The energy lost by helium atoms axially scattered from a Ag(110) surface is studied in order to investigate the influence of dissipative processes on fast atom diffraction spectra. In this work inelastic projectile distributions are evaluated within a semiclassical formalism that includes dissipative effects due to electron-hole excitations through a friction force. For incidence along the $\left[1\overline{1}2\right]$ and $\left[1\overline{1}0\right]$ directions the model predicts the presence of multiple peaks in the energy-loss spectrum for a given impact energy. But these structures are completely washed out when the experimental dispersion of the incident beam is taken into account, giving rise to a smooth energy-loss distribution. Simulations including the experimental energy spread are in fairly good agreement with available experimental data for the $\left[1\overline{1}2\right]$ channel. In addition, our results suggest that inelastic processes produce an almost constant background in the transverse momentum distribution, except in the extremes of the momentum range where classical rainbow maxima appear. By adding elastic and inelastic contributions, experimental diffraction patterns are well reproduced.

Continuing previous work, we have measured the projectile scattering-angle dependency for transfer excitation of fast protons (300–1200 keV$/$u) colliding with helium ($p+$ He $\to$ H $+$ He${}^{+*}$). Our high-resolution fully differential data are accompanied by calculations, performed in the plane-wave first Born approximation and the eikonal wave Born approximation. Experimentally, we find a deep minimum in the differential cross section around 0.5 mrad. The comparison with our calculations shows that describing the scattering-angle dependence of transfer excitation in fast collisions requires us to go beyond the first Born approximation and in addition to use the initial-state wave function, which contains some degree of angular correlations.

The weak-field asymptotic theory of tunneling ionization in a static electric field is applied to ${\mathrm{H}}_2\mathrm{O}$. The orientation dependence of the ionization rate is studied. The use of polarization-consistent basis sets with up to heptuble-zeta accuracy and variationally optimized exponents improves the asymptotic form of the wave function and allows for an accurate extraction of the structure factor defining the ionization rate. The results are presented based on Hartree-Fock wave functions and density functional theory. The density functional theory reproduces closely the experimental vertical ionization potential. We find that the rate peaks at an angle of ${81}^{\circ }$ between the field and molecular principal inertial axis through the O atom. The predictions for the orientation dependence of the rate are compared to available theoretical results.

The growing successes in performing quantum control experiments motivated the development of control landscape analysis as a basis to explain these findings. When a quantum system is controlled by an electromagnetic field, the observable as a functional of the control field forms a landscape. Theoretical analyses have revealed many properties of control landscapes, especially regarding their slopes, curvatures, and topologies. A full experimental assessment of the landscape predictions is important for future consideration of controlling quantum phenomena. Nuclear magnetic resonance (NMR) is exploited here as an ideal laboratory setting for quantitative testing of the landscape principles. The experiments are performed on a simple two-level proton system in a H${}_2$O-D${}_2$O sample. We report a variety of NMR experiments roving over the control landscape based on estimation of the gradient and Hessian, including ascent or descent of the landscape, level set exploration, and an assessment of the theoretical predictions on the structure of the Hessian. The experimental results are fully consistent with the theoretical predictions. The procedures employed in this study provide the basis for future multispin control landscape exploration where additional features are predicted to exist.

We study degenerate mixtures of heavy bosons and light superfluid bosons using a variational polaron transformation. We consider the Mott-insulator–superfluid transition of the heavy species and find that at $T=0$ interaction favors the superfluid phase of the heavy species. Our analytic results agree well with numerically exact quantum Monte Carlo simulations in two dimensions. We then show that in three dimensions the variational polaron transformation can be combined with a Gutzwiller approximation to give good results.

We study the Bose-Hubbard and Fermi-Hubbard models in the (formal) limit of large coordination numbers $Z\gg 1$. Via an expansion into powers of $1/Z$, we establish a hierarchy of correlations which facilitates an approximate analytical derivation of the time evolution of the reduced density matrices for one and two sites, etc. With this method, we study the quantum dynamics (starting in the ground state) after a quantum quench, i.e., after suddenly switching the tunneling rate $J$ from zero to a finite value, which is still in the Mott regime. We find that the reduced density matrices approach a (quasi)equilibrium state after some time. For one lattice site, this state can be described by a thermal state (within the accuracy of our approximation). However, the (quasi)equilibrium state of the reduced density matrices for two sites including the correlations can not be described by a thermal state. Thus, real thermalization (if it occurs) should take a much longer time. This behavior has already been observed in other scenarios and is sometimes called “prethermalization”. Finally, we compare our results to numerical simulations for finite lattices in one and two dimensions and find qualitative agreement.

We investigate the uniform spin susceptibility ${\chi }_{\mathrm{s}}$ in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas. Including pairing fluctuations within the framework of an extended $T$-matrix approximation, we show that ${\chi }_{\mathrm{s}}$ exhibits nonmonotonic temperature dependence in the normal state. In particular, ${\chi }_{\mathrm{s}}$ is suppressed near the superfluid phase transition temperature $T_{\mathrm{c}}$ due to strong pairing fluctuations. To characterize this anomalous behavior, we introduce the spin-gap temperature $T_{\mathrm{s}}$ as the temperature at which ${\chi }_{\mathrm{s}}$ takes a maximum value. Determining $T_{\mathrm{s}}$ in the whole BCS-BEC crossover region, we identify the spin-gap regime in the phase diagram of a Fermi gas in terms of the temperature and the strength of a pairing interaction. We also clarify how the spin-gap phenomenon is related to the pseudogap phenomenon appearing in the single-particle density of states. Our results indicate that an ultracold Fermi gas in the BCS-BEC crossover region is a very useful system to examine the pseudogap phenomenon and the spin-gap phenomenon in a unified manner.

Harmonic generation of laser radiation from ionized particles in the extreme ultraviolet range at the conditions of the phase matching between the femtosecond driving field and harmonics is an advanced concept for the development of efficient coherent sources in this spectral region. Here we present experimental evidence of quasi-phase-matched high-order harmonic generation in multiple plasma jets produced during laser ablation of the silver target. We observed both the enhancement of some groups of harmonics along the plateau range and the variation of maximally enhanced harmonic order; additionally we analyzed the influence of the number of plasma jets on the harmonic yield. The enhancement factor of 13 was achieved for the 33th harmonic of Ti:sapphire laser using the five-jet plasma configuration.

We report on three-photon electromagnetically induced absorption (TPEIA) due to three-photon coherence in ladder-type atomic systems for the $5S_{1/2}$-$5P_{3/2}$-$5D_{5/2}$ transition in Rb atoms. When a counterpropagating coupling field was added to the typical ladder-type electromagnetically induced transparency (EIT) experiment, both EIT and two-photon absorption (TPA) switched to TPEIA. Considering three-photon coherence in a Doppler-broadened three-level ladder-type atomic system, the spectrum of the switch from EIT and TPA to TPEIA was numerically calculated and could be understood by decomposing the calculated spectrum into two-photon coherence and three-photon coherence components.

We consider the simultaneous propagation of a pair of Raman-resonant, frequency-modulated (chirped) laser pulses in an optically thick medium, modeled by an ensemble of $\Lambda$ atoms. A self-organization (“matching”) effect is shown for the chirped-pulse pair, which leads to a quasilossless propagation. Furthermore, we demonstrate that a well-defined coherent superposition of the atomic ground states and, correspondingly, a coherence are robustly created in the medium that can be controlled by amplitudes of the laser pulses. The proposed scheme can be applied to substantially increase the efficiency of the optical wave mixing processes, as well as in other nonlinear processes where the initial preparation of a spatially extended medium in a coherent superposition state is required.

We discuss the role of pseudofermions in the analysis of some two-dimensional models, recently introduced in connection with non-self-adjoint Hamiltonians. Among other aspects, we discuss the appearance of exceptional points in connection with the validity of the extended anticommutation rules which define the pseudofermionic structure.

Semi-device-independent randomness expansion is the protocol which can generate and certify perfect free random numbers, without making detailed assumptions on the internal working of quantum devices, except the bound on the dimension of the Hilbert space. The two-dimensional (2D) quantum witness violation has been used to generate guaranteed fresh randomness from untrusted devices in the prepare-and-measure configuration. In this paper we first show that the 2D quantum witness violation can be simulated by a classical deterministic process by relaxing the assumption of the dimension. A deterministic process to simulate the generation of random numbers is given, under the hidden variables model, with just the high dimensional measurement device and one qubit communication. This makes the generated number essentially deterministic, which is said to be pseudorandom. Then we also show that when we relax the freedom of choice, there is the exact region for which quantum correlations can be simulated by the classical process in the task of randomness expansion.

We introduce a class of bipartite entangled continuous variable states that are positive under a partial transposition (PPT) operation, i.e., PPT bound entangled. These states are based on realistic preparation procedures in optical systems, being thus a feasible option to generate and observe genuinely bipartite bound entanglement in high precision experiments. One fundamental step in our scheme is to perform a non-Gaussian operation over a single-mode Gaussian state. This degaussification procedure is achieved through a modified single-photon addition, which is a procedure that is currently being investigated in diverse optical setups. Although dependent on a single-photon detection in an idler channel, the preparation can be made unconditional after a calibration of the apparatus. The detection and proof of bound entanglement is made by means of the range criterion, theory of Hankel operators and Gerschgorin Disk's perturbation theorems.

An electric dipole placed in an anisotropic medium experiences a torque due to the induced polarization of the medium. The torque tends to align the dipole in the appropriate direction determined by the medium properties. Here we present simple quantitative theory of this effect and predict splitting of the emission spectrum lines of the elementary quantum emitter in an anisotropic environment due to the self-induced torque.

We investigate the parity nonconservation (PNC) effect in the dielectronic recombination of a polarized electron with a heavy He-like ion into doubly excited ${\left[{\left(1s2p_{1/2}\right)}_{0}n\kappa \right]}_{1/2}$ and ${\left[{\left(1s2s\right)}_{0}n\kappa \right]}_{1/2}$ states of a Li-like ion. We determine the nuclear charge number $Z$ for which these opposite-parity levels are almost crossing and therefore the PNC effect will be significantly enhanced. Calculations are performed for quantum numbers $n\ge 4$ and $\kappa =\pm 1$.

We derive the expressions for the virial coefficients up to the third order for two-species fermion mixtures, and investigate the fugacity of the species and the interaction energy density around the $s$-wave Feshbach resonance in ${}^6\mathrm{Li}$-${}^{40}\mathrm{K}$ mixtures. The theoretical results show that the magnitude of the virial coefficients depends on the mass ratio in a mass-imbalanced system. We find that the contribution of two-body bound states to high-order virial coefficients is very small at temperatures of microkelvin order. With the decrease of $T/T_F$, the positions of the maxima of fugacity and interaction energy density shift away from the resonance position in the third-order virial expansion calculation. The mass ratio determines the relevance between the fugacity and number density of heavy species. Our conclusion is also effective for other mass-imbalanced systems.

The development of strong x-ray lasers motivates the advancement of pair production studies into regions of higher laser frequency. In this paper, a resonant electron-positron pair production process with the absorption of two x-ray photons is considered in the impact of an energetic electron at the overlap region of two colliding x-ray laser beams. The laser-dressed QED method is justified to tackle the complexity of the corresponding multiple Feynman diagrams calculation. The dependence of the production rate as well as the positron energy distribution on the relative angles among the directions of the two laser wave vectors and the incoming electron momentum is revealed. It is shown that the non-plane-wave laser field configuration arouses additional features in the pair production process compared to the plane-wave case.

We obtain a rigorous solution of universal Bose gases near resonance and offer an answer to one of the long-standing challenges of quantum gases at large scattering lengths, where the standard dilute theory breaks down. The solution was obtained by using an $\epsilon$ expansion near four spatial dimensions. In dimension $d=4-\epsilon$, the chemical potential of Bose gases near resonance is shown to approach the universal value ${\epsilon }^{\frac{2}{4-\epsilon }}{\epsilon }_F\sqrt{\frac{2}{3}}(1+0.474\epsilon -i1.217\epsilon +\cdots )$, where ${\epsilon }_F$ is the Fermi energy defined for a Fermi gas of density $n$, and the condensation fraction is equal to $\frac{2}{3}(1+0.0877\epsilon +\cdots )$. We also discuss the implications on ultracold gases in physical dimensions.

Layered structures consisting of finite numbers of charge sheets separating dielectric layers are considered as physically realizable media for the control of dipole deexcitation due to spontaneous emission. The dispersion relation and the corresponding field distributions of a general structure are determined using transfer-matrix techniques. An excited dipole emitter localized in the vicinity of the truncation layer is coupled to the fields supported by such a structure, and its deexcitation rate is evaluated for a number of scenarios in which the structure is characterized by different numbers of charge sheets for varying excitation frequency and varying emitter position. The analysis highlights significant enhancements of the deexcitation rate which can be readily controlled through the adjustable parameters of the structure.