We have measured the absolute photoionization cross sections of ${\text{Fe}}^{2+}$ to ${\text{Fe}}^{6+}$ ions in the photon energy range covering the first ionization threshold up to 160 eV. Particular interest is emphasized on the region of the $3p\to 3d$ excitations. The experimental data are compared to the results of calculations we have performed using a spectral opacity code, as well as to available $R$-matrix and one-electron calculations. It is shown that often the theoretical results tend to overestimate the intensity of the $3p\to 3d$ photoexcitations. An anomalously low value of the integrated oscillator strength is measured for ${\text{Fe}}^{2+}$ ion.

We compare the Hanbury Brown–Twiss (HBT) and the thermal light ghost imaging schemes in both near and far fields. Both effects arise as a result of the intensity fluctuations of the thermal light and we find that the essential physics behind the two effects is the same. The difference however is that, in the ghost imaging, large number of bits information of an object needs to be treated together, whereas, in the HBT, there is only one bit information required to be obtained. In the HBT experiment far field is used for the purpose of easy detection, while in the ghost image experiment near (or not far) field is used for good quality image.

The 1996 experiment by Brune et al. [Phys. Rev. Lett. 76, 1800 (1996)] on vacuum Rabi oscillation is analyzed by means of alternative models of atom-reservoir interaction. Agreement with experimental Rabi oscillation data can be obtained if one defines jump operators in the dressed-state basis and takes into account thermal fluctuations between dressed states belonging to the same manifold. Such low-frequency transitions could be ignored in a closed cavity, but the cavity employed in the experiment was open, which justifies our assumption. The cavity quality factor corresponding to the data is approximately $Q=3.31\times {10}^{10}$, whereas $Q$ reported in the experiment was $Q=7\times {10}^7$. The rate of decoherence arising from opening of the cavity can be of the same order as an analogous correction coming from finite time resolution $\Delta t$ (formally equivalent to collisional decoherence). Peres-Horodecki separability criterion shows that the rate at which the atom-field state approaches a separable state is controlled by fluctuations between dressed states from the same manifold and not by the rate of transitions toward the ground state. In consequence, improving the $Q$ factor we do not improve the coherence properties of the cavity.

A nonlinear extension of geometric optics is used to derive a modulation theory solution for the trajectory of an optical solitary wave in a nematic liquid crystal—i.e., a nematicon—in which a wide waveguide has been defined by an externally applied static electric field. This solution is used to find the power threshold for the solitary wave to escape the trapping waveguide. This threshold is found to be in excellent agreement with experimental results.

We have measured the spin-forbidden decay rate from $\left(3s3d\right){{}^{1}D}_2\to \left(3s3p\right){{}^{3}P}_{2,1}$ in ${}^{24}\text{M}\text{g}$ atoms trapped in a magneto-optical trap. The total decay rate, summing up both exit channels $\left(3s3p\right){{}^{3}P}_1$ and $\left(3s3p\right){{}^{3}P}_2$, yields $196\pm 10{\text{s}}^{-1}$ in excellent agreement with resent relativistic many-body calculations of Porsev et al. [Phys. Rev. A 64, 012508 (2001)]. The characterization of this decay channel is important as it may limit the performance of quantum optics experiments carried out with this ladder system as well as two-photon cooling experiments currently explored in several groups.

We present a simple scheme using two identical cross-phase modulation processes in the decoherence environment to generate superpositions of two coherent states with the opposite phases, which are known as cat states. The scheme is shown to be robust against decoherence due to photon absorption losses and other errors, and the design of its experimental setup is also discussed.

Linear mechanical oscillators have been applied to measure very small forces, mostly with the help of noise suppression. In contrast, adding noise to nonlinear oscillators can improve the measurement conditions. Here, this effect of stochastic resonance is demonstrated in a macroscopic torsion oscillator, for an optomechanical nonlinear potential. The signal output is enhanced for a subthreshold electronic signal. This nonlinear oscillator serves as a model system for the enhancement of signal-to-noise ratio in high precision optomechanical experiments.

We investigate nonequilibrium entanglement generation in a chain of harmonic oscillators with time-dependent linear coupling. We use optimal control theory to determine the coupling modulation that leads to maximum logarithmic negativity for a pair of opposite oscillators, and show that it corresponds to a synchronization of the eigenmodes of the chain. We further analytically relate the maximum attainable entanglement to the irreversible work done to produce it. We find that the energy cost of optimal entanglement production scales exponentially with the amount of entanglement.

We explore the quantum-classical crossover in the behavior of a quantum field mode. The quantum behavior of a two-state system—a qubit—coupled to the field is used as a probe. Collapse and revival of the qubit inversion form the signature for quantum behavior of the field and continuous Rabi oscillations form the signature for classical behavior of the field. We demonstrate both limits in a single model for the full coupled system, for field states with the same average field strength, and so for qubits with the same Rabi frequency.

An accurate QED calculation of transition probabilities for the low-lying two-electron configurations of multicharged ions is presented. The calculation is performed for the nondegenerate states $\left(1s2s\right){{}^{3}S}_1$, $\left(1s2p_{3/2}\right){{}^{3}P}_2$ ($M1$ and $M2$ transitions, respectively) and for the quasidegenerate states $\left(1s2p\right){{}^{1}P}_1$, $\left(1s2p\right){{}^{3}P}_1$ ($E1$ transitions) decaying to the ground state $\left(1s1s\right){{}^{1}S}_0$. Two-electron ions with nuclear-charge numbers $Z=10–92$ are considered. The line profile approach is employed for the description of the process in multicharged ions within the framework of QED.

In the improved strong-field approximation, which describes high-order above-threshold ionization (HATI), the rescattering of the ionized electron on the parent ion is described within the first-order Born approximation. The low-frequency approximation for laser-assisted scattering goes beyond the first Born approximation. In the present paper, we derive the low-frequency approximation for HATI. The rescattering amplitude in the first Born approximation is replaced by the exact scattering amplitude calculated on the energy shell. Our numerical results for the angle-resolved HATI energy spectra show that the difference between the improved strong-field approximation and the low-frequency approximation is significant for scattering away from the laser polarization axis. In the context of quantum-orbit theory and the uniform approximation, we also show that on the back-rescattering ridge, the rescattering $T$-matrix element can be factorized into the product of the incoming flux and the elastic-scattering cross section so that the latter can be extracted from the angle- and energy-resolved HATI spectra.

We discuss the dynamics of a Bose-Einstein condensate in a double-well trap subject to phase noise and particle loss. The phase coherence of a weakly interacting condensate, experimentally measured via the contrast in an interference experiment, as well as the response to an external driving becomes maximal for a finite value of the dissipation rate matching the intrinsic time scales of the system. This can be understood as a stochastic resonance of the many-particle system. Even stronger effects are observed when dissipation acts in concurrence with strong interparticle interactions, restoring the purity of the condensate almost completely and increasing the phase coherence significantly. Our theoretical results are backed by Monte Carlo simulations, which show a good qualitative agreement and provide a microscopic explanation for the observed stochastic resonance effect.

Linear stability of two-dimensional radial ground and excited nonlinear states of Bose-Einstein condensate is considered. Following the approach of the paper of Zezyulin et al. [Phys. Rev. A 78, 013606 (2008)] it is shown that the excited states of small enough amplitude become stable when the harmonic trap potential is replaced with an anharmonic one. However, in contrast to one-dimensional case, small anharmonic perturbation of the harmonic potential does not cause considerable stabilization effect: excited states lose stability as the number of particles exceeds some threshold. Some exact solutions of linear stability eigenvalue problems are reported.

Generation of wave structures by a two-dimensional (2D) object (laser beam) moving in a 2D two-component Bose-Einstein condensate with a velocity greater than the two sound velocities of the mixture is studied by means of analytical methods and systematic simulations of the coupled Gross-Pitaevskii equations. The wave pattern features three regions separated by two Mach cones. Two branches of linear patterns similar to the so-called “ship waves” are located outside the corresponding Mach cones, and oblique dark solitons are found inside the wider cone. An analytical theory is developed for the linear patterns. A particular dark-soliton solution is also obtained, its stability is investigated, and two unstable modes of transverse perturbations are identified. It is shown that for a sufficiently large flow velocity, this instability has a convective character in the reference frame attached to the moving body, which makes the dark soliton effectively stable. The analytical findings are corroborated by numerical simulations.

We give the optimal bounds on the phase-estimation precision for mixed Gaussian states in the single-copy and many-copy regimes. Specifically, we focus on displaced thermal and squeezed thermal states. We find that while for displaced thermal states an increase in temperature reduces the estimation fidelity, for squeezed thermal states a larger temperature can enhance the estimation fidelity. The many-copy optimal bounds are compared with the minimum variance achieved by three important single-shot measurement strategies. We show that the single-copy canonical phase measurement does not always attain the optimal bounds in the many-copy scenario. Adaptive homodyning schemes do attain the bounds for displaced thermal states, but for squeezed states they yield fidelities that are insensitive to temperature variations and are, therefore, suboptimal. Finally, we find that heterodyne measurements perform very poorly for pure states but can attain the optimal bounds for sufficiently mixed states. We apply our results to investigate the influence of losses in an optical metrology experiment. In the presence of losses squeezed states cease to provide the Heisenberg limited precision, and their performance is close to that of coherent states with the same mean photon number.

We demonstrate how the dynamical coarse-graining approach can be systematically extended to higher orders in the coupling between system and reservoir. Up to second order in the coupling constant, we explicitly show that dynamical coarse-graining unconditionally preserves positivity of the density matrix—even for bath density matrices that are not in equilibrium and also for time-dependent system Hamiltonians. By construction, the approach correctly captures the short-time dynamics; i.e., it is suitable for analyzing non-Markovian effects. We compare the dynamics with the exact solution for highly non-Markovian systems and find a remarkable quality of the coarse-graining approach. The extension to higher orders is straightforward but rather tedious. The approach is especially useful for bath correlation functions of simple structure and for small system dimensions.

We propose an approach to implement quantum repeaters for long-distance quantum communication. Our protocol generates a backbone of encoded Bell pairs and uses the procedure of classical error correction during simultaneous entanglement connection. We illustrate that the repeater protocol with simple Calderbank-Shor-Steane encoding can significantly extend the communication distance, while still maintaining a fast key generation rate.

The generalized $n$-qubit Greenberger-Horne-Zeilinger (GHZ) states and their local unitary equivalents are the only pure states of $n$ qubits that are not uniquely determined (among arbitrary states, pure or mixed) by their reduced density matrices of $n-1$ qubits. Thus, the generalized GHZ states are the only ones containing information at the $n$-party level.

Static dipole polarizabilities for the electronic ground state of neutral Yb and No are investigated by all-electron relativistic coupled cluster theory. We find a significant reduction in the polarizability due to both electron correlation and scalar-relativistic effects. Whereas electron correlation effects are of the same size as relativistic contributions for Yb, for No relativistic effects are dominant and result in smaller polarizability for No compared to Yb. In comparison the contributions from spin-orbit coupling and Gaunt interaction are rather small. Our recommended dipole polarizabilities for Yb and No are 140.7 and 110.8 a.u., respectively.