We analyze the dependence of attosecond streaked photoelectron spectra and photoemission time delays from valence band (VB) and 2$p$ core-level (CL) states of a single-crystalline Mg(0001) thin film on the (i) modeling of the substrate electronic structure, (ii) electron mean free path (MFP), (iii) screening of the near-infrared (NIR) streaking laser field, and (iv) chirp of the attosecond extreme ultraviolet (XUV) pulse. Our quantum-mechanical numerical simulations predict streaked photoemission spectra that depend sensitively on the XUV chirp and weakly on the screening of the streaking laser field by the substrate. They furthermore show that streaking time delays for VB emission are relatively insensitive to the modeling of the initial quantum states, electron MFP, and NIR skin depth of the Mg substrate, in contrast to the stronger dependence of streaking time delays for 2$p$ CL emission on these factors.

Nonlinear Thomson and Compton processes, in which energetic electrons collide with an intense optical pulse, are investigated in the framework of classical and quantum electrodynamics. Spectral modulations of the emitted radiation, appearing as either oscillatory or pulsating structures, are observed and explained. It is shown that both processes generate a bandwidth radiation spanning the range of a few MeV, which occurs in a small cone along the propagation direction of the colliding electrons. Most importantly, these broad bandwidth structures are temporarily coherent, which proves that Thomson and Compton processes lead to the generation of a supercontinuum. It is demonstrated that the radiation from the supercontinuum can be synthesized into zeptosecond (possibly even yoctosecond) pulses. This confirms that Thomson and Compton scattering can be used as sources of an ultrashort radiation, which opens routes to new physical domains for strong laser physics. We study properties of generated pulses arising in the classical and in the exclusively quantum regime of electron-laser-field interaction, and we attribute them to the behavior of global phases of Thomson and Compton scattering probability amplitudes.

We study the entanglement properties of the shape phase transitions between different geometric limits of two coupled equivalent molecular benders modeled with the two-dimensional limit of the vibron model. This system has four possible geometric configurations: linear, cis-bent, trans-bent, and nonplanar. We show how the entanglement, accessed through the calculation of the linear entropy, between benders and between rotational and vibrational degrees of freedom changes sensitively in the critical regions of this two-fluid bosonic model. The numeric calculation is complemented with a variational approach to the ground-state wave function in terms of symmetry-adapted coherent states.

We present fully relativistic convergent close-coupling calculations of differential cross sections, spin-asymmetries, and Stokes parameters for inelastic electron-cesium scattering at intermediate energies. Comparison is made with the differential cross section and spin asymmetry measurements of Baum et al. [Phys. Rev. A 70, 012707 (2004)] and the Stokes parameter measurements of Slaughter et al. [Phys. Rev. A 75, 062717 (2007)]. Comparison is also made with previous semirelativistic and nonrelativistic theories. With a relatively high atomic number for cesium ($Z=55$) we find surprisingly excellent agreement between the relativistic, semirelativistic, and nonrelativistic theories for most observables. The overall agreement with the measurements is very good, with isolated discrepancies for some observables.

Orientation-dependent photoelectron holography from the one-electron polar molecular ion HeH${}^{2+}$ in linearly polarized few-cycle laser fields is studied numerically. We identify four dominant holography patterns simultaneously with one shot of lasers and show that these are sensitive to the molecular structure and the orientation angle between the molecular axis and laser polarization direction. Due to the preferential enhanced ionization from one direction of the molecular axis, two kinds of forward photoelectron holography and two backward holography interference patterns predicted by a semiclassical model show up simultaneously for parallel orientation. Coulomb focusing effects are shown to be crucial for the backward scattering holography and a weak forward scattering holography channel with relatively small overlap. In contrast, after increasing the orientation angle, the backward scattering patterns disappears gradually because of the smaller scattering cross sections. Only one forward scattering holography can be clearly identified in the perpendicular orientation. Ultrashort and intense laser pulses are shown to be preferred for photoelectron holography. Intercycle interference rings in the above-threshold ionization are suppressed and consequently the intracycle rescattering signals are enhanced.

The experimental observations of sequential double ionization (SDI) of Ar [A. N. Pfeiffer et al., Nat. Phys. 7, 428 (2011)], such as the four-peak momentum distribution and the ionization time of the first and second electrons, are investigated and reproduced with a quantum model by including and excluding the electron-electron correlation effect. Based on the comparison of experiment and simulation, the role of electron-electron correlation in SDI is discussed. It is shown that the inclusion of electron-electron correlation is necessary to reproduce the momentum distribution of electrons.

We numerically simulate the creation process of two-dimensional skyrmionic excitations in antiferromagnetic spin-1 Bose-Einstein condensates by solving the full three-dimensional dynamics of the system from the Gross-Pitaevskii equation. Our simulations reproduce quantitatively the experimental results of Choi et al. [Phys. Rev. Lett. 108, 035301 (2012)] without any fitting parameters. Furthermore, we examine the stability of the skyrmion by computing the temporal evolution of the condensate in a harmonic potential. The presence of both the quadratic Zeeman effect and dissipation in the simulations is vital for reproducing the experimentally observed decay time.

We investigate the quantum mechanics of a single particle constrained to move along an arbitrary smooth reference curve by a confinement that is allowed to vary along the waveguide. The Schrödinger equation is evaluated in the adapted coordinate frame and a transverse-mode decomposition is performed, taking into account both curvature and torsion effects and the possibility of a cross-section potential that changes along the curve in an arbitrary way. We discuss the adiabatic structure of the problem, and examine nonadiabatic couplings that arise due to the curved geometry, the varying transverse profile, and their interplay. The exact multimode matrix Hamiltonian is taken as the natural starting point for few-mode approximations. Such approximate equations are provided, and it is worked out how these recover known results for twisting waveguides and can be applied to other types of waveguide designs. The quantum waveguide Hamiltonian is recast into a form that clearly illustrates how it generalizes the Born-Oppenheimer Hamiltonian encountered in molecular physics. In analogy to the latter, we explore the local gauge structure inherent to the quantum waveguide problem and suggest the usefulness of diabatic states, giving an explicit construction of the adiabatic-to-diabatic basis transformation.

We present an improved superatom model for examining nonlinear optical responses of cold Rydberg atoms in the regime of electromagnetically induced transparency (EIT). By going beyond the weak-probe approximation, we find that several higher-order collective states should be included to correctly describe the coherent Rydberg excitation of superatoms. Otherwise, numerical results based on the simple ladder system of superatoms will contribute wrong predictions on light intensity and photon correlation of the transmitted probe field. In particular, a great photon-bunching effect will be improperly expected somewhere out of the EIT window in one dilute atomic sample. The essence of this improved superatom model lies in that it can provide reliable predictions on the nonlinear Rydberg-EIT phenomena even in dense atomic samples and may be extended to realize lossless conditional light interactions in appropriate multilevel systems exhibiting dipole blockade.

A recent $S$-matrix-based theory of the quantum-limited linewidth, which is applicable to general lasers, including spatially nonuniform laser cavities operating above threshold, is analyzed in various limits. For broadband gain, a simple interpretation of the Petermann and bad-cavity factors is presented in terms of geometric relations between the zeros and poles of the $S$ matrix. When there is substantial dispersion, on the frequency scale of the cavity lifetime, the theory yields a generalization of the bad-cavity factor, which was previously derived for spatially uniform one-dimensional lasers. This effect can lead to sub–Schawlow-Townes linewidths in lasers with very narrow gain widths. We derive a formula for the linewidth in terms of the lasing mode functions, which has accuracy comparable to the previous formula involving the residue of the lasing pole. These results for the quantum-limited linewidth are valid even in the regime of strong line pulling and spatial hole burning, where the linewidth cannot be factorized into independent Petermann and bad-cavity factors.

We evaluate optical forces and torques induced by a surface plasmon to a sphere of arbitrary size, i.e., beyond the pointlike dipolar limit. Through a multipolar decomposition of the plasmonic field, we demonstrate that the induced torque is purely transverse to the plasmon propagation direction. Our approach removes the inherent ambiguities of the dipolar regime with respect to rotations and emphasizes the crucial role played by dissipation in the onset of the plasmonic torque. We also give realistic estimates of such plasmon-induced spinning of gold spheres immersed in water or air.

We propose a scheme to construct a two-dimensional Aubry-André (AA) model and realize two-dimensional AA localization of light waves via electromagnetically induced transparency (EIT). The system we suggest is a cold, resonant atomic gas with an $N$-type level configuration and interacting with probe, control, assisted, and far-detuned laser fields. We show that under EIT conditions the probe-field envelope obeys a modified nonlinear Schrödinger equation with a quasiperiodic potential, which becomes a two-dimensional nonlinear AA model when the system parameters are suitably chosen. The quasiperiodic potential is obtained by the cross-phase modulation of the assisted field and the Stark shift of the far-detuned laser field. In addition, the cubic nonlinearity term appearing in the model is contributed by the self-phase modulation of the probe field. We demonstrate that the system can be used to not only realize various two-dimensional AA localizations of light waves, but also to display nonlinearity and dimensionality effects on the AA localizations.

We derive the equation of motion for a Bose-Einstein condensate of photons in a dye-microcavity system, starting from Maxwell's equations. Our theory takes into account mirror shape, Kerr-type intensity-dependent refractive index and incoherent pumping and loss. The resulting equation is remarkably similar to the Gross-Pitaevskii equation for exciton-polariton condensates, despite the different microscopic origins. We calculate the incoherent photoluminescence spectrum of the photon condensate, which shows the Bogoliubov-type excitations around the mean field at thermal equilibrium. Both open- and closed-system models are presented to account for, respectively dissipation and inhomogeneities. Considering realistic parameters and experimental resolution, we estimate that by observing the angle-resolved spectrum of incoherent photoluminescence it is possible to resolve dimensionless interaction parameters of order ${10}^{-5}$, two orders of magnitude below current estimates. Thus we expect that this technique will lead to accurate measurements of the interactions in photon condensates.

Collapse and revivals in the Jaynes-Cummings model are studied within the context of the Mollow transformation for a single-mode coherent-state cavity field. The Mollow-transformed Jaynes-Cummings Hamiltonian has two atom-field terms, one corresponding to a classical field and the other to a quantized field driving the atomic transition. As such it maps onto the complementary problem of an atom in an optical cavity, whose field is initially in the vacuum state, driven by an external classical field. Both problems have the same atomic-state dynamics. It is shown that the revivals can be associated with two distinct properties of the quantized field of the transformed Hamiltonian. Revivals occurring at even (odd) multiples of the revival time are correlated with field states that are close to the initial field state and for which the average energy in the field is a minimum (maximum). Using semiclassical dressed states, we are able to map the problem for the field states onto one involving two uncoupled oscillators driven by off-resonant fields, provided effects related to dispersion are neglected. Analytic expressions for the photon number distribution of the transformed cavity field, including dispersion, are obtained for the even revival times.

We report an experimental and numerical investigation of third- and fifth-harmonic generation in a CaF${}_2$ crystal with 20 fs (three-optical-cycle), 2 $\mu$m driving pulses. The double-peaked temporal profile of the third-harmonic pulse and its propagation dynamics was captured by means of the cross-correlation technique, showing that the third-harmonic pulse naturally consists of free and driven components propagating with different group velocities, and which occur without the splitting of the driving pulse at the fundamental frequency. Relevant characteristics of the harmonics generation process, such as the harmonics spectra, energy oscillations, and conversion efficiency, were measured as functions of propagation length and input-pulse energy and intensity. Our results demonstrate that the fifth harmonic is generated solely via cascaded four-wave mixing between the fundamental and third-harmonic frequency pulses due to cubic nonlinearity, without any detectable contribution of six-wave mixing due to quintic nonlinearity in the process.

It is well known that a particle cannot freely share entanglement with two or more particles. This restriction is generally called monogamy. However the formal quantification of such restriction is only known for some measures of entanglement and for two-level systems. The first and broadly known monogamy relation was established by Coffman, Kundu, and Wootters for the square of the concurrence. Since then, it is usually said that the entanglement of formation is not monogamous, as it does not obey the same relation. We show here that despite that, the entanglement of formation cannot be freely shared and therefore should be said to be monogamous. Furthermore, the square of the entanglement of formation does obey the same relation of the squared concurrence, a fact recently noted for three particles and extended here for N particles. Therefore the entanglement of formation is as monogamous as the concurrence. We also numerically study how the entanglement is distributed in pure states of three qubits and the relation between the sum of the bipartite entanglement and the classical correlation.

Considering one-body and two-body currents, we study the photoassociation and photodissociation of universal bosonic trimers. Analyzing the relative importance of these currents, we identify two physical scenarios: (i) normal hierarchy, where naive power counting holds and the one-body current dominates, and (ii) strong hierarchy, where the one-body current is suppressed. For both scenarios we observe that at the high-frequency tail, the response function exhibits log-periodic oscillations in transition to or from any continuum state regardless of the reaction partial-wave channel. In contrast, near-threshold log-periodic oscillations appear only in the leading $s$-wave components. These oscillations are the fingerprints of universal Efimov physics. We discuss the relevance of this effect to contemporary experiments in ultracold atoms.

We test experimentally the quantum “paradox” proposed by L. Hardy [Phys. Rev. Lett. 71, 1665 (1993)] by using single photons instead of photon pairs. This is achieved by addressing two compatible degrees of freedom of the same particle, namely, its spin angular momentum, determined by the photon polarization, and its orbital angular momentum, a property related to the optical transverse mode. Because our experiment involves a single particle, we cannot use locality to logically enforce noncontextuality, which must therefore be assumed based only on the observables' compatibility. On the other hand, our single-particle experiment can be implemented more simply and allows larger detection efficiencies than typical two-particle ones, with a potential future advantage in terms of closing the detection loopholes.

We propose Bell inequalities for discrete or continuous quantum systems which test the compatibility of quantum physics with an interpretation in terms of deterministic hidden-variable theories. The wave function collapse that occurs in a sequence of quantum measurements enters the upper bound via the concept of quantum conditional probabilities. The resulting hidden-variable inequality is applicable to an arbitrary observable that is decomposable into a weighted sum of noncommuting projectors. We present local and nonlocal examples of violation of generalized Bell inequalities in phase space, which sense the negativity of the Wigner function.