that the masses of the constituent particles differ from the masses of the particles in the free state. The increasing of the constituent mass of electron is comparatively larger than the increasing of constituent mass of the proton, deuteron, and triton. The constituent masses of the electron differ between for the molecular ions of hydrogen ${\text{H}}_2^{+}$, ${\text{D}}_2^{+}$, ${\text{T}}_2^{+}$, ${\text{HD}}^{+}$, ${\text{DT}}^{+}$, and ${\text{HT}}^{+}$. Thus, our results on the energy spectrum of molecular hydrogen ions in the ground and excited states show good agreement with the existing results of precision spectroscopy; this is achieved by taking into account the values of the masses of constituent particles.

The linear polarization of the characteristic photon emission from few-electron ions is studied for its sensitivity with regard to the nuclear spin and magnetic moment of the ions. Special attention is paid, in particular, to the $K{\alpha }_1$ ($1s2p_{3/2}{}^{1,3}{P}_{1,2}\to 1s^2{}^1{S}_0$) decay of selected heliumlike ions following the radiative electron capture into initially hydrogenlike species. Based on the density matrix theory, a unified description is developed that includes both the many-electron and hyperfine interactions as well as the multipole-mixing effects arising from the expansion of the radiation field. It is shown that the polarization of the $K{\alpha }_1$ line can be significantly affected by the mutipole mixing between the leading $M2$ and hyperfine-induced $E1$ components of $1s2p{}^3{P}_2,F_i\to 1s^2{}^1{S}_0,F_f$ transitions. This $E1$-$M2$ mixing strongly depends on the nuclear properties of the considered isotopes and can be addressed experimentally at existing heavy-ion storage rings.

We compare electron-scattering data from Ne with first-order Born and distorted-wave calculations in an energy range between 300 and 2500 eV and for scattering angles up to 135${}^{\circ }$ and for energy losses up to the ionization energy. At small angles the distorted-wave calculations and first-order Born calculations are in good agreement, but at large momentum transfer the intensity predicted by the first-order Born theory drops off much faster than the experiment and distorted-wave calculations. The present distorted-wave calculations reproduce most of the experimental observations quite well, except for monopole transitions and near the minima in dipole-allowed $s$-to-$p$ transitions. The first-order Born approximation fails completely at larger momentum transfer.

We investigate the photoionization spectrum of helium by attosecond XUV pulses both in the spectral region of doubly excited resonances as well as above the double ionization threshold. In order to probe for convergence, we compare three techniques to extract photoelectron spectra from the wave packet resulting from the integration of the time-dependent Schrödinger equation in a finite-element discrete variable representation basis. These techniques are projection on products of hydrogenic bound and continuum states, projection onto multichannel scattering states computed in a $B$-spline close-coupling basis, and a technique based on exterior complex scaling implemented in the same basis used for the time propagation. These methods allow one to monitor the population of continuum states in wave packets created with ultrashort pulses in different regimes. Applications include photo cross sections and anisotropy parameters in the spectral region of doubly excited resonances, time-resolved photoexcitation of autoionizing resonances in an attosecond pump-probe setting, and the energy and angular distribution of correlated wave packets for two-photon double ionization.

In this contribution, we investigate the femtosecond filament interactions in gas by employing Hermite-Gaussian TEM${}_{02}$ mode focused by lens in argon gas by means of numerically evaluating the $3+1$ dimension nonlinear Schrödinger equation. High-resolution spatiotemporal characteristics has been obtained. The TEM${}_{02}$ mode laser pulse, which has three lobes in the plane perpendicular to the propagation direction, can form three filaments. In the filamentation region, interaction between the lobes occurs at the trailing parts of the laser filaments which correspond to the blue-shifted supercontinuum generated by self-phase modulation and plasma. We find the energetic spatiotemporal fragments are more inclined to repulsion rather than fusion when close to each other, which may imply that two adjacent single filaments dislike to combine into a big filament in multifilaments. It seems that the phenomenon of repulsion bears no relation to the phase difference between the input subpulses. Our finding may help to understand why one cannot supply energy to a single filament by multifilament fusion until to now.

We report a combined experimental and theoretical investigation of near-resonance light scattering from a high-density and ultracold atomic ${}^{87}$Rb gas. The atomic sample, having a peak density $\sim$$5\times {10}^{13}$ atoms/cm${}^3$, temperature $\sim$65 $\mu$K, and initially prepared in the $F=1$ lower-energy ${}^{87}$Rb hyperfine component, is optically pumped to the higher-energy $F=2$ hyperfine level. Measurements are made of the transient hyperfine pumping process and of the time evolution of scattering of near-resonance probe radiation on the $F=2\to F^{\prime }=3$ transition. Features of the density, detuning, and temporal dependence of the signals are attributed to the high density and consequent large optical depth of the samples.

We study an optomechanical system in which a microwave field and an optical field are coupled to a common mechanical resonator. We explore methods that use these mechanical resonators to store quantum-mechanical states and to transduce states between the electromagnetic resonators from the perspective of the effect of mechanical decoherence. Besides being of fundamental interest, this coherent quantum state transfer could have important practical implications in the field of quantum information science, as it potentially allows one to overcome intrinsic limitations of both microwave and optical platforms. We discuss several state transfer protocols and study their transfer fidelity using a fully quantum-mechanical model that utilizes quantum state diffusion techniques. This work demonstrates that mechanical decoherence should not be an insurmountable obstacle in realizing high-fidelity storage and transduction.

We investigate correlations between orthogonally polarized cavity modes of a bimodal micropillar laser with a single layer of self-assembled quantum dots in the active region. While one emission mode of the microlaser demonstrates a characteristic S-shaped input-output curve, the output intensity of the second mode saturates and even decreases with increasing injection current above threshold. Measuring the photon autocorrelation function $g^{\left(2\right)}\left(\tau \right)$ of the light emission confirms the onset of lasing in the first mode with $g^{\left(2\right)}\left(0\right)$ approaching unity above threshold. In contrast, strong photon bunching associated with superthermal values of $g^{\left(2\right)}\left(0\right)$ is detected for the other mode for currents above threshold. This behavior is attributed to gain competition of the two modes induced by the common gain material, which is confirmed by photon cross-correlation measurements revealing a clear anticorrelation between emission events of the two modes. The experimental studies are in qualitative agreement with theoretical studies based on a microscopic semiconductor theory, which we extend to the case of two modes interacting with the common gain medium. Moreover, we treat the problem by a phenomenological birth-death model extended to two interacting modes, which reveals that the photon probability distribution of each mode has a double-peak structure, indicating switching behavior of the modes for pump rates around threshold.

The stability of tripositronium, a system consisting of three electrons and three positrons, has been investigated systematically by varying the repulsion strength between like-charged particles. The possibility of the existence of a Ps${}_3$ bound state that is stable against dissociation appears utterly unlikely based on the results of variational calculations employing all-particle explicitly correlated Gaussian basis functions.

We use the previously developed radiative potential method to calculate quantum electrodynamic (QED) corrections to energy levels and electric dipole transition amplitudes for atoms which are used for the study of the parity nonconservation (PNC) in atoms. The QED shift in energies and dipole amplitudes leads to noticeable change in the PNC amplitudes. This study compliments the previously considered QED corrections to the weak matrix elements. We demonstrate that the QED corrections due to the change in energies and dipole matrix elements are comparable in value to those due to the change in weak matrix elements and therefore must be included.