Photoelectron angular distributions (PADs) from two-photon near-threshold ionization of Ne atoms by the combined action of femtosecond pulses from an extreme ultraviolet (EUV) free-electron laser and infrared (IR) laser have been studied experimentally and theoretically. Solutions of the time-dependent Schrödinger equation indicate that the PADs strongly depend on the time delay between EUV and IR pulses. The experimental results obtained for two extreme cases of completely overlapping and nonoverlapping pulses fully confirm the prediction, illustrating that the measurements of the time-delay dependence of the PAD provide a tool for investigating the fundamental problem of the relative importance of the resonant and nonresonant pathways in the two-color two-photon processes.

The total photodetachment cross section of a negative ion near a repulsive center is studied based on the closed-orbit theory. An analytical expression, written as a product of the zero-field photodetachment cross section and a modulation function, is obtained for energy above and below threshold. The expression also incorporates the different wave sources produced by different negative ions. The present results are shown to be accurate by comparing with quantum calculations based on the exact Coulomb Green's function. We also compared the photodetachment cross section near a repulsive center with that in a homogeneous electric field and found interesting connections between the two.

The results of three-dimensional numerical simulations of strong-field atomic stabilization with arbitrarily polarized light beyond the electric dipole approximation are presented. The study of the long-term evolution of the ground-state hydrogen atom exposed to an intense high-frequency laser field reveals the persistence of the metastable bound states up to relativistic intensities. The population of these states in a pulsed field is examined as a function of the pulse peak intensity. These calculations resolve recent contradictions in the literature related to the role of magnetic field in the high-frequency stabilization phenomenon.

The dipole blockade of multiple Rydberg excitations in mesoscopic atomic ensembles allows the implementation of various quantum information tasks using collective states of cold, trapped atoms. Precise coherent manipulations of the collective ground and single Rydberg excitation states of an atomic ensemble requires the knowledge of the number of atoms with small uncertainty. We present an efficient method to acquire such information by interrogating the atomic ensemble with resonant pulses while monitoring the resulting Rydberg excitations. We show that after several such steps accompanied by feedback the number of atoms in an ensemble can be assessed with high accuracy. This will facilitate the realization of high-fidelity quantum gates, long-term storage of quantum information, and deterministic sources of single photons with Rydberg-blockaded atomic ensembles.

We study a system of hard-core boson on a one-dimensional lattice with frustrated next-nearest-neighbor hopping and nearest-neighbor interaction. At half filling, for equal magnitude of nearest- and next-nearest-neighbor hopping, the ground state of this system exhibits a first-order phase transition from a bond-ordered solid to a charge-density-wave solid as a function of the nearest-neighbor interaction. Moving away from half filling we investigate the system at incommensurate densities, where we find a supersolid phase which has concurrent off-diagonal long-range order and density-wave order which is unusual in a system of hard-core bosons in one dimension. Using the finite-size density-matrix renormalization group method, we obtain the complete phase diagram for this model.

In ultracold Fermi gases, the effect of spin imbalance on superfluidity has been the subject of intense study. One of the reasons for this is that spin imbalance frustrates the Bardeen-Cooper-Schrieffer (BCS) superfluid pairing mechanism, in which fermions in different spin states combine into Cooper pairs with zero momentum. In 1964, it was proposed that an exotic superfluid state called the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, in which the Cooper pairs have nonzero momentum, could exist in a spin-imbalanced Fermi gas. At the saddle-point (mean-field) level, it has been shown that the FFLO state only occupies a very small sliver in the ground-state phase diagram of a three-dimensional (3D) Fermi gas. However, a question that remains to be investigated is as follows: What is the influence of phase fluctuations on the FFLO state? In this work, we show that phase fluctuations only lead to relatively small quantitative corrections to the presence of the FFLO state in the saddle-point phase diagram of a 3D spin-imbalanced Fermi gas. Starting from the partition function of the system, we calculate the effective action within the path-integral adiabatic approximation. The action is then expanded up to second order in the fluctuation field around the saddle point, leading to the fluctuation free energy. Using this free energy, we calculate corrections due to phase fluctuations to the BCS-FFLO transition in the saddle-point phase diagram. At temperatures at which the FFLO state exists, we find only small corrections to the size of the FFLO area. Our results suggest that fluctuations of the phase of the FFLO order parameter, which can be interpreted as an oscillation of its momentum vector, do not cause an instability of the FFLO state with respect to the BCS state.

We show that the third Goldstone mode, which emerges in binary condensates at phase separation, persists to higher interspecies interaction for density profiles where one component is surrounded on both sides by the other component. This is not the case with symmetry-broken density profiles where one species is entirely to the left and the other is entirely to the right. We, then, use Hartree-Fock-Bogoliubov theory with Popov approximation to examine the mode evolution at $T\ne 0$ and demonstrate the existence of mode bifurcation near the critical temperature. The Kohn mode, however, exhibits deviation from the natural frequency at finite temperatures after the phase separation. This is due to the exclusion of the noncondensate atoms in the dynamics.

We consider the integration of quantum emitters into a negative permeability metamaterial design in order to introduce tunability as well as nonlinear behavior. The unit cell of our metamaterial is a ring of metamolecules, each consisting of a metal nanoparticle and a two-level semiconductor quantum dot (QD). Without the QDs, the ring of the unit cell is known to act as an artificial optical magnetic resonator. By adding the QDs we show that a Fano interference profile is introduced into the magnetic field scattered from the ring. This induced interference is shown to cause an appreciable effect in the collective magnetic resonance of the unit cell. We find that the interference provides a means to tune the response of the negative permeability metamaterial. The exploitation of the QD's inherent nonlinearity is proposed to modulate the metamaterial's magnetic response with a separate control field.

X-ray free-electron lasers have had an enormous impact on x-ray science by achieving femtosecond pulses with unprecedented intensities. However, present-day facilities operating by the self-amplified spontaneous emission principle have a number of shortcomings; namely, their radiation has a chaotic pulse profile and short coherence times. We put forward a scheme for a neon-based atomic inner-shell x-ray laser (XRL) which produces temporally and spatially coherent subfemtosecond pulses that are controlled by and synchronized to an optical laser with femtosecond precision. We envision that such an XRL will allow for numerous applications such as nuclear quantum optics and the study of ultrafast quantum dynamics of atoms, molecules, and condensed matter.

Spectral singularities are ubiquitous with $\mathcal{PT}$ symmetry leading to infinite transmission and reflection coefficients. Since physical systems cannot have singularities, these must be the artifact of some missing physics. The singularities imply the divergence of the fields in the medium, thereby breaking the very assumption of the linearity of the medium used to obtain such singularities. We show how the incorporation of the correct all-order nonlinear behavior of the medium regularizes the spectral singularity. The all-order nonlinear $\mathcal{PT}$-symmetric device is shown to exhibit very effective isolation or optical diode action since transmission through such a system is nonreciprocal. In contrast, a linear system or a system with Kerr nonlinearity is known to have only reciprocal transmission.

The analysis of plane waves normally incident upon moving media yields a velocity-dependent net flow of electromagnetic energy within the system such that optical pressures are deduced. Two common formulations (Chu and Minkowski) are employed to model the phenomena with respect to the Abraham-Minkowski debate. Within the framework of relativistic electrodynamics, two perfect reflectors submerged in a dielectric are used to compare the calculated optical work and pressure exerted to the system for both formulations via momentum and energy conservation. The perfect reflectors are represented by having zero electric field and zero magnetic field at the surface of the perfect electrical conductor and the perfect magnetic conductor, respectively. Comparison of these results gives valuable insight into the differences between Abraham and Minkowski momenta and provides theoretical evidence as to observable effects in actual optical moment transfer to media.

We study the spatiotemporal effects in waveguide arrays with Kerr and Raman nonlinearities when a short pulse is launched into the system where both dispersive and diffractive resonant radiations can be simultaneously emitted by solitons. We show that it is possible to generate and control the supercontinuum not only in the frequency domain, but also in the wavenumber domain. This work could potentially pave the way for designing unique optical devices that generate spectrally broad supercontinua with a controllable directionality by taking advantage of the combined physics of optical fibers and waveguide arrays.

We analyze the nature and performance of clocks formed by stabilizing an oscillator to the phase difference between two paths of an atom interferometer. The phase evolution has been modeled as being driven by the proper-time difference between the two paths, although it has an ambiguous origin in the nonrelativistic limit and it requires a full quantum-field-theory treatment in the general case. We present conditions for identifying deviations from the nonrelativistic limit as a way of testing the proper-time-driven phase evolution model. We show that the system performance belies the premise that an atom-interferometer clock is referenced to a divided-down Compton oscillation, and we suggest that this implies there is no physical oscillation at the Compton frequency.

By using first-principles arguments, based on the time-energy uncertainty principle in the form given by Mandelstam and Tamm, we show that the Keldysh time represents a lower limit for the tunnel time. We use the definition of the tunnel time as identified in a recent numerical investigation [C. R. McDonald et al., Phys. Rev. Lett. 111, 090405 (2013)]; it is the time it takes for the unperturbed initial ground-state wave function to evolve into the field-perturbed final eigenstate which is the quasistatic field dressed resonance.

Two qubits in pure entangled states going through separate paths and interacting with their own individual environments will gradually lose their entanglement. Here we show that the entanglement change of a two-qubit state due to amplitude damping noises can be recovered by entanglement swapping. Some initial states can be asymptotically purified into maximally entangled states by iteratively using our protocol.

We develop a numerical method to calculate the Bethe logarithm for resonant states. We use the complex coordinate rotation (CCR) formalism to describe resonances as time-independent Schrödinger solutions. To get a proper expression for the Bethe logarithm we apply the generalization of the second order perturbation theory to an isolated CCR eigenstate. Using the developed method we perform a systematic calculation of the Bethe logarithm for metastable states in the antiprotonic helium He${}^{+}\overline{p}$ atoms with a precision of 7–8 significant digits. We also recalculate the nonrelativistic energies with improved precision using CODATA10 recommended values of masses. Along with a complete set of corrections of $m{\alpha }^7$ order and the leading contributions of $m{\alpha }^8$ order, that has allowed us to get theoretical values for ro-vibrational transition frequencies for the He${}^{+}\overline{p}$ atoms with an uncertainty of 0.1–0.3 MHz.

We propose an experimental scheme to realize the valley-dependent gauge fields for ultracold fermionic atoms trapped in a state-dependent square optical lattice. Our scheme relies on two sets of Raman laser beams to engineer the hopping between adjacent sites populated by two-component fermionic atoms. One set of Raman beams is used to realize a staggered $\pi$-flux lattice, where low-energy atoms near two inequivalent Dirac points should be described by the Dirac equation for spin-$\frac{1}{2}$ particles. Another set of laser beams with proper Rabi frequencies is added to further modulate the atomic hopping parameters. The hopping modulation will give rise to effective gauge potentials with opposite signs near the two valleys, mimicking the interesting strain-induced pseudogauge fields in graphene. The proposed valley-dependent gauge fields are tunable and provide an alternative route to realize an uncommon type of quantum Hall effects and atomic devices.

An exact calculation of the local electric potential field $\psi \left(\mathbf{r}\right)$ in the quasistatic limit is described for the case of a point electric charge $q$ in a two-constituent composite medium. In the case of an ${\epsilon }_2$, ${\epsilon }_1$, ${\epsilon }_2$ three-parallel-slab microstructure, where $q$ is in the top ${\epsilon }_2$ layer and both ${\epsilon }_2$ layers are infinitely thick while the ${\epsilon }_1$ layer has a finite thickness $L_1$, a perfect imaging of the point charge is expected if ${\epsilon }_1=-{\epsilon }_2$ is real [J. B. Pendry, Phys. Rev. Lett. 85, 3966 (2000); R. J. Blaikie and D. O. S. Melville, J. Opt. A 7, S176 (2005); , New J. Phys. 11, 093040 (2009)]. Among our results we find that an infinite resolution image of the point charge $q$ is only achievable if the actual charge is situated at a distance that is between $L_1/2$ and $L_1$ away from the ${\epsilon }_1$ layer.

Imagine three parties, Alice, Bob, and Charlie, who share a state of three qubits such that all two-party reduced states $A$-$B$, $A$-$C$, and $B$-$C$ are separable. Suppose that they have information only about these marginals but not about the global state. According to recent results, there exists an example for a set of three separable two-party reduced states that is only compatible with an entangled global state. In this paper, we show a stronger result by exhibiting separable two-party reduced states $A$-$B$, $A$-$C$, and $B$-$C$, such that any global state compatible with these marginals is nonlocal. Hence, we obtain that nonlocality of multipartite states can be certified from information only about separable marginals.

We propose a criterion which defines whether a superposition of two photonic components is macroscopic. It is based on the ability to discriminate these components with a particular class of “classical” detectors, namely, a photon number measurement with a resolution coarse-grained by noise. We show how our criterion can be extended to a measure of the size of macroscopic superpositions by quantifying the amount of noise that can be tolerated and taking the distinctness of two Fock states differing by N photons as a reference. After applying our measure to several well-known examples, we demonstrate that the superpositions which meet our criterion are very sensitive to phase fluctuations. This suggests that quantifying the macroscopicity of a superposition state through the distinguishability of its components with “classical” detectors not only is a natural measure but also explains why it is difficult to observe superpositions at the macroscopic scale.