We investigate numerically the generation of water window x-rays under phase-matched conditions at very low ionization level. We show that at high-gas pressure and short-wavelength infrared driving pulse, phase matching condition is obtained only if one accounts for the effects ionization has on both driving and harmonic fields. Recent experimental results obtained in these conditions by Takahashi et al. [Phys. Rev. Lett. 101, 253901 (2008)] are also discussed.

We study the biorthogonality between decaying and growing eigenmodes in one-dimensional potential barrier problems. It is shown that Petermann factors $K_n$ of the eigenmodes, a measure of nonorthogonality, are involved in decaying mechanism of an initially confined particle. We also show that the decay tail of the growing modes at an exceptional point (EP), where $K_n$ become infinite, is not exponential, but $\sim t^2{e}^{-{\Gamma }_{EP}t}$, ${\Gamma }_{EP}$ the decay rate of the decaying mode at EP. In addition, the geometrical phase near an EP is illustrated by the evolution of wave function.

There exist multiple branch points in the energy spectrum for some $PT$-symmetric periodic potentials, where the real eigenvalues turn into complex ones. By studying the transmission amplitude for a localized complex potential, we elucidate the physical origin of the breakdown of the perturbation method and Born approximation. Most importantly, we derive an analytic criterion to determine why, when, and where the bifurcation will occur.

We present an alternative approach to the derivation of benchmarks for quantum channels, such as memory or teleportation channels. Using the concept of effective entanglement and the verification thereof, a testing procedure is derived which demands very few experimental resources. The procedure is generalized by allowing for mixed test states. By constructing optimized measure and reprepare channels, the benchmarks are found to be very tight in the considered experimental regimes.

We show how to apply Lyapunov control design to the problem of entanglement creation between two atoms in distant cavities connected by optical fibers. The Lyapunov control design is optimal in the sense that the distance from the target state decreases monotonically and exponentially, and the concurrence increases accordingly. This method is far more robust than simple geometric schemes.

We propose a general method to calculate the full Green’s function of multielectron atomic ions. The key point exists in the usage of $L^2$ integrable functions as a complete basis set in a finite region together with an optical potential to guaranty the outgoing scattering boundary condition. In such a way, the cumbersome procedure of adjusting boundary conditions in solving the differential Schrödinger equation is avoided. To show the validity of the method, we studied the radiative recombination involving dielectronic recombinations of Be-like Hg $\left(\text{Z}=80\right)$ ions. The radiative damping effect is taken into account naturally in the present method. The calculated results reproduce well the asymmetric line profile observed in the experiments.

We have measured the projectile scattering-angle dependency for various electronic final states for single-electron capture in $\text{p}+\text{He}$ and ${{}^3\text{H}\text{e}}^{+}+\text{He}$ collisions at incident energies between 60 and 630 keV/u. We find pronounced peak structures in the scattering-angle dependence of some of the ratios of different capture channels. We interpret this as experimental evidence that an excitation process of the target is caused by a transverse momentum transfer that leads to an additional projectile deflection.

We report experimental investigations of a single-ion optical frequency standard based on ${{}^{171}\text{Y}\text{b}}^{+}$. The ion is confined in a cylindrically symmetric radiofrequency Paul trap. The reference transition is the ${{}^{2}S}_{1/2}\left(F=0\right)–{{}^{2}D}_{3/2}\left(F^{\prime }=2\right)$ electric quadrupole transition at 688 THz. Using a differential measurement scheme, we determine the shift of the reference transition frequency that occurs due to the interaction of the electric quadrupole moment of the ${{}^{2}D}_{3/2}$ state with the gradient of the electrostatic stray field in the trap. We determine an upper limit for the instability of the quadrupole shift over times between 100 s to 20 h. We also observe the variations in the shift and in the applied stray-field compensation voltages that result from loading a new ion into the trap and during a subsequent storage period of 74 days. This information is utilized to measure the absolute frequency of the reference transition with an uncertainty that is a factor of 3 smaller than that of the previous measurement. Using a fiber laser based optical frequency comb generator and the cesium fountain clock CSF1 of PTB (Physikalisch-Technische Bundesanstalt), the frequency at 300 K temperature is determined as $688358979309306.62\pm 0.73\text{Hz}$.

Invariant entangled states remain unchanged under simultaneous identical unitary transformations of all their subsystems. We experimentally generate and characterize such invariant two-, four-, and six-photon polarization entangled states. This is done only with a suitable filtering procedure of multiple emissions of entangled photon pairs from a single source without any interferometric overlaps. We get the desired states utilizing bosonic emission enhancement due to indistinguishability. The setup is very stable and gives high interference contrasts. Thus, the process is a very likely candidate for various photonic demonstrations of quantum information protocols.

We present an optoelectrical cooling scheme for polar molecules based on a Sisyphus-type cooling cycle in suitably tailored electric trapping fields. Dissipation is provided by spontaneous vibrational decay in a closed level scheme found in symmetric-top rotors comprising six low-field-seeking rovibrational states. A generic trap design is presented. Suitable molecules are identified with vibrational decay rates on the order of 100 Hz. A simulation of the cooling process shows that the molecular temperature can be reduced from 1 K to 1 mK in approximately 10 s. The molecules remain electrically trapped during this time, indicating that the ultracold regime can be reached in an experimentally feasible scheme.

We study the dynamics of the relative phase of a bilayer of two-dimensional superfluids after the two superfluids have been decoupled, using truncated Wigner approximation. On short time scales the relative phase shows “light-cone”-like thermalization and creates a metastable superfluid state, which can be supercritical. On longer time scales this state relaxes to a disordered state due to dynamical vortex unbinding. This scenario of dynamically suppressed vortex proliferation constitutes a reverse-Kibble-Zurek effect. We observe dynamics of creation of vortex-antivortex pairs and their consequent motion. Our predictions can be directly measured in interference experiments [Z. Hadzibabic et al., Nature (London) 441, 1118 (2006)].

We develop an exact method for computing Casimir forces and the power of radiative heat transfer between two arbitrary nanostructured surfaces out of thermal equilibrium. The method is based on a generalization of the scattering approach recently used in investigations on the Casimir effect. Analogously to the equilibrium case, we find that also out of thermal equilibrium the shape and composition of the surfaces enter only through their scattering matrices. The expressions derived provide exact results in terms of the scattering matrices of the intervening surfaces.

We analyze the quantum process tomography (QPT) in the presence of decoherence, focusing on distinguishing local and nonlocal decoherence mechanisms for a two-partite system from experimental QPT data. In particular, we consider the $\sqrt{i\text{SWAP}}$ gate realized with superconducting phase qubits and calculate the QPT matrix $\chi$ in the presence of several local and nonlocal decoherence processes. We determine specific patterns of these decoherence processes, which can be used for a fast identification of the main decoherence mechanisms from an experimental $\chi$ matrix.

In this work the ion’s velocity dependence of surface-plasmon-induced transition rates for proton neutralization at metal surfaces is analyzed. In particular we study the opening of the collective channel for the ${\text{H}}^{+}\text{-Al}$ system, which is known to be closed in the fixed ion approximation. The threshold velocity of the collective rates, as a function of the surface-plasmon wave vector and the atomic energy shift, is also analyzed. Afterward, we have computed outgoing $\text{H}\left(1s\right)$ fractions in the velocity range $0.1\le v\le 2\text{a}\text{.u}\text{.}$ for protons impinging on Al(111) with an angle of incidence of $0.50°$. Capture and loss for resonant and Auger processes are taken into account and the role of the surface-plasmon-assisted electron capture mechanism is investigated. Consideration of all these processes allows us to obtain a good agreement with experimental data. A detailed analysis of our results shows that resonant processes dominate for $v\le 1.5\text{a}\text{.u}\text{.}$ although the surface-plasmon-assisted capture mechanism has a significant contribution around the Fermi velocity $\left(v_F\simeq 0.9\text{a}\text{.u}\text{.}\right)$. For higher velocities, Auger processes mainly explain the experimental findings.

We describe a scheme for probing a gas of ultracold atoms trapped in an optical lattice and moving in the presence of an external potential. The probe is nondestructive and uses the existing lattice fields as the measurement device. Two counterpropagating cavity fields simultaneously set up a conservative lattice potential and a weak quantum probe of the atomic motion. Balanced heterodyne detection of the probe field at the cavity output along with integration in time and across the atomic cloud yield information about the atomic dynamics in a single run. The scheme is applied to a measurement of the Bloch oscillation frequency for atoms moving in the presence of the local gravitational potential. Signal-to-noise ratios are estimated to be as high as ${10}^4$.

In this paper we analyze some important characteristics of the spatial Wigner function of entangled photon pairs. We show that the Wigner description of quantum states that live in the infinite-dimensional space-momentum degree of freedom proves to be particularly useful. We propose an experimental configuration that can be used to retrieve the Wigner function of paired photons entangled in the spatial degree of freedom. In particular, it allows the full characterization of the paired photons emitted from a spontaneous parametric down-conversion source.

This work presents experimental measurements of the $2{{}^{3}P}_2\to 1{{}^{1}S}_0$ and $2{{}^{3}P}_0\to 1{{}^{1}S}_0$ transition rates for helium. It completes the determination of the decay rates to the ground state from the $2{}^{3}P$ manifold and also completes the $2{{}^{3}P}_2$ decay rates for the metastable rare-gas series. The rate of photon emission from the $2{{}^{3}P}_2$ and $2{{}^{3}P}_0$ states to the $1{{}^{1}S}_0$ ground state is measured relative to the emission rate from the $2{{}^{3}P}_1\to 1{{}^{1}S}_0$ transition, which we have determined previously for the first time. The value for the $2{{}^{3}P}_2\to 1{{}^{1}S}_0$ transition is $0.324\pm 0.016{\text{s}}^{-1}$. An upper bound of $\sim 0.01{\text{s}}^{-1}$ is placed on the emission rate for the $2{{}^{3}P}_0\to 1{{}^{1}S}_0$ transition, which is predicted to be strictly forbidden. Along with our previous measurement of the $2{{}^{3}S}_1$ lifetime, the $2{}^{3}P$ manifold decay rates are in excellent agreement with current quantum-electrodynamic theory.

Is it possible to prove that the properties of Hamiltonians, such as the ground-state energy, results of dynamical evolution, or thermal state expectation values, can be efficiently calculated when the Hamiltonians have physically motivated constraints such as translational or rotational invariance? We report that rotational invariance does not reduce the difficulty of finding the ground-state energy of the system. Crucially, the construction preserves the translational invariance of a Hamiltonian. The failure of the construction for the properties of thermal states at finite temperatures is discussed.

We study the routing of quantum information in qubit chains. This task is achieved by suitably chosen time-dependent local fields acting on the qubits. Employing the physics of coherent destruction of tunneling, we demonstrate that a driving-induced renormalization of the coupling between neighboring qubits provides the key for controlling the transduction of quantum information between permanently coupled qubits. We employ this idea for building a quantum router. Moreover, we discuss the experimental implementation with Penning traps and study the robustness of our protocol under realistic experimental conditions, such as fabrication uncertainties and decoherence.