Quantum memories have been realized in different physical systems, such as atomic ensembles or solid systems. To date, all quantum memories have realized the storage and retrieval of photons encoded using only a two-dimensional space spanned, for example, by the orthogonal polarization states, and hence can only store a quantum bit. Using electromagnetically induced transparency in a cold atomic ensemble, we report the experimental realization of a quantum memory storing a photon encoded in a three-dimensional space spanned by orbital angular momentum (OAM) states. We experimentally reconstruct the storage process density matrix with a fidelity of $85.3\%\pm 1.8\%$ using a 4-f imaging system. We also realize storage of two special photonic qutrit states as examples. Toward storing a higher-dimensional state encoded in OAM space, the efficiency difference between different OAM states should be considered according to the experimental results. The capability to store high-dimensional quantum states with high fidelity is a key step towards building high-dimensional quantum networks.

We present experimental and theoretical results for electron-impact-induced ionization and excitation of helium atoms. Using a high-sensitivity and high-resolution $(e,2e)$ spectrometer, we measured the differential cross sections of ionization-excitation of He to the $n=1-5$ states of ${\mathrm{He}}^{+}$ at an impact energy of 600 eV. The experimental results are compared with 4-body distorted wave (4DW) calculations and first Born approximation (FBA) calculations. It was found that 4DW can well describe the shapes of the cross sections of the ionization with a simultaneous excitation. The cross-section ratios of excited states $n=2-5$ to the ground state $n=1$ were underestimated by FBA and 4DW calculations, indicating the need for further improvement of the theoretical methods.

Jaynes-Cummings-Hubbard lattices provide unique properties for the study of correlated phases as they exhibit convenient state preparation and measurement, as well as in situ tuning of parameters. We show how to realize charge density and supersolid phases in Jaynes-Cummings-Hubbard lattices in the presence of long-range interactions. The long-range interactions are realized by the consideration of Rydberg states in coupled atom-cavity systems and the introduction of additional capacitive couplings in quantum-electrodynamics circuits. We demonstrate the emergence of supersolid and checkerboard solid phases, for calculations which take into account nearest-neighbor couplings, through a mean-field decoupling.

The nonreciprocal propagation of light at the single-photon level is essential for building a quantum network. Bulk optical schemes are lossy and difficulty to integrate onto a chip. We propose a single-photon optical diode and a three-port circulator without a magnetic field by coupling an unbalanced quantum impurity to a passive, linear optical waveguide or a whispering-gallery-mode microresonator which supports a locally or globally circularly polarized photon. Thanks to the unbalanced quantum Jaynes-Cummings coupling, the optical nonreciprocal propagation of single photons can be achieved without an external magnetic field. In particular, the three-port single-photon circulator can be accomplished using the existing experimental technology. The optical isolation can be reversed by selectively populating the initial state of the quantum impurity. Moreover, by using an ensemble of identical atoms filled in a hollow-core microbottle resonator, nonreciprocal propagation of weak light pulses can be achieved.

Two aspects of the classic two-level Landau-Zener (LZ) problem are considered. First, we address the LZ problem when one or both levels decay, i.e., ${\varepsilon }_j\left(t\right)\to {\varepsilon }_j\left(t\right)-i{\Gamma }_j/2$. We find that if the system evolves from an initial time $-T$ to a final time $+T$ such that $|{\varepsilon }_1(\pm T)-{\varepsilon }_2(\pm T)|$ is not too large, the LZ survival probability of a state $|j\rangle$ can increase with increasing decay rate of the other state $|i\ne j\rangle$. This surprising result occurs because the decay results in crossing of the two eigenvalues of the instantaneous non-Hermitian Hamiltonian. On the other hand, if $|{\varepsilon }_1(\pm T)-{\varepsilon }_2(\pm T)|\to \infty$ as $T\to \infty$, the probability is independent of the decay rate. These results are based on analytic solutions of the time-dependent Schrödinger equations for two cases: (a) the energy levels depend linearly on time, and (b) the energy levels are bounded and of the form ${\varepsilon }_{1,2}\left(t\right)=\pm \varepsilon tanh(t/\mathcal{T})$. Second, we study LZ transitions affected by dephasing by formulating the Landau-Zener problem with noise in terms of a Schrödinger-Langevin stochastic coupled set of differential equations. The LZ survival probability then becomes a random variable whose probability distribution is shown to behave very differently for long and short dephasing times. We also discuss the combined effects of decay and dephasing on the LZ probability.

We determine the role of entanglement for nonlocal memory effects induced by initial correlations in a composite environment. The dephasing dynamics of two two-level systems which are coupled locally to an environment consisting of nonlocally correlated field modes is studied with respect to the occurrence of reviving nonlocal coherences. Employing two-mode Gaussian states correlating pairs of modes of the bosonic baths we observe strong nonlocal memory effects in this locally Markovian dynamics for entangled as well as separable quantum states for the environment. We present an explanation for this phenomenon linking it to the correlation coefficient of the environmental coupling operators and derive a necessary and sufficient condition for the occurrence of nonlocal memory effects for sufficiently short interaction times. Finally, we comment on how the magnitude of the nonlocal rephasing may also be used to estimate parameters of the two-mode Gaussian states.

Using the high-resolution spectroscopic method of collinear laser–ion-beam spectroscopy (CLIBS), the Zeeman effect of singly ionized praseodymium spectral lines has been studied at relatively small magnetic fields up to 330 G. With this unusual method for studying the Zeeman effect of ionic lines we recorded Zeeman-hyperfine structure patterns with clearly resolved components with linewidths as low as 60 MHz, which is only sometimes the natural linewidth. From the Zeeman patterns of 30 lines, improved Landé $g_J$ factors were determined for 39 Pr ii levels of the $4f^{3}5d$, $4f^{2}5d^2$, and $4f^{3}6p$ configurations.

We report a high-precision calculation for the hydrogen molecular ion ${\text{H}}_2^{+}$ by using a variational method in Hylleraas coordinates. The calculated nonrelativistic ground-state energy is accurate to 2 parts in ${10}^{34}$, which represents a more than 3 orders of magnitude improvement over the best previous value of Hijikata et al. [J. Chem. Phys. 130, 024102 (2009)]. For the first excited states of $S$ and $P$ symmetries, the achieved precision is about 2 parts in ${10}^{31}$ and 3 parts in ${10}^{29}$, respectively. Comparison with other calculations are made.

Electron elastic scattering off a spin-polarized Cr($...3d^{5}4s^1$, ${}^{7}S$) atom is theoretically studied in the region of electron energies up to 15 eV using both a one-electron “spin-polarized” Hartree-Fock approximation and a multielectron spin-polarized random-phase approximation with exchange. It is found that scattering phase shifts of oppositely spin-polarized incoming electrons and corresponding cross sections of the scattering reactions significantly differ from each other, in general, even without accounting for spin-orbit interaction. This is shown to be associated with the presence of two semifilled $3d^5$ and $4s^1$ subshells in the Cr configuration which induce considerably different exchange in the interaction of oppositely spin-polarized incoming electrons with the atom target. The importance of electron correlation in the $e^{-}+\mathrm{Cr}$ elastic-scattering process is revealed. Moreover, correlation is shown to induce strong differences between scattering of oppositely spin-polarized electrons off Cr. A physically transparent interpretation for the latter is provided.

Quantum correlations between two neighboring atoms are studied. It is assumed that one atomic system comprises a single auto-ionizing level and the other atom does not contain any auto-ionizing level. The excitation of both atoms is achieved by the interaction with the same mode of the quantized field. It is shown that the long-time behavior of two atoms exhibits quantum correlations even when the atoms do not interact directly. This can be shown using the optical excitation of the neighboring atom. Also a measure of entanglement of two atoms can be applied after reduction of the continuum to two levels.

We consider a two-component pancake-shaped, i.e., effectively two-dimensional (2D), Bose-Einstein condensate coupled by the spin-orbit (SO) and Rabi terms. The SO coupling adopted here is of the mixed Rashba-Dresselhaus type. For this configuration, we derive a system of two 2D nonpolynomial Schrödinger equations (NPSEs), for both attractive and repulsive interatomic interactions. In the low- and high-density limits, the system amounts to previously known models, namely, the usual 2D Gross-Pitaevskii equation, or the Schrödinger equation with the nonlinearity of power $7/3$. We present simple approximate localized solutions, obtained by treating the SO and Rabi terms as perturbations. Localized solutions of the full NPSE system are obtained in a numerical form. Remarkably, in the case of the attractive nonlinearity acting in free space (i.e., without any 2D trapping potential), we find parameter regions where the SO and Rabi couplings make 2D fundamental solitons dynamically stable.

We discuss the possibility of realizing a non-Hermitian, i.e., an open two-well system of ultracold atoms by enclosing it within additional time-dependent wells that serve as particle reservoirs. With the appropriate design of the additional wells, $\mathcal{PT}$-symmetric currents can be induced to and from the inner wells, which support stable solutions. We show that interaction in the mean-field limit does not destroy this property. As a first method we use a simplified variational ansatz leading to a discrete nonlinear Schrödinger equation. A more accurate and more general variational ansatz is then used to confirm the results.

We exploit geometric properties of quantum states of light in optical cavities to carry out quantum nondemolition measurements. We generalize the “mode-invisibility” method to obtain information about the Wigner function of a squeezed coherent state in a nondestructive way. We also simplify the application of this nondemolition technique to measure single-photon and few-photon states.

Enhanced ionization (EI) of molecules has been extensively studied over the past two decades as a common process in molecular dissociative ionization in strong laser fields. Direct evidence for EI has been found only in ${\mathrm{I}}_2$ and ${\mathrm{H}}_2$. However, in this work we perform a direct study of EI in CO and ${\mathrm{N}}_2$, and find enhanced ionization in an alternate dissociation channel in each of these two molecules following double ionization. Surprisingly, EI does not happen in the commonly seen dissociation channels that were previously assigned undergoing EI. Instead, EI occurs only in the alternate channels seen here with a lower kinetic-energy release.

We study the two-dimensional weakly repulsive Bose gas at zero temperature in the presence of correlated disorder. Using large-scale simulations, we show that the low-energy Bogoliubov cumulative density of states remains quadratic up to a critical disorder strength, beyond which a power law with disorder-dependent exponent $\beta <2$ sets in. We associate this threshold behavior with the transition from superfluid to Bose glass, and compare the resulting mean-field phase diagram with scaling laws and the Thomas-Fermi percolation threshold of the mean-field density profile.

The generalized Hartmann effect (GHE) predicts a strict inequality between the traversal times across a contiguous and a separated double-barrier system. This is compared to the implications of the time-of-arrival (TOA) operator approach to barrier traversal time [E. A. Galapon, Phys. Rev. Lett. 108, 170402 (2012)]. It is shown that, for initial wave packets with compact supports in the far incident side of the barrier system, the expectation value of the traversal time is independent of the separation between the barriers. On the other hand, for wave packets with supports extending inside the first barrier, the contribution of the barrier separation to the traversal time exponentially increases with the barrier height. Our result shows that if the support of the incident wave packet is far from the barrier region, the GHE inequality is violated. However, if the support of the wave packet extends inside the barrier region, the GHE inequality is consistent with the TOA operator approach, but only when the particle's incident energy is very small.

Various widely used approaches for the exchange-correlation energy of Kohn-Sham theory, such as the generalized gradient approximation and hybrid methods, suffer from the one-electron self-interaction error. This error manifests itself in one-electron or two-electron spin-unpolarized systems. We propose new approximations to the exchange energy that are exact for the mentioned problems. Starting from the weighted density approximation (WDA), we introduce several weighting factors (that we refer to as exchange factors) which are designed to reproduce various properties of the exact exchange hole. Furthermore, a new exact condition for the exchange factor is presented that concerns the asymptotic behavior of the exchange hole as one electron coordinate moves out to infinity. Observance of this and other conditions results in exchange functionals that improve upon the WDA.

The stability of antiproton-hydrogen compound states $\left(\overline{p}\mathrm{H}\right)$ is investigated theoretically. The $\overline{p}\mathrm{H}$ compound state has decay channels of atomic electron detachment $(\to e^{-}+\overline{p}p)$ and hadronic pair $\left(\overline{p}\text{−}p\right)$ annihilation, and hence it cannot be permanently stable even if the dissociation channel $(\to \overline{p}+\mathrm{H})$ is energetically closed. In this paper, information on the metastable $\overline{p}\mathrm{H}$ states at energies below the dissociation limit is obtained by analyzing low-energy resonances in $e^{-}+\overline{p}p$ scattering. The scattering calculation is carried out by using an R-matrix method, which also allows for the annihilation channel simultaneously. The metastable states are found to be identified as vibrational levels supported by a Born-Oppenheimer (BO) potential. The energy-level widths attributed to the electron detachment and the annihilation are also calculated using a model based on the BO picture.

We present detailed analysis of the recently proposed technique of high-order-harmonic generation spectroscopy of correlation-driven electron hole dynamics in atoms and molecules. This novel technique resolves Auger-type processes with attosecond-scale resolution by clocking the decay process with high-harmonic generation. The harmonic generation is driven by an attosecond, XUV pump pulse and a long-duration, infrared pulse. We present the strong-field-approximation-based theory of such an XUV-initiated high-order-harmonic generation process. We detail different ways of recovering the hole survival probability by altering experimental parameters to change the time-energy mapping of the harmonics. The various reconstruction methods are then simulated for $M_{4,5}\mathit{NN}$ Auger decay in krypton and molecular-orbital breakdown dynamics in trans-butadiene and propanal.