We studied simultaneous electron ejection from both collision partners in $200\text{−}\mathrm{keV}$ ${\mathrm{H}}^{-}+\mathrm{He}$ collisions in a kinematically complete experiment by measuring the fully momentum-analyzed recoil ions and both active electrons in coincidence. The data were analyzed in terms of Dalitz spectra, in which the momentum exchange between three particles is plotted simultaneously in a single spectrum. We found that the energy transfer occurs predominantly between the active electrons, but most of the momentum is exchanged in elastic scattering between the cores of the collision partners.

Finite quantal systems at high angular momenta may exhibit vortex formation and localization. These phenomena occur independent of the statistics of the repulsively interacting particles, which may be of bosonic or fermionic nature. We analyze the relation between vortex localization and formation of stable Wigner molecules at high angular momenta in the view of particle-hole duality. Trial wave functions for the vortex states and the corresponding fermion-boson relations are discussed.

We study the macroscopic quantum tunneling of magnetization of the $F=1$ spinor condensate interacting through dipole-dipole interaction with an external magnetic field applied along the longitudinal or transverse direction. We show that the ground state energy and the effective magnetic moment of the system exhibit an interesting macroscopic quantum oscillation phenomenon originating from the oscillating dependence of thermodynamic properties of the system on the vacuum angle. Tunneling between two degenerate minima are analyzed by means of an effective potential method and the periodic instanton method.

We present a theoretical formulation of the competition between electromagnetically induced transparency (EIT) and Raman processes. The latter become important when the medium can no longer be considered to be dilute. Unlike the standard formulation of EIT, we consider all fields applied and generated as interacting with both the transitions of the $\Lambda$ scheme. We solve the Maxwell equations for the net generated field using a fast-Fourier-transform technique and obtain predictions for the probe, control, and Raman fields. We show how the intensity of the probe field is depleted at higher atomic number densities due to the buildup of multiple Raman fields. Furthermore, we find that the generated fields and the input fields acquire oscillatory behavior as a function of the density of the medium due to dynamical coupling of the various Raman processes.

We consider spontaneous emission in dielectric microcavities with multiple resonances that may overlap in both position and frequency. By projecting the Green tensor onto a set of quasimodes associated with the resonances, we derive the reduced spontaneous emission rate. To demonstrate the accuracy and computational efficiency of our approach, we model spontaneous emission in a two-dimensional photonic crystal with two coupled defects. Employing a quantum mechanical interpretation, we show that the spontaneous emission involves both direct decay into the individual quasimodes and photon transfer between the quasimodes. The quantum path interference between these different emission channels can cause the spontaneous emission rate to differ dramatically from the rate obtained by the summation over resonant line shapes. We also show how our approach can be used to determine the emission into particular quasimodes of multiple-resonance microcavities.

We analyze the energy transformation of light propagating with slow group velocity in a three-level medium. Depending on the excitation scheme, the energy of the probe can be transformed into the coupling field or into the atomic excitation. As a result a zero-energy adiabaton develops for the lambda scheme or an energy carrying adiabaton is formed for the ladder scheme. In both cases slow light is produced by the atoms, also feeding this light by the energy stored in the metastable state (ladder scheme) or borrowing the energy from the coupling field (lambda scheme).

Universal behavior of Gaussian-Klauder states emerges near soft classical turning points, as expressed through a complex-valued Airy transformation that approximates the wave function. Study of these classical turning points provides analytic evidence that Gaussian-Klauder states generally display recurrent localization for many classical orbital periods. Analytic position and momentum moments of the wave function are determined from this approximation, leading in part to connections with the traditionally chosen positional Gaussian wave functions as the limit of large energy uncertainty. Application of this procedure to hydrogenic states of maximal eccentricity leads to the classical limit of recurrent collisional bouncing in the Kepler problem, via the explicit construction of states that maintain phase space localization for many orbital periods.

We point out a simplifying but mildly inconsistent assumption of the Perdew-Burke-Ernzerhof (PBE) correlation functional, which should be corrected when evaluating the high-density limit of the PBE energy for nonsinglet systems under uniform density scaling. The discussion also concerns high-density limits of all density functionals that use PBE as an ingredient, including the Tao-Perdew-Staroverov-Scuseria (TPSS) approximation. We revisit the nonrelativistic correlation energies of isoelectronic atomic ions in the limit of infinite nuclear charge and explain small discrepancies between the PBE and TPSS values of these limits existing in the literature.

In addition to photon pairs entangled in polarization or other variables, quantum mechanics also allows optical beams that are entangled through the absence of the photons themselves. These correlated absences, or “entangled photon holes,” can lead to counterintuitive nonlocal effects analogous to those of the more familiar entangled photon pairs. Here we report an experimental observation of photon holes generated using quantum interference effects to suppress the probability that two photons in a weak laser pulse will separate at an optical beam splitter.

We demonstrate an information hiding and retrieval scheme with the relative phases between states in a Rydberg wave packet acting as the bits of a data register. We use a terahertz half-cycle pulse (HCP) to transfer phase-encoded information from an optically accessible angular momentum manifold to another manifold which is not directly accessed by our laser pulses, effectively hiding the information from our optical interferometric measurement techniques. A subsequent HCP acting on these wave packets reintroduces the information back into the optically accessible data register manifold which can then be read out.

We present measurements of the different quantum path contributions to the high-order harmonic emission. Through spatial and spectral filtering, we evidence the strong correlation between the spatial and spectral distributions, which allows us to quantify the contribution of each quantum path. A systematic analysis as a function of the generating parameters has been done to identify the conditions for efficient generation and selection of a single quantum path. We show that combining phase matching and spatial filtering allows maximizing and selecting the short quantum path contribution, condition for the generation of “clean” and intense attosecond pulses.

The statistical properties of the Salerno model are investigated. In particular, a comparison between the coherent and partially coherent wave modes is made for the case of a random-phased wave packet. It is found that the random-phased-induced spectral broadening gives rise to the damping of instabilities, but also a broadening of the instability region in quasiparticle momentum space. The results can be of significance for the condensation of magnetic-moment bosons in deep optical lattices.

There are two complementary approaches to realizing quantum information so that it is protected from a given set of error operators. Both involve encoding information by means of subsystems. One is initialization-based error protection, which involves a quantum operation that is applied before error events occur. The other is operator quantum error correction, which uses a recovery operation applied after the errors have occurred. Together, the two approaches make it clear how quantum information can be stored at all stages of a process involving alternating error and quantum operations. In particular, there is always a subsystem that faithfully represents the desired quantum information. We give a definition of faithful realization of quantum information and show that it always involves subsystems. This supports the “subsystems principle” for realizing quantum information. In the presence of errors, one can make use of noiseless, (initialization) protectable, or error-correcting subsystems. We give an explicit algorithm for finding optimal noiseless subsystems. Finding optimal protectable or error-correcting subsystems is in general difficult. Verifying that a subsystem is error correcting is known to involve only linear algebra. We discuss the verification problem for protectable subsystems and reduce it to a simpler version of the problem of finding error-detecting codes.

The problem of the reliable transfer of entanglement from one pure bipartite quantum state to another using local operations is analyzed. It is shown that in the case of qubits the amount that can be transferred is restricted to the difference between the entanglement of the two states. In the presence of a catalytic state the range of the transferable amount broadens to a certain degree.

We study the relationship between the entanglement, mixedness, and energy of two-qubit and two-mode Gaussian quantum states. We parametrize the set of allowed states of these two fundamentally different physical systems using measures of entanglement, mixedness, and energy that allow us to compare and contrast the two systems using a phase diagram. This phase diagram enables one to clearly identify not only the physically allowed states, but the set of states connected under an arbitrary quantum operation. We pay particular attention to the maximally entangled mixed states of each system. Following this we investigate how efficiently one may transfer entanglement from two-mode to two-qubit states.

We introduce the concept of a quantum walk with two particles and study it for the case of a discrete time walk on a line. A quantum walk with more than one particle may contain entanglement, thus offering a resource unavailable in the classical scenario and which can present interesting modifications on quantum walks with single particles. In this work, we show both numerically and analytically how the entanglement and the relative phase between the states describing the coin degree of freedom of each particle will influence the evolution of the quantum walk. In particular, the probability to find at least one particle in a certain position after $N$ steps of the walk, as well as the average distance and the squared distance between the two particles, can be larger or smaller than the case of two unentangled particles, depending on the initial conditions we choose. This resource can then be tuned according to our needs to modify the features of a quantum walk. Experimental implementations are briefly discussed.

The quantum $N$-electron structure problem is expressed as a classical $N$-body structure problem plus a quantum many-body problem with a reduced number of variables. This forms the foundation of a method for solving the electronic structure problem using $k$-electron distribution functions instead of many-electron wave functions. The method represents a unified approach to the entire hierarchy of generalized density-functional theories, beginning with standard density-functional theory $(k=1)$ and pair density-functional theory $(k=2)$ and culminating in an exact treatment of the entire $N$-electron system $(k=N)$.

Differential cross sections for elastic electron scattering in neon have been measured in the angular range of backward scattering from 110°–180° at incident energies of 7, 10, and $15\mathrm{eV}$. These measurements combined the use of a magnetic angle changer and an electrostatic electron spectrometer. The differential cross sections measured in the above scattering angle range, together with results obtained previously in the range below 110°, have been integrated to obtain integral elastic and momentum transfer cross sections. Detailed comparison is presented of the measured differential and integral cross sections with the results of various theoretical calculations.

In this work we calculate doubly differential cross sections (DDCS) for single ionization of helium by highly charged ion impact. We study the importance of two-particle interactions in these processes by considering the cross sections as a function of all two-particle subsystems momenta. Experimental DDCSs were obtained recently from kinematically complete experiments on single ionization of He by $100\mathrm{MeV}∕\mathrm{amu}$ ${\mathrm{C}}^{6+}$ and $3.6\mathrm{MeV}∕\mathrm{amu}$ ${\mathrm{Au}}^{24,53+}$ impact. Furthermore, we evaluated the importance of three-particle interactions by plotting the squared momenta of all three collision fragments simultaneously in a Dalitz plot. Using the first Born and distorted-wave approximations for fully differential cross sections, together with Monte Carlo integration techniques, we were able to reproduce the main features observed in experimental data and to assess the quality of the models implied by the different employed approximations.