The dissipative dynamics of a quantum system weakly coupled to one or several reservoirs is usually described in terms of a Lindblad generator. The popularity of this approach is certainly due to the linear character of the latter. However, while such linearity finds justification from an underlying Hamiltonian evolution in some scaling limit, it does not rely on solid physical motivations at small but finite values of the coupling constants, where the generator is typically used for applications. The Markovian quantum master equations we propose are instead supported by very natural thermodynamic arguments. They themselves arise from Markovian master equations for the system and the environment which preserve factorized states and mean energy and generate entropy at a non-negative rate. The dissipative structure is driven by an entropic map, called modular, which introduces nonlinearity. The generated modular dynamical semigroup (MDS) guarantees for the positivity of the time evolved state the correct steady state properties, the positivity of the entropy production, and a positive Onsager matrix with symmetry relations arising from Green-Kubo formulas. We show that the celebrated Davies Lindblad generator, obtained through the Born and the secular approximations, generates a MDS. In doing so we also provide a nonlinear MDS which is supported by a weak coupling argument and is free from the limitations of the Davies generator.

The multipartite setting offers much more complexity of nonlocality than the bipartite one. We analyze the structure of tripartite nonlocal correlations by proposing inequalities satisfied by each of type bilocal, broadcasting, and quantum but violated by the other two. One of the inequalities satisfied by broadcasting correlations is generalized for multipartite systems. The study of its quantum mechanical violation reveals that Greenberger-Horne-Zeilinger-like states exhibit new, powerful correlations.

We derive steerability criteria applicable for both finite- and infinite-dimensional quantum systems using covariance matrices of local observables. We show that these criteria are useful to detect a wide range of entangled states particularly in high-dimensional systems and that the steering criterion for general $M\times N$ modes of continuous variables, known as Gaussian criterion but applicable also for non-Gaussian states, are obtained as a special case. Extending from the approach of entanglement detection via covariance matrices, our criteria are based on the local uncertainty principles incorporating the asymmetric nature of steering scenario. Specifically, we apply the formulation to the case of local orthogonal observables and obtain some useful criteria that can be readily computable, and testable in experiment, with no need for numerical optimization.

We investigate radiative processes of inertial two-level atoms in an entangled state interacting with a quantum electromagnetic field. Our intention is to clarify and to analyze the contributions of vacuum fluctuations and radiation reaction to the decay rate of the entangled state. The possible relevance of the findings in the present work is discussed.

In this work we probe the usefulness of non-Gaussian entangled states as a resource for quantum communication through atmospheric channels. We outline the initial conditions in which non-Gaussian state transfer leads to enhanced entanglement transfer relative to that obtainable via Gaussian state transfer. However, we conclude that in (anticipated) operational scenarios—where most of the non-Gaussian states to be transferred over the air are created just in time via photonic subtraction, addition, or replacement from incoming Gaussian states—the entanglement-generation rate between stations via non-Gaussian state transfer will be substantially less than that created by direct Gaussian state transfer. The role of postselection, distillation, and quantum memory in altering this conclusion is discussed, and comparison with entanglement rates produced via single-photon technologies is provided. Our results suggest that in the near term entangled Gaussian states, squeezed beyond some modest level, offer the most attractive proposition for the distribution of entanglement through high-loss atmospheric channels. The implications of our results for entanglement-based quantum key distribution to low-Earth orbit are presented.

We propose the continuous-variable quantum key distribution protocol based on the Gaussian modulation of a single quadrature of the coherent states of light, which is aimed to provide simplified implementation compared to the symmetrically modulated Gaussian coherent-state protocols. The protocol waives the necessity in one of the quadrature modulations and the corresponding channel transmittance estimation. The security of the protocol against collective attacks in a generally phase-sensitive Gaussian channel is analyzed and is shown achievable upon certain conditions. Robustness of the protocol to channel imperfections is compared to that of the symmetrical coherent-state protocol. The simplified unidimensional protocol is shown possible at a reasonable quantitative cost in terms of key rate and of tolerable channel excess noise.

The problems of genuine multipartite entanglement detection and classification are challenging. We show that a multipartite quantum state is genuine multipartite entangled if the multipartite concurrence is larger than certain quantities given by the number and the dimension of the subsystems. This result also provides a classification of various genuine multipartite entanglements. We present a lower bound of the multipartite concurrence in terms of bipartite concurrences. While various operational approaches are available for providing lower bounds of bipartite concurrences, our results give an effective operational way to detect and classify genuine multipartite entanglement. As applications, genuine multipartite entanglement of tripartite systems is analyzed in detail.

We investigate the nonlocality distributions among multiqubit systems based on the maximal violations of the Clauser-Horne-Shimony-Holt (CHSH) inequality of reduced pairwise qubit systems. We present a trade-off relation satisfied by these maximal violations, which gives rise to restrictions on the distribution of nonlocality among the subqubit systems. For a three-qubit system, it is impossible that all pairs of qubits violate the CHSH inequality, and once a pair of qubits violates the CHSH inequality maximally, the other two pairs of qubits must both obey the CHSH inequality. Detailed examples are given to illustrate the trade-off relations, and the trade-off relations are generalized to arbitrary multiqubit systems.

Entanglement is the resource to overcome the natural limitations of spatially separated parties restricted to local operations assisted by classical communication (LOCC). Recently, two new classes of operational entanglement measures, the source and the accessible entanglement, for arbitrary multipartite states have been introduced. Whereas the source entanglement measures from how many states the state of interest can be obtained via LOCC, the accessible entanglement measures how many states can be reached via LOCC from the state at hand. We consider here pure bipartite as well as multipartite states and derive explicit formulas for the source entanglement. Moreover, we obtain explicit formulas for a whole class of source entanglement measures that characterize the simplicity of generating a given bipartite pure state via LOCC. Furthermore, we show how the accessible entanglement can be computed numerically. For generic four-qubit states we first derive the necessary and sufficient conditions for the existence of LOCC transformations among these states and then derive explicit formulas for their accessible and source entanglement.

The purity entanglement measure introduced by Los Alamos group a decade ago is reexamined in the light of interesting features. The role played by purity, reaching a real thermodynamic limit, in detecting quantum phase transitions is studied with a different system, the bond-alternating $XY$ model in an external magnetic field. The properties of this system are described as well. By considering the dynamics of the original $XY$ model, we observe that nonergodicity is also grasped by the purity measure, in accordance with other quantum correlation measures that have no common physical or mathematical relation. Adiabaticity is not recovered from the dynamic to the static case, in accordance with one of the consequences of the celebrated Kibble-Zurek mechanism.

We demonstrate Feshbach optimized photoassociation (FOPA) into the $0_g^{-}(5S_{1/2}+5P_{1/2})$ state in ${}^{85}\mathrm{Rb}_2$. FOPA uses the enhancement of the amplitude of the initial atomic scattering wave function due to a Feshbach resonance to increase the molecular formation rate from photoassociation. We observe three vibrational levels, $v=127$, 140, and 150, with previously unmeasured binding energies of 256, 154, and $96{\text{cm}}^{-1}$. We measure the frequency, central magnetic field position, and magnetic field width of each Feshbach resonance. Our findings experimentally confirm that this technique can measure vibrational levels lower than those accessible to traditional photoassociative spectroscopy.

Above-threshold ionization (ATI) results from strong-field laser-matter interaction and it is one of the fundamental processes that may be used to extract electron structural and dynamical information about the atomic or molecular target. Moreover, it can also be used to characterize the laser field itself. Here we develop an analytical description of ATI, which extends the theoretical strong-field approximation (SFA), for both the direct and rescattering transition amplitudes in atoms. From a nonlocal, but separable potential, the bound-free dipole and the rescattering transition matrix elements are analytically computed. In comparison with the standard approaches to the ATI process, our analytical derivation of the rescattering matrix elements allows us to study directly how the rescattering process depends on the atomic target and laser-pulse features; we can turn on and off contributions having different physical origins or corresponding to different physical mechanisms. We compare SFA results with the full numerical solutions of the time-dependent Schrödinger equation (TDSE) within the few-cycle pulse regime. Good agreement between our SFA and TDSE model is found for the ATI spectrum. Our model captures also the strong dependence of the photoelectron spectra on the carrier envelope phase of the laser field.

We conduct a systematic study of the dipole phase and the photoelectron group delay (Wigner time delay) in inner shell photoionization of noble gas atoms from Ne to Xe. Our study encompasses the tender x-ray spectral range and extends to 1 keV photoelectron energy. We employ both the relativistic and the nonrelativistic versions of the random-phase approximation with exchange. We identify the long-range Coulomb and short-range Hartree-Fock contributions to the dipole phase which governs the Wigner time delay variation from the threshold to the whole considered range of photoelectron energies.

We numerically study models of ${\mathrm{H}}_2$ and $\mathrm{LiH}$ molecules, aligned collinearly with the linear polarization of the external field, to elucidate the possible role of correlation in the enhanced-ionization (EI) phenomena. Correlation is considered at different levels of approximation with the time-dependent generalized-active-space configuration-interaction method. The results of our studies show that enhanced ionization occurs in multielectron molecules and that correlation is important, and they also demonstrate significant deviations between the results of the single-active-electron approximation and more accurate configuration-interaction methods. We further investigate the role of low-lying excited states in the EI phenomena. With the inclusion of correlation we show strong carrier-envelope-phase effects in the enhanced ionization of the asymmetric heteronuclear $\mathrm{LiH}$-like molecule. The correlated calculation shows an intriguing feature of crossover in enhanced ionization with two carrier-envelope phases at critical internuclear separation.

Absolute single-photoionization cross-section measurements for ${\mathrm{Se}}^{2+}$ ions were performed at the Advanced Light Source at Lawrence Berkeley National Laboratory using the merged-beams photo-ion technique. Measurements were made at a photon energy resolution of $24\pm 3$ meV in the photon energy range 23.5–42.5 eV, spanning the ground state and low-lying metastable state ionization thresholds. To clearly resolve the resonant structure near the ground-state threshold, high-resolution measurements were made from 30.0 to 31.9 eV at a photon energy resolution of $6.7\pm 0.7$ meV. Numerous resonance features observed in the experimental spectra are assigned and their energies and quantum defects tabulated. The high-resolution cross-section measurements are compared with large-scale, state-of-the-art theoretical cross-section calculations obtained from the Dirac Coulomb $R$-matrix method. Suitable agreement is obtained over the entire photon energy range investigated. These results are an experimental determination of the absolute photoionization cross section of doubly ionized selenium and include a detailed analysis of the photoionization resonance spectrum of this ion.

The ability to characterize static and time-dependent electric fields in situ is an important prerequisite for quantum-optics experiments with atoms close to surfaces. Especially in experiments which aim at coupling Rydberg atoms to the near field of superconducting circuits, the identification and subsequent elimination of sources of stray fields are crucial. We present a technique that allows the determination of stray-electric-field distributions $[F_{\mathrm{x}}^{\mathrm{str}}\left(\vec{r}\right),F_{\mathrm{y}}^{\mathrm{str}}\left(\vec{r}\right),F_{\mathrm{z}}^{\mathrm{str}}\left(\vec{r}\right)]$ at distances of less than 2 mm from (cryogenic) surfaces using coherent Rydberg-Stark spectroscopy in a pulsed supersonic beam of metastable $1s^{1}2s^1{}^1{S}_0$ helium atoms. We demonstrate the capabilities of this technique by characterizing the electric stray field emanating from a structured superconducting surface. Exploiting coherent population transfer with microwave radiation from a coplanar waveguide, the same technique allows the characterization of the microwave-field distribution above the surface.

The variation of the hyperfine populations $N{\rho }_{\mu }$ of the ground state (g.s.) of spin-2 condensates against a magnetic field $B$ is studied. A $N$-body theory which is rigorous in dealing with the spin degrees of freedom has been adopted. The conservation of the total magnetization $M$ is assumed. A combined hyperfine population $N{\rho }_{\text{comb}}=N({\rho }_0-3{\rho }_2-3{\rho }_{-2})$, which is an observable, is defined. The lower bound for this population has been derived analytically. It turns out that the lower bound is identical to the actual population when $B=0$ and $\infty$, and the population is monotonically uprising in between. Thus, via the analytical form of the lower bound, the variation of the population against $B$ can be roughly known in advance. Numerical examples are given to demonstrate the applicability of the bound. When $M=0$, the g.s. in the polar phase was found to be highly unstable against $B$ because the neighboring level density is extremely high. When $M=N$, the g.s. in the cyclic phase was also found to be highly unstable. This is because the appearance of $B$, even if it is very weak, will break the high degeneracy of the g.s. and result in having numerous levels emerging in the neighborhood of the g.s.

We present evidence of Kelvin excitations in space-time resolved spectra of numerical simulations of quantum turbulence. Kelvin waves are transverse and circularly polarized waves that propagate along quantized vortices, for which the restitutive force is the tension of the vortex line, and which play an important role in theories of superfluid turbulence. We use the Gross-Pitaevskii equation to model quantum flows, letting an initial array of well-organized vortices develop into a turbulent bundle of intertwined vortex filaments. By achieving high spatial and temporal resolution we are able to calculate space-time resolved mass density and kinetic energy spectra. Evidence of Kelvin and sound waves is clear in both spectra. Identification of the waves allows us to extract the spatial spectrum of Kelvin waves, clarifying their role in the transfer of energy.

We previously proposed [K. G. L. Pedersen, B. M. Andersen, G. M. Bruun, O. F. Syljuåsen, and A. S. Sørensen, Phys. Rev. A 84, 041603 (2011)] a method to utilize a temporally dependent superlattice potential to mediate spin-selective transport and thereby probe long- and short-range magnetic correlations in optical lattices. Specifically, this can be used for detecting antiferromagnetic ordering in repulsive fermionic optical lattice systems, but more generally it can serve as a means of directly probing correlations among the atoms by measuring the mean value of an observable, the number of double occupied sites. Here we provide a detailed investigation of the physical processes that limit the effectiveness of this “conveyer belt method.” Furthermore, we propose a simple way to improve the procedure, resulting in an essentially perfect (error-free) probing of the magnetic correlations. These results shows that suitably constructed superlattices constitute a promising way of manipulating atoms of different spin species as well as probing their interactions.

We theoretically investigate the influence of time-dependent phase matching on high-order-harmonic generation (HHG). The results show that the phase-matching degrees of the short and long paths are different at the leading and falling edges of the driving laser pulse. By using the strong-field approximation model and solving the Maxwell equations, we demonstrate that the time-dependent phase matching leads to the harmonic splitting with either a blue- or a red-frequency shift in the HHG spectra. Moreover, we also discuss the influence of the laser intensity, the laser duration, and the position of the gas medium on the time-dependent phase matching as well as the spatial and spectral properties of the HHG spectra.