The angular distribution and the photon-photon angular correlation have been investigated for the x-ray emission from two-step radiative cascades that proceed via overlapping intermediate resonances. In particular, density matrix theory is applied in order to explore how the splitting of these intermediate levels affects the subsequent x-ray emission and whether measurements of photon angular distributions may help reveal level crossings in highly charged ions, if analyzed along isoelectronic sequences. Detailed computations within the multiconfiguration Dirac-Fock method were performed especially for the two-step $1s2p^2{J}_i=1/2,3/2\to 1s2s2pJ=1/2,3/2+{\gamma }_1\to 1s^{2}2s{J}_f=1/2+{\gamma }_1+{\gamma }_2$ cascade of lithiumlike ions, for which a level crossing of the two $1s2s2pJ=1/2,3/2$ intermediate resonances occurs in the range $74\le Z\le 79$. For this cascade, a remarkably strong depolarization effect, associated with the finite lifetime of these intermediate levels, is found for the angular distribution and the photon-photon correlation function for all level splittings $\Delta \omega \lesssim 0.2\text{a.u.}\approx 5.4$ eV. We therefore suggest that accurate angle-resolved measurements of the x-ray emission may serve also as a tool for determining small level splittings in excited highly charged ions.

Time delay of photoemission from valence $ns$, $n{p}_{3/2}$, and $n{p}_{1/2}$ subshells of noble-gas atoms is theoretically scrutinized within the framework of the dipole relativistic random phase approximation. The focus is on the variation of time delay in the vicinity of the Cooper minima in photoionization of the outer subshells of neon, argon, krypton, and xenon, where the corresponding dipole matrix element changes its sign while passing through a node. It is revealed that the presence of the Cooper minimum in one photoionization channel has a strong effect on time delay in other channels. This is shown to be due to interchannel coupling.

The semiclassical description of the dynamics of wave packets in periodic potentials and subject to an applied force relies on the concepts of effective mass and anomalous transport. This picture is valid if the force changes slowly in time and space, so that the particle described by the wave packet has time to respond according to the properties of the lattice. We analyze the dynamical corrections to this picture when a uniform force is suddenly applied, identifying separate corrections to the usual group and anomalous velocities. We find approximate semianalytical expressions for generalized “dynamical” group and anomalous velocities and the associated accelerations. We use a two-dimensional optical lattice with finite Berry curvature to illustrate the semianalytical approximation in a regime where the dynamical corrections are significant, suggesting the possibility of experiments to detect them; we compare the results with a full numerical solution, showing excellent agreement for weak forces.

We consider the metastable dynamics of a flattened dipolar condensate. We develop an analytic model that quantifies the energy barrier to the system undergoing local collapse to form a density spike. We also develop a stochastic Gross-Pitaevskii equation theory for a flattened dipolar condensate, which we use to perform finite-temperature simulations verifying the local collapse scenario. We predict that local collapses play a significant role in the regime where rotons are predicted to exist and will be an important consideration for experiments looking to detect these excitations.

We present measurements on pulsed four-wave mixing involving a Rydberg state in an atomic vapor cell. The excitation to the Rydberg state is conducted by a pulsed two-photon excitation on the nanosecond time scale that is combined with a third cw laser in phase-matched geometry yielding light emission on the same time scale. An additional signal peak is observed shortly after the pulse that can be attributed to a revival of constructive interference between different velocity classes of the radiating atomic dipoles. Furthermore we investigate the density dependence of the four-wave mixing signal. From the shape of the respective curve we are able to confirm energy and momentum conservation in the photonic part of the system.

Self-testing is a device-independent method which can be used to determine the nature of a physical system or device, without knowing any detail of the inner mechanism or the physical dimensions of the Hilbert space of the system. The only information required comprises the number of measurements, the number of outputs of each measurement, and the statistics of each measurement. Earlier works on self-testing were restricted either to a two-party scenario or multipartite graph states. Here we construct a method to self-test the three-qubit $W$ state and show how to extend it to other pure three-qubit states. Our bounds are robust against the inevitable experimental errors.

In the usual tomography of multipartite entangled quantum states one assumes that the measurement devices used in the laboratory are under perfect control of the experimenter. In this paper, using the so-called swap concept introduced recently, we show how one can remove this assumption in realistic experimental conditions and nevertheless be able to characterize the produced multipartite state based only on observed statistics. Such a black-box tomography of quantum states is termed self-testing. As a function of the magnitude of the Bell violation, we are able to self-test emblematic multipartite quantum states such as the three-qubit $W$ state, the three- and four-qubit Greenberger-Horne-Zeilinger states, and the four-qubit linear cluster state.

We report a nonlocal interferometer capable of detecting entanglement and identifying Bell states statistically. This is possible due to the interferometer's unique correlation dependence on the antidiagonal elements of the density matrix, which have distinct bounds for separable states and unique values for the four Bell states. The interferometer consists of two spatially separated balanced Mach-Zehnder or Sagnac interferometers that share a polarization-entangled source. Correlations between these interferometers exhibit nonlocal interference, while single-photon interference is suppressed. This interferometer also allows for a unique version of the Clauser-Horne-Shimony-Holt–Bell test where the local reality is the photon polarization. We present the relevant theory and experimental results.

We present a state-of-the-art reanalysis of experimental results on Efimov resonances in the three-fermion system of ${}^6\mathrm{Li}$. We discuss different definitions of the three-body parameter (3BP) for Efimov states and adopt a definition that excludes effects due to deviations from universal scaling for low-lying states. We develop a finite-temperature model for the case of three distinguishable fermions and apply it to the excited-state Efimov resonance to obtain the most accurate determination to date of the 3BP in an atomic three-body system. Our analysis of ground-state Efimov resonances in the same system yields values for the three-body parameter that are consistent with the excited-state result. Recent work has suggested that the reduced 3BP for atomic systems is a near-universal quantity, almost independent of the particular atom involved. However, the value of the 3BP obtained for ${}^6\mathrm{Li}$ is significantly $(\sim 20\%)$ different from that previously obtained from the excited-state resonance in Cs. The difference between these values poses a challenge for theory.

We introduce a general mapping for encoding quantum communication protocols involving pure states of multiple qubits, unitary transformations, and projective measurements into another set of protocols that employ a coherent state of light in a linear combination of optical modes, linear-optics transformations, and measurements with single-photon threshold detectors. This provides a general framework for transforming protocols in quantum communication into a form in which they can be implemented with current technology. We explore the similarity between properties of the original qubit protocols and the coherent-state protocols obtained from the mapping and make use of the mapping to construct additional protocols in the context of quantum communication complexity and quantum digital signatures. Our results have the potential of bringing a wide class of quantum communication protocols closer to their experimental demonstration.

We derive the ground-state energy for a small number of ultracold atoms in an isotropic harmonic trap using effective quantum field theory (EFT). Atoms are assumed to interact through pairwise energy-independent and energy-dependent delta-function potentials with strengths proportional to the scattering length $a$ and effective range volume $V$, respectively. The calculations are performed systematically up to the order of $l^{-4}$, where $l$ denotes the harmonic-oscillator length. The effective three-body interaction contains a logarithmic divergence in the cutoff energy, giving rise to a nonuniversal three-body interaction in the EFT. Our EFT results are confirmed by nonperturbative numerical calculations for a Hamiltonian with finite-range two-body Gaussian interactions. For this model Hamiltonian, we explicitly calculate the nonuniversal effective three-body contribution to the energy.

The electron loss to the continuum has been studied for the collision system ${\mathrm{U}}^{88+}+{\mathrm{N}}_2\to {\mathrm{U}}^{89+}+{\left[{\mathrm{N}}_2\right]}^{*}+e^{-}$ at the low-relativistic projectile energy of 90 MeV/u. Using a magnetic electron spectrometer, the energy distribution of cusp electrons emitted at an angle of $0^{\circ }$ with respect to the projectile beam was measured in coincidence with the up-charged projectile. At the experimental collision energy ionization of the berylliumlike ${\mathrm{U}}^{88+}$ projectile proceeds predominantly from the $L$ shell, but a contribution from the $K$ shell could also be identified experimentally. The measurement is shown to be in accordance with fully relativistic Dirac calculations applying first-order perturbation theory. Furthermore, the underlying continuum electron distribution in the projectile frame is illustrated.

Superconducting atom chips and Rydberg atoms are promising tools for quantum information processing operations based on the dipole blockade effect. Nevertheless, one has to face the severe problem of stray electric fields in the vicinity of the chip. We demonstrate a simple method circumventing this problem. Microwave spectroscopy reveals extremely long coherence lifetimes (in the millisecond range) for a qubit stored in a Rydberg level superposition close to the chip surface. This is an essential step for the development of quantum simulations with Rydberg atoms and of a hybrid quantum information architecture based on atomic ensembles and superconducting circuits.

A theoretical model of the influence of detection-bandwidth properties on observed line shapes in laser absorption spectroscopy is described. The model predicts artificial frequency shifts, extra broadenings, and line asymmetries that must be taken into account in order to obtain accurate central frequencies and other spectroscopic parameters. This reveals sources of systematic effects most probably underestimated, so far potentially affecting spectroscopic measurements. This may impact many fields of research, from atmospheric and interstellar physics to precision spectroscopic measurements devoted to metrological applications, tests of quantum electrodynamics, or other fundamental laws of nature. Our theoretical model is validated by linear absorption experiments performed on ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{NH}}_3$ molecular lines recorded by precision laser spectroscopy in two distinct spectral regions, near- and midinfrared. Possible means of recovering original line-shape parameters or experimental conditions under which the detection bandwidth has a negligible impact, given a targeted accuracy, are proposed. Particular emphasis is put on the detection-bandwidth adjustments required to use such high-quality molecular spectra for a spectroscopic determination of the Boltzmann constant at the 1 ppm level of accuracy.

We propose a scheme for the detection of quantum phase transitions in the one-dimensional (1D) Bose-Hubbard (BH) and 1D Extended Bose-Hubbard (EBH) models, using the nondemolition measurement technique of quantum polarization spectroscopy. We use collective measurements of the effective total angular momentum of a particular spatial mode to characterize the Mott insulator to superfluid phase transition in the BH model and the transition to a density wave state in the EBH model. We extend the application of collective measurements to the ground states at various deformations of a superlattice potential.

We present an alternative theoretical model for a recent experiment [A. Fleischer et al., Nature Photon. 8, 543 (2014)] which used bichromatic, counter-rotating high-intensity laser pulses to probe the conservation of spin angular momentum in high-order-harmonic generation. We separate elliptical polarizations into independent circular fields with definite angular momentum, instead of using the expectation value of spin for each photon in the conservation equation, and we find good agreement with the experimental results. In our description the generation of each individual harmonic conserves spin angular momentum, in contrast to the model proposed by Fleischer et al. Our model also correctly describes analogous processes in standard perturbative optics.

We propose a stroboscopic method to dynamically decouple the effects of two-body atom-atom interactions for ultracold bosonic atoms, and realize a system dominated by elastic three-body interactions. Using this method, we show that it is possible to achieve the optimal scaling behavior predicted for interaction-based quantum metrology with three-body interactions. Specifically, we show that for ultracold bosons quenched in an optical lattice, we can measure the three-body interaction strength with a precision proportional to ${\overline{n}}^{-5/2}$ using homodyne quadrature interferometry, and ${\overline{n}}^{-7/4}$ using conventional collapse-and-revival techniques, where $\overline{n}$ is the mean number of atoms per lattice site. Both precision scalings surpass the nonlinear scaling of ${\overline{n}}^{-3/2}$, which was previously proposed and achieved with a physical system. Our method of achieving a decoupled three-body interacting system may also have applications in the creation of exotic three-body states and phases.

Quantum properties of correlations have a key role in disparate fields of physics including quantum information processing, quantum foundations, and strongly correlated systems. We tackle a specific aspect of the fundamental quantum marginal problem: We address the issue of deducing the global properties of correlations of tripartite quantum states based on the knowledge of their bipartite reductions, focusing on relating specific properties of bipartite correlations to global correlation properties. We prove that strictly classical bipartite correlations may still require global entanglement and that unentangled (albeit not strictly classical) reductions may require global genuine multipartite entanglement rather than simple entanglement. On the other hand, for three qubits, the strict classicality of the bipartite reductions rules out the need for genuine multipartite entanglement. Our work sheds light on the relation between local and global properties of quantum states and on the interplay between classical and quantum properties of correlations.

The observation of features, often referred to as “Wigner cusps” in a scattering cross section at the opening of a new scattering channel has been widely reported in electron scattering, and has been attributed to strong coupling between the two scattering channels. Recently a series of cusps were observed in positron scattering from the noble gases [A. C. L. Jones et al., Phys. Rev. Lett. 105, 073201 (2010)]. In this case, the strong opening of the positronium formation cross section resulted in a cusplike feature, or bump, in the total elastic scattering cross section. The positronium formation channel is typically the largest partial scattering cross section in the region of the electronic excitation and ionization thresholds in most atoms and molecules, and it is plausible that similar features may exist in other atomic and molecular species. We have undertaken measurements of the isoelectronic targets of helium—${\mathrm{H}}_2$, and neon—${\mathrm{H}}_2\mathrm{O}$, ${\mathrm{NH}}_3$, and ${\mathrm{CH}}_4$, in an effort to better understand the mechanisms leading to these threshold features.

We analyze some crucial questions regarding the practical feasibility of quantum simulation for lattice gauge models. Our analysis focuses on two models suitable for the quantum simulation of the Schwinger Hamiltonian, or QED in $1+1$ dimensions, which we investigate numerically using tensor networks. In particular, we explore the effect of representing the gauge degrees of freedom with finite-dimensional systems and show that the results converge rapidly; thus even with small dimensions it is possible to obtain a reasonable accuracy. We also discuss the time scales required for the adiabatic preparation of the interacting vacuum state and observe that for a suitable ramping of the interaction the required time is almost insensitive to the system size and the dimension of the physical systems. Finally, we address the possible presence of noninvariant terms in the Hamiltonian that is realized in the experiment and show that for low levels of noise it is still possible to achieve a good precision for some ground-state observables, even if the gauge symmetry is not exact in the implemented model.