We measured differential cross sections for exciting the three lowest electronically excited states in furan, as functions both of electron energy and of scattering angle. Emphasis of the present work is on recording detailed excitation functions, revealing resonances in the excitation process. The cross section for the first triplet state has a shoulder in the first 2 eV above threshold, assigned to the high-energy tails of the shape resonances, followed by two peaks at 6.5 and 8.0 eV, assigned to two ${}^2(\pi ,{\pi }^{*2})$ core-excited shape resonances with substantial configuration mixing. The excitation mechanism thus closely follows that of the prototype case of ethene, except that as the number of double bonds doubles, the numbers of both shape and core-excited resonances also double. The cross section for the singlet states around 6 eV (primarily the $S_2{}^1{B}_2$ state) rises linearly with energy and attains very high values in the forward direction as expected for a dipole-allowed transition, but has a vertical onset at higher scattering angles, possibly due to a threshold core-excited ${}^2(\pi ,{\pi }^{*}3\overline{s})$ resonance responsible also for dissociative attachment. Elastic cross sections are also presented. Both elastic and inelastic absolute values compare favorably with existing measurements where available, except very near threshold. The results are compared to existing calculations.

Advanced pulsed cryogenic molecular-beam electric deflection methods involving position-sensitive mass spectrometry and 7.87-eV ionizing radiation were used to measure the polarizabilities of more than half of the metallic elements in the Periodic Table. Concurrent Stern-Gerlach deflection measurements verified the ground-state condition of the measured atoms. Comparison with state-of-the-art calculations exposes significant systematic and isolated discrepancies throughout the Periodic Table.

Ab initio QED calculations of the ground-state binding energies of berylliumlike ions are performed for the wide range of the nuclear charge number: $Z=18–96$. The calculations are carried out in the framework of the extended Furry picture starting with three different types of the screening potential. The rigorous QED calculations up to the second order of the perturbation theory are combined with the third- and higher-order electron-correlation contributions obtained within the Breit approximation by the use of the large-scale configuration-interaction Dirac-Fock-Sturm method. The effects of nuclear recoil and nuclear polarization are taken into account. The ionization potentials are obtained by subtracting the binding energies of the corresponding lithiumlike ions. In comparison with the previous calculations the accuracy of the binding energies and the ionization potentials is significantly improved.

We provide an elementary description of the dynamics of defect centers in crystals in terms of a quantum optical master equation which includes spontaneous decay and a simplified vibronic interaction with lattice phonons. We present the general solution of the dynamical equation by means of the eigensystem of the Liouville operator and exemplify the usage of this damping basis to calculate the dynamics of the electronic and vibrational degrees of freedom and to provide an analysis of the spectra of scattered light. The dynamics and spectral features are discussed with respect to the applicability for color centers, especially for negatively charged nitrogen-vacancy centers in diamond.

We show theoretically that it is possible to create and manipulate a pair of bound states in the continuum in ultracold atoms by two lasers in the presence of a magnetically tunable Feshbach resonance. These bound states are formed due to coherent superposition of two electronically excited molecular bound states and a quasibound state in the ground-state potential. These superposition states are decoupled from the continuum of two-atom collisional states. Hence, in the absence of other damping processes they are nondecaying. We analyze in detail the physical conditions that can lead to the formation of such states in cold collisions between atoms and discuss the possible experimental signatures of such states. An extremely narrow and asymmetric shape with a distinct minimum of the photoassociative absorption spectrum or the scattering cross section as a function of collision energy will indicate the occurrence of a bound state in the continuum (BIC). We prove that the minimum will occur at the energy at which the BIC is formed. We discuss how a BIC will be useful for efficient the creation of Feshbach molecules and manipulation of cold collisions. Experimental realizations of BIC will pave the way for a new kind of bound-bound spectroscopy in ultracold atoms.

We use fidelity susceptibility to calculate quantum critical scaling exponents for a system consisting of $N$ identical bosons interacting with a single impurity atom in a double-well potential (bosonic Josephson junction). Above a critical value of the boson-impurity interaction energy there is a spontaneous breaking of ${\mathbb{Z}}_2$ symmetry corresponding to a second-order quantum phase transition from a balanced to an imbalanced number of particles in either the left- or the right-hand well. We show that the exponents match those in the Lipkin-Meshkov-Glick and Dicke models, suggesting that the impurity model is in the same universality class. The phase transition can be interpreted as a measurement of the position of the impurity by the bosons.

A framework is established for evaluating cphase gates that use single-photon cross-phase modulation (XPM) originating from the Kerr nonlinearity. Prior work [J. H. Shapiro, Phys. Rev. A 73, 062305 (2006)], which assumed that the control and target pulses propagated at the same group velocity, showed that the causality-induced phase noise required by a noninstantaneous XPM response function precluded the possibility of high-fidelity $\pi$-radian conditional phase shifts. The framework presented herein incorporates the more realistic case of group-velocity disparity between the control and target pulses, as employed in existing XPM-based fiber-optical switches. Nevertheless, the causality-induced phase noise identified by Shapiro [J. H. Shapiro, Phys. Rev. A 73, 062305 (2006)] still rules out high-fidelity $\pi$-radian conditional phase shifts. This is shown to be so for both a reasonable theoretical model for the XPM response function and for the experimentally measured XPM response function of silica-core fiber.

We consider a single-impurity atom confined to an optical lattice and immersed in a homogeneous Bose-Einstein condensate (BEC). Interaction of the impurity with the phonon modes of the BEC leads to the formation of a stable quasiparticle, the polaron. We use a variational mean-field approach to study dispersion renormalization and derive equations describing nonequilibrium dynamics of polarons by projecting equations of motion into mean-field-type wave functions. As a concrete example, we apply our method to study dynamics of impurity atoms in response to a suddenly applied force and explore the interplay of coherent Bloch oscillations and incoherent drift. We obtain a nonlinear dependence of the drift velocity on the applied force, including a sub-Ohmic dependence for small forces for dimensionality $d>1$ of the BEC. For the case of heavy impurity atoms, we derive a closed analytical expression for the drift velocity. Our results show considerable differences with the commonly used phenomenological Esaki-Tsu model.

Many predictions of Doppler-cooling theory of two-level atoms have never been verified in a three-dimensional geometry, including the celebrated minimum achievable temperature $\hslash \Gamma /2k_B$, where $\Gamma$ is the transition linewidth. Here we show that, despite their degenerate level structure, we can use helium-4 atoms to achieve a situation in which these predictions can be verified. We make measurements of atomic temperatures, magneto-optical trap sizes, and the sensitivity of optical molasses to a power imbalance in the laser beams, finding excellent agreement with Doppler theory. We show that the special properties of helium, particularly its small mass and narrow transition linewidth, prevent effective sub-Doppler cooling with red-detuned optical molasses. This discussion can be generalized to identify when a given species is likely to be subject to the same limitation.

We study a distribution of thermal states given by random Hamiltonians with a local structure. We show that the ensemble of thermal states monotonically approaches the unitarily invariant ensemble with decreasing temperature if all particles interact according to a single random interaction and achieves a state $t$-design at temperature $O(1/\log \left(t\right))$. For the system where the random interactions are local, we show that the ensemble achieves a state 1-design. We then provide numerical evidence indicating that the ensemble undergoes a phase transition at finite temperature.

We simulate the time-dependent Schrödinger equation and observe the rescattering dissociation of ${\mathrm{H}}_2$ in strong infrared laser fields. Two dissociation pathways are identified, i.e., the dissociation of ${\mathrm{H}}_2{}^{+}$ in the $2p{\sigma }_u$ state and the dissociation of ${\mathrm{H}}_2$ in doubly excited states. The former accounts for larger proportions as the rescattering energy is larger. The kinetic energy release of dissociative fragments reflects the temporal internuclear distance at the moment the rescattering happens.

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.