We provide an alternative simple proof of the necessity of entanglement in quantum teleportation by using the no-disentanglement theorem. We show that this is true even when the state to be teleported is known to be among two noncommuting qubits. We further show that to teleport any set of commuting qubits, it is sufficient to have a classically correlated channel. Using this result we provide a simple proof of the fact that any set of bipartite entangled states can be exactly disentangled if the single-particle density matrices of any one party commute.

A study of the influence of spin-orbit coupling and dynamical electron correlation on the polarizabilities of the halogen atoms in a variational treatment is presented. It is shown that for the heavier homologs it is necessary to replace the averaged values by individual component polarizabilities for the $J=\frac{3}{2}$ and $J=\frac{1}{2}$ atomic states due to the large spin-orbit coupling. A high level of correlation treatment by large-scale direct configuration interaction with up to quadruple excitations is maintained throughout. We, furthermore, put our relativistic two-component approach to the test by evaluating the change of picture effect on the polarizability values of the astatine atom arising from the neglect of a unitary transformation of the property operator to an approximate relativistic theory.

A high-resolution study of the K x-ray emission spectrum of the metal manganese is presented. The spectrum of the ${3d}^6{4s}^1$ configuration is simulated with the help of a multiconfiguration Dirac-Fock program. Good agreement is obtained between the experimental and simulated shapes of the well resolved $K{\alpha }_{1,2}$ doublet as well as of the strongly asymmetrical $K\beta$ emission. The experimental intensity of $K{\beta }_5$ is larger than the predicted value because a mixing of the s, p, and d valence states exists in the solid. Satellite emissions, due to the transitions in doubly ionized atoms created by shake-off processes, are observed toward the higher energies of the normal emissions, at the predicted energies. In contrast, no radiative Auger satellite is observed toward the lower energies. We conclude that this process cannot be described in the shake approximation as previously assumed. The measured and theoretical $I\left(K\beta \right)/I\left(K\alpha \right)$ intensity ratios agree within 3%. Both are almost independent of the satellite emissions, while they vary appreciably with the external Mn configuration.

The influence of the excitation and the alignment of a laser-prepared target on the final $n\mathcal{l}{m}_{\mathcal{l}}$ distributions are studied in ${\mathrm{Ar}}^{8+}-\mathrm{Cs}\left(6p\Sigma \right)$ and ${\mathrm{Ar}}^{8+}-\mathrm{Cs}\left(6p\Pi \right)$ collisions. These collisions are studied experimentally by means of near UV and visible photon spectroscopy (200–600 nm) in the 0.4–4.0 keV/amu energy range, and theoretically by using the three-body classical trajectory Monte Carlo (CTMC) method at 0.1–4.0 keV/amu. Calculations of electronic energies for the one-electron $\{{\mathrm{Ar}}^{7+}-\mathrm{Cs}{\}}^{+}$ molecular system have allowed an analysis of the results in terms of dynamical couplings. Polarization rates for single-electron-capture lines corresponding to transitions between high-l values states are also measured. The comparison with those calculated from the CTMC results provides information about the magnetic sublevel distributions and the influence of the initial alignment of a laser-prepared target.

The analytic properties of Nilsson’s modified oscillator, which was first introduced in nuclear structure, and of the recently introduced, based on quantum algebraic techniques, three-dimensional q-deformed harmonic oscillator (3D $q-\mathrm{HO})$ with ${\mathrm{u}}_q\mathrm{}\left(3\right)\mathrm{}\supset {\mathrm{so}}_q\mathrm{}\left(3\right)\mathrm{}$ symmetry, which is known to reproduce correctly in terms of only one parameter the magic numbers of alkali clusters up to 1500 (the expected limit of validity for theories based on the filling of electronic shells), are considered. Exact expressions for the total energy of closed shells are determined and compared among them. Furthermore, the systematics of the appearance of supershells in the spectra of the two oscillators is considered, showing that the 3D $q-\mathrm{HO}$ correctly predicts the first supershell closure in alkali clusters without use of any extra parameter.

An interferometric method is proposed to controllably split an atomic condensate in two spatial components with strongly reduced population fluctuations. All steps in our proposal are currently in use in cold-atom laboratories, and we show with a theoretical calculation that our proposal is very robust against imperfections of the interferometer.

Multiply quantized vortices in trapped Bose-Einstein condensates are studied using the Bogoliubov theory. Suitable combinations of a localized pinning potential and an external rotation of the system are found to energetically stabilize, both locally and globally, vortices with multiple circulation quanta. We present a phase diagram for stable multiply quantized vortices in terms of the angular rotation frequency of the system and the width of the pinning potential. We argue that multiquantum vortices could be experimentally created using a suitable choice of these two parameters.

An experiment is proposed to visualize stroboscopically in real time the dynamics of a photon oscillating between two cavities. The visualization is implemented by a sequence of weak measurements, which are carried out by probing one of the cavities with a Rydberg atom and detecting a resulting phase shift by Ramsey interferometry. This way to measure the number of photons in a cavity was experimentally realized by Brune et al. [Phys. Rev. Lett. 65, 976, 1990]. We suggest a feedback mechanism which minimizes the disturbance due to the measurement and enables a detection of the original evolution of the radiation field.

An experimental search for the electron electric-dipole moment using the $a\left(1\right)\mathrm{}{[}^3{\Sigma }^{+}]$ state of PbO is currently underway and promises to yield high precision. Several experimental parameters are required as input to theoretical determinations of the sensitivity of the proposed experiment. We use molecular-beam spectroscopy to determine the hyperfine constant, molecular dipole moment, and Lande g factor of the $a\left(1\right)\mathrm{}$ state, and the electronic isotope shifts of the $X-a$ transition.

We study the robustness of quantum computers under the influence of errors modeled by strictly contractive channels. A channel T is defined to be strictly contractive if, for any pair of density operators ρ, σ in its domain, $\Vert T\rho -T\sigma {\Vert }_1<~k\Vert \rho -\sigma {\Vert }_1$ for some $0<~k<\mathrm{}1\mathrm{}$ (here $\Vert \cdot {\Vert }_1$ denotes the trace norm). In other words, strictly contractive channels render the states of the computer less distinguishable in the sense of quantum detection theory. Starting from the premise that all experimental procedures can be carried out with finite precision, we argue that there exists a physically meaningful connection between strictly contractive channels and errors in physically realizable quantum computers. We show that, in the absence of error correction, sensitivity of quantum memories and computers to strictly contractive errors grows exponentially with storage time and computation time, respectively, and depends only on the constant k and the measurement precision. We prove that strict contractivity rules out the possibility of perfect error correction, and give an argument that approximate error correction, which covers previous work on fault-tolerant quantum computation as a special case, is possible.

By analogy to classical cryptography, we develop a quantum cryptographic scheme in which the two public and private keys consist in each of two entangled beams of squeezed light. An analog secret information is encrypted by modulating the phase of the beam sent in public. The knowledge of the degree of nonclassical correlation between the beam quadratures measured in private and in public allows only the receiver to decrypt the secret information. Finally, in a view towards absolute security, we formally prove that any external intervention of an eavesdropper makes him vulnerable to any subsequent detection.

The ${(2s}_{1/2}{-2p}_{1/2})$ Lamb shift in lithiumlike krypton was obtained through a recombination measurement. The Lamb shift was extracted from the recombination spectrum by comparison with a calculation of the dielectronic-recombination resonance positions. The energy calibration used in this work is based on an accurate energy determination of one arbitrary point in the spectrum. Our value for the ${2s}_{1/2}{-2p}_{1/2}$ energy difference is 71.243(8) eV. The relativistic many-body contributions to the Lamb shift can be calculated very precisely, which enabled an extraction of an “experimental” quantum-electrodynamical contribution of 1.539(8) eV. This precision is beyond what can be achieved with conventional estimates of the screened quantum-electrodynamical effects. The main error originates from the uncertainty in the length of the ion-beam orbit. Improved knowledge of this value can improve our result significantly.

We have measured the pressure broadening and shift of the cesium $D1\mathrm{}$ (894 nm) and $D2\mathrm{}$ (852 nm) lines by nitrogen and helium buffer gases at pressures of 0–160 Torr using high-resolution spectroscopy with tunable external-cavity diode lasers. This measurement has a precision of about 1%, which significantly improves upon previously published measurements for the Cs $D1\mathrm{}$ and $D2\mathrm{}$ lines in ${\mathrm{N}}_2$ and He buffer gases.

The dynamics of helium double ionization by 2 keV electron impact has been investigated experimentally and theoretically at large momentum transfer of $|\mathbf{q}|=2\mathrm{}\mathrm{a}.\mathrm{u}.\mathrm{}$ Fully resolved fivefold differential cross sections (FDCS’s) are presented for symmetric and asymmetric energy sharing between the two ejected electrons at excess energies from 10 to 40 eV, and for the coplanar as well as the out-of-plane scattering geometries. Experimentally, a multielectron–recoil-ion coincidence technique has been applied and a large part of the final-state momentum space has been mapped. The presently employed theoretical model treats the interaction between the two slow ejected electrons nonperturbatively using the convergent close-coupling method, whereas the projectile-target interaction is described in the first Born approximation. The experimental and theoretical FDCS’s agree well in shape. The cross section is dominated by two pairs of strong peaks. From this pattern it can be concluded that the two-step 1 mechanism, which is due to interelectron interaction after a single ionizing collision, is the dominant ionization process for the present kinematics.

Collisions between argon atoms and hydrogen molecules are investigated at very low temperatures. Quantum-mechanical calculations are performed using two different potential-energy surfaces. Rate coefficients for all possible combinations of initial vibrational and rotational level have been computed in the limit of zero temperature. Resonant and quasiresonant behavior is found using both potential-energy surfaces. The results are interpreted and estimates are made of the reliability of the calculations.

The double photoionization of He is investigated using the hyperspherical $\mathcal{R}$ matrix with semiclassical outgoing waves method. Triply, doubly, and singly differential, as well as fully integrated, cross sections are computed in a variety of geometrical and dynamical situations. The results are found to be in excellent agreement with absolute measurements both in shape and, more importantly, in magnitude. This demonstrates the robustness and accuracy of this ab initio method, which also provides a visualization of the formation of the various cross sections during the expansion of the system. This visualization reveals that, for very asymmetric energy sharings, the cross sections take their final form when the electrons are thousands of atomic units away from the ionic core, a distance where no other method is able to describe the double continuum wave function accurately.

In this work, we investigate experimentally the excitable properties of a solid-state laser with an intracavity saturable absorber. The main test of excitability is the characterization of the system response to short-pulse external stimuli. We present an original method to excite the system by injecting light pulses into the saturable absorber. This technique avoids imposing frequency tuning between the input laser and the passive laser, which is a necessary condition for field injection. We show that under certain conditions the laser behaves as an excitable system. The observed excitable regime is consistent with theoretical predictions for class-$B$ lasers with saturable absorber.

We describe the resonance fluorescence spectrum of an atomic three-level system where two of the states are coupled by a single monochromatic laser field. The influence of the third energy level, which interacts with the two laser-coupled states only via radiative decays, is studied in detail. For a suitable choice of parameters, this system gives rise to a very narrow structure at the laser frequency in the fluorescence spectrum which is not present in the spectrum of a two-level atom. We find those parameter ranges by a numerical analysis and use the results to derive analytical expressions for the additional narrow peak. We also derive an exact expression for the peak intensity under the assumption that a random telegraph model is applicable to the system. This model and a simple spring model are then used to describe the physical origins of the additional peak. Using these results, we explain the connection between our system, a three-level system in the V configuration where both transitions are laser driven, and a related experiment that was recently reported.

We prove that all purifications of a nonfactorable state (i.e., the state that cannot be expressed in a form ${\rho }_{\mathrm{AB}}={\rho }_A\otimes {\rho }_B)$ are entangled. We also show that for any bipartite state there exists a pair of measurements that are correlated on this state if and only if the state is nonfactorable.

We report the experimental observation of a strongly selective population of the spin-orbit components in the ${\mathrm{HCl}}^{+}$ $\tilde{X}{}^2{\Pi }_{\Omega }$ states after excitation into the dissociative ${2p}^{-1}6{\sigma }^{*}$ core excited state. A progression of highly excited vibrational states with either the $\Omega =3/2\mathrm{}$ or $\Omega =1/2\mathrm{}$ component of the final state is populated, respectively, when the excitation is tuned in the ${2p}_{3/2}^{-1}6{\sigma }^{*}$ or ${2p}_{1/2}^{-1}6{\sigma }^{*}$ part of the resonance. This effect is explained theoretically to be due to the orientational selectivity of the excitation process and the preference of the $L_{2,3}\mathrm{VV}$ Auger process to produce valence holes with the same orientation as the core holes.