In an experiment combining a state-of-the-art visible laser system with the vacuum ultraviolet beamline I411 at the storage ring MAX II, shakedown satellite lines in the $2p$ photoelectron emission of excited Na atoms have been observed. A close investigation of these lines shows a strong dependency of the line intensities from the linear polarization angle of the laser light. The shakedown electrons are preferably emitted into the direction of the laser electric field vector.

We present a theoretical interpretation of radio frequency (RF) pairing gap experiments in trapped atomic Fermi gases, over the entire range of the BCS–Bose-Einstein condensation (BEC) crossover, for temperatures above and below $T_c$. Our calculated RF excitation spectra, as well as the density profiles on which they are based, are in semiquantitative agreement with experiment. We provide a detailed analysis of the physical origin of the two different peak features seen in RF spectra, one associated with nearly free atoms at the edge of the trap, and the other with (quasi-)bound fermion pairs.

A SWAP operation between different types of qubits of single photons is essential for manipulating hyperentangled photons for a variety of applications. We have implemented an efficient SWAP gate for the momentum and polarization degrees of freedom of a single photon. The SWAP gate was utilized in a single-photon two-qubit quantum logic circuit to deterministically transfer momentum entanglement between a pair of down-converted photons to polarization entanglement. The polarization entanglement thus obtained violates Bell’s inequality by more than 150 standard deviations.

The broadening and shifting of spectral lines induced by collisions with the five noble gases in both the intercombination system and the triplet manifold of Sr are studied using tunable dye laser absorption spectroscopy. Cross sections for impact broadening and line shifting are determined from an examination of the spectral line profiles. These results are utilized in an analysis to compute difference potentials modeled by the Lennard-Jones (6-12) potential and the coefficients and derived from this analysis are reported.

The dissociative recombination (DR) of ${}^3\mathrm{He}{}^4\mathrm{He}^{+}$ has been investigated at the heavy-ion Test Storage Ring (TSR) in Heidelberg by observing neutral products from electron-ion collisions in a merged beams configuration at relative energies from near-zero (thermal electron energy about 10 meV) up to 40 eV. After storage and electron cooling for 35 s, an effective DR rate coefficient at near-zero energy of $3\times {10}^{-9}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ is found. The temporal evolution of the neutral product rates and fragment imaging spectra reveals that the populations of vibrational levels in the stored ion beam are nonthermal with fractions of $\sim 0.1\text{–}1\%$ in excited levels up to at least $v=4$, having a significant effect on the observed DR signals. With a pump-probe-type technique using DR fragment imaging while switching the properties of the electron beam, the vibrational excitation of the ions is found to originate mostly from ion collisions with the residual gas. Also, the temporal evolution of the DR signals suggests that a strong electron induced rotational cooling occurs in the vibrational ground state, reaching a rotational temperature near or below 300 K. From the absolute rate coefficient and the shape of the fragment imaging spectrum observed under stationary conditions, the DR rate coefficient from the vibrational ground state is determined; converted to a thermal electron gas at 300 K it amounts to $(3.3\pm 0.9)\times {10}^{-10}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$. The corresponding branching ratios from $v=0$ to the atomic final states are found to be $(3.7\pm 1.2)\%$ for $1s2s{}^{3}S,(37.4\pm 4.0)\%$ for $1s2s{}^{1}S,(58.6\pm 5.2)\%$ for $1s2p{}^{3}P$, and $(2.9\pm 3.0)\%$ for $1s2p{}^{1}P$. A DR rate coefficient in the range of $2\times {10}^{-7}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ or above is inferred for vibrational levels $v=3$ and higher. As a function of the collision energy, the measured DR rate coefficient displays a structure around 0.2 eV. At higher energies, it has one smooth peak around 7.3 eV and a highly structured appearance at 15–40 eV. The small size of the observed effective DR rate coefficient at near-zero energy indicates that the electron induced rotational cooling is due to inelastic electron-ion collisions and not due to selective depletion of rotational levels by DR.

Energy loss per unit path in grazing collisions with metal surfaces is studied by using the collisional and dielectric formalisms. Within both theories we make use of the band-structure-based (BSB) model to represent the surface interaction. The BSB approach is based on a model potential and provides a precise description of the one-electron states and the surface-induced potential. The method is applied to evaluate the energy lost by 100 keV protons impinging on aluminum surfaces at glancing angles. We found that when the realistic BSB description of the surface is used, the energy loss obtained from the collisional formalism agrees with the dielectric one, which includes not only binary but also plasmon excitations. The distance-dependent stopping power derived from the BSB model is in good agreement with available experimental data. We have also investigated the influence of the surface band structure in collisions with the Al(100) surface. Surface-state contributions to the energy loss and electron emission probability are analyzed.

We analyze the behavior of the coarse-grained entropy for classical probabilities in nonlinear Hamiltonians. We focus on the result that if the trajectory dynamics are integrable, the probability ensemble shows transient increases in the coherence, corresponding to an increase in localization of the ensemble and hence the phase-space density of the ensemble. We discuss the connection of these dynamics to the problem of cooling in atomic ensembles. We show how these dynamics can be understood in terms of the behavior of individual trajectories, allowing us to manipulate ensembles to create “cold” dense final ensembles. We illustrate these results with an analysis of the behavior of particular nonlinear integrable systems, including discussions of the spin-echo effect and the seeming violation of Liouville's theorem.

We show the rather counterintuitive result that entangled input states can strictly enhance the distinguishability of two entanglement-breaking channels.

Current research on sources for extreme ultraviolet lithography (EUVL) has converged on the use of discharge or laser produced plasmas containing xenon, tin, or lithium with tin showing by far the most promise. Because of their density, radiation transport from these plasmas is a major issue and accurate photoabsorption cross sections are required for the development of the plasma models needed to optimize conditions for source operation. The relative EUV photoionization cross sections of Sn I through Sn IV have been measured and from a comparison with the results of many body calculations, the cross section has been estimated to be close to 11 Mb in each species at 13.5 nm (91.8 eV), the wavelength of choice for EUVL.

Many protocols for quantum computation require a memory element to store qubits. We discuss the speed and accuracy with which quantum states prepared in a superconducting qubit can be stored in and later retrieved from an attached high-$Q$ resonator. The memory fidelity depends on both the qubit-resonator coupling strength and the location of the state on the Bloch sphere. Our results show that a quantum memory demonstration should be possible with existing superconducting qubit designs, which would be an important milestone in solid-state quantum information processing. Although we specifically focus on a large-area, current-biased Josesphson-junction phase qubit coupled to the dilatational mode of a piezoelectric nanoelectromechanical disk resonator, many of our results will apply to other qubit-oscillator models.

Efficient teleportation is a crucial step for quantum computation and quantum networking. In the case of qubits, four different entangled Bell states have to be distinguished. We have realized a probabilistic, but in principle deterministic, Bell-state analyzer for two photonic quantum bits by the use of a nondestructive controlled-NOT gate based on entirely linear optical elements. This gate was capable of distinguishing between all of the Bell states with higher than 75% fidelity without any noise substraction due to utilizing quantum interference effects.

Along with attosecond pulses comes a growing need for broadband amplification in the extreme ultraviolet spectral regime. Ultrafast laser pulses interacting with high symmetry molecules produce electronic excitations that are approaching this energy regime. Although there is some theoretical work regarding such excitation, experimental progress had been hampered in a fundamental way. Ion time-of-flight spectroscopy measures dissociation pathways but provides no specific information regarding the final state of the fragments. Vacuum-ultraviolet spectroscopy provides complete information about excited fragments but says nothing of the dissociation channel. In this paper we measure the internal states of nitrogen ion fragments and we associate them with specific dissociation pathways.

We investigate the nonlinear interaction of tightly focused femtosecond infrared laser pulses in fused silica and sapphire. For fused silica, emission occurs in the form of a cone with a structured spectrum centered near the third harmonic of the fundamental laser frequency. The emission spectrum evolves with successive laser pulses, suggesting a Raman origin for the emission with laser-induced structural changes of the material. For sapphire, only the third-harmonic component is observed, also in the form of conical emission. The proposed explanation for these observations, three-photon-excited hyper-Raman emission, provides a tool for probing the vibrational and structural properties of solid-state materials.

The notion of entangling power of unitary matrices was introduced by Zanardi et al., [Phys. Rev. A 62, 030301 (2000)]. We study the entangling power of permutations, given in terms of a combinatorial formula. We show that the permutation matrices with zero entangling power are, up to local unitaries, the identity and the swap. We construct the permutations with the minimum nonzero entangling power for every dimension. With the use of orthogonal latin squares, we construct the permutations with the maximum entangling power for every dimension. Moreover, we show that the value obtained is maximum over all unitaries of the same dimension, with a possible exception for 36. Our result enables us to construct generic examples of 4-qudit maximally entangled states for all dimensions except for 2 and 6. We numerically classify, according to their entangling power, the permutation matrices of dimensions 4 and 9, and we give some estimates for higher dimensions.

We present a practical and general scheme of remote preparation for pure and mixed states, which is proposed with an auxiliary qubit and controlled-NOT gate. We discuss the remote state preparation (RSP) in two important types of decoherent channel (depolarizing and dephasing). We realize RSP in the dephasing channel in our experiment by using spontaneous parametric down-conversion, linear optical elements, and single photon detector. Our experimental results match the theoretical prediction well.

The time-evolution equation of a one-dimensional quantum walker is exactly mapped to the three-dimensional Weyl equation for a zero-mass particle with spin $1∕2$, in which each wave number $k$ of the walker’s wave function is mapped to a point $\mathbf{q}\left(k\right)$ in the three-dimensional momentum space and $\mathbf{q}\left(k\right)$ makes a planar orbit as $k$ changes its value in $[-\pi ,\pi )$. The integration over $k$ providing the real-space wave function for a quantum walker corresponds to considering an orbital state of a Weyl particle, which is defined as a superposition (curvilinear integration) of the energy-momentum eigenstates of a free Weyl equation along the orbit. Konno’s novel distribution function of a quantum walker’s pseudovelocities in the long-time limit is fully controlled by the shape of the orbit and how the orbit is embedded in the three-dimensional momentum space. The family of orbital states can be regarded as a geometrical representation of the unitary group U(2) and the present study will propose a new group-theoretical point of view for quantum-walk problems.

A necessary and sufficient condition for characterization and quantification of entanglement of any bipartite Gaussian state belonging to a special symmetry class is given in terms of classicality measures of one-party states. For Gaussian states whose local covariance matrices have equal determinants it is shown that separability of a two-party state and classicality of a one-party state are completely equivalent to each other under a nonlocal operation, allowing entanglement features to be understood in terms of any available classicality measure.

The quantum ghost imaging phenomena, experimentally demonstrated a decade ago, exploited the apparent spooky action at a distance of entangled photon pairs and offered a novel approach toward imaging. Can ghost imaging effects be produced by “classical” light sources, such as separable systems of photon pairs or thermal light? If so, can these sources achieve the same accuracy achieved by entangled states? In order to answer these questions, we formulate the different physics behind entangled and separable systems in terms of a set of inequalities derived from the historical argument of Einstein, Podolsky, and Rosen. We show that the ghost images produced by separable sources are subject to the standard statistical limitations. However, entangled states offer the possibility of overcoming such limitations. Imaging can, therefore, achieve its fundamental limit through the high spatial resolution and nonlocal behavior of entangled systems.

The self-stability of multiple four-wave mixing (FWM) processes in the optical fibers is studied both theoretically and experimentally. The coupled-mode equations for the complex amplitudes of four waves are derived. The nonlinear interaction between four optical fields involves a single nondegenerate and two degenerate FWM processes in the third-order nonlinear mixings. The proposed models are compared to the conventional coupled-mode theory of FWM processes and are proven by the experiments. The energy conservation of four waves is achieved, and the power flow relationship between two pump waves and two created waves is obtained. The analytic solutions reveal the self-stability mechanism of multiple FWM processes. The self-stability prediction is experimentally testified by using a kind of photonic-crystal fiber, and such excellence can be applied to achieve dual-wavelength erbium-doped fiber lasers with the excellent uniformity and stability.

We consider a physical system in which the description of states and measurements follow the usual quantum mechanical rules. We also assume that the dynamics is linear, but may not be fully quantum (i.e., unitary). We show that in such a physical system, certain complementary evolutions, namely, cloning and deleting operations that give a better fidelity than quantum mechanically allowed ones, in one (inaccessible) region, lead to signaling to a far-apart (accessible) region. To show such signaling, one requires certain two-party quantum correlated states shared between the two regions. Subsequent measurements are performed only in the accessible part to detect such phenomenon.