A control strategy incorporating a fixed carrier frequency constraint on the optimal field is presented. As an illustrative example we consider the creation of maximum coherence in a six-level $\Lambda$ system by solving the Schrödinger equation without the rotating-wave approximation in the field-interaction representation. We demonstrate that application of the optimal control theory optimization reformulated in the field-interaction representation allows one to keep the carrier frequency of the control field constant and successfully optimize off-resonant processes in multilevel quantum systems.

We analyze the correlation and spectral properties of two-photon states resonantly transmitted by a nonlinear optical microcavity. We trace the correlation properties of transmitted two-photon states to the decay spectrum of multiphoton resonances in the nonlinear microcavity.

In optical interferometry multimode entanglement is often assumed to be the driving force behind quantum enhanced measurements. Recent work has shown this assumption to be false: single-mode quantum states perform just as well as their multimode entangled counterparts. We go beyond this to show that when photon losses occur, an inevitability in any realistic system, multimode entanglement is actually detrimental to obtaining quantum enhanced measurements. We specifically apply this idea to a superposition of coherent states, demonstrating that these states show a robustness to loss that allows them to significantly outperform their competitors in realistic systems. A practically viable measurement scheme is then presented that allows measurements close to the theoretical bound, even with loss. These results promote an alternate way of approaching optical quantum metrology using single-mode states that we expect to have great implications for the future.

We have investigated the effect of an external magnetic field on the angular distribution and the linear polarization of the $2s^{2}2p^{5}3s{}^3{P}_2^o\to 2s^{2}2p^6{}^1{S}_0$ emission line in Ne-like ions. Since an electric dipole decay channel is opened by the magnetic field and competes with the inherent magnetic quadrupole transition, these properties strongly depend on the magnetic field strength, $B$. As an example, we discuss in some detail this transition in neonlike magnesium. This is of special interest, since the $B$-dependent angular distributions and linear polarization degrees are potentially unique diagnostic tools for the magnetic field in different plasmas.

In this paper, we treat an asymptotic hypothesis testing (or state discrimination with asymmetric treatment of errors) between an arbitrary fixed bipartite pure state $|\Psi \rangle$ and the completely mixed state by one-way local operation and classical communication (LOCC), two-way LOCC, and separable positive operator-valued measures (POVMs). As a result, we derive single-letterized formulas for the Stein's lemma type of optimal error exponents under one-way LOCC, two-way LOCC, and separable POVMs, the Chernoff bounds under one-way LOCC POVMs and separable POVMs, and the Hoeffding bounds under one-way LOCC POVMs in the whole region of a parameter and under separable POVMs on a restricted region of a parameter. We also numerically calculate the Chernoff and the Hoeffding bounds under a class of three-step LOCC protocols in low-dimensional systems and show that these bounds not only outperform the bounds for one-way LOCC POVMs but also almost approximate the bounds for separable POVMs in the parameter region where analytical bounds for separable POVMs are derived.

We study in theory and experiment the verification of the Schmidt number of quantum correlated radiation fields. This measure of entanglement counts the number of global superpositions of product states which are needed to describe a compound quantum system. For unraveling a certain value of this measure, a family of entanglement and Schmidt number tests is constructed. The measurement of the whole class of witnesses is realized. By generating a path entangled two-photon state, we demonstrate the feasibility to directly count quantum superpositions.

We present an ab initio three-dimensional precision calculation and analysis of high-order-harmonic generation (HHG) of the hydrogen molecular ion subject to intense elliptically polarized laser pulses by means of the time-dependent generalized pseudospectral method in two-center prolate spheroidal coordinates. The calculations are performed for the ground and first excited electronic states of ${\mathrm{H}}_2{}^{+}$ at the equilibrium internuclear separation $R=2$ a.u. as well as for the stretched molecule at $R=7$ a.u. The spectral and temporal structures of the HHG signal are explored by means of the wavelet time-frequency analysis. Several aspects of ellipticity-dependent dynamical behaviors are uncovered. We found that the production of above-threshold harmonics for nonzero ellipticity is generally reduced, as compared with linearly polarized fields. However, below-threshold harmonics still appear quite strong except when the polarization plane is perpendicular to the molecular axis. Weak even harmonics are detected in the HHG spectra of stretched molecules. This effect can be explained by the broken inversion symmetry due to dynamic localization of the electron density near one of the nuclei. Multiphoton resonance and two-center interference effects are analyzed for the exploration of the quantum origin of the predicted HHG spectral and dynamical behavior.

Excitation energies, term designations, and $g$ factors of Th, ${\mathrm{Th}}^{+}$, and ${\mathrm{Th}}^{2+}$ are determined using a relativistic hybrid configuration-interaction (CI) plus all-order approach that combines configuration-interaction and linearized coupled-cluster methods. The results are compared with other theoretical and experimental values where available. We find some vanishing $g$ factors, similar to those known in lanthanide spectra. Reduced matrix elements, oscillator strengths, transition rates, and lifetimes are determined for ${\mathrm{Th}}^{2+}$. To estimate the uncertainties of our results, we compare our values with the available experimental lifetimes for higher $5f7p{}^3{G}_4$, $7s7p{}^3{P}_0$, $7s7p{}^3{P}_1$, and $6d7p{}^3{F}_4$ levels of ${\mathrm{Th}}^{2+}$. These calculations provide a benchmark test of the CI plus all-order method for heavy systems with several valence electrons and yield recommended values for transition rates and lifetimes of ${\mathrm{Th}}^{2+}$.

We present experimentally measured and theoretically calculated rate coefficients for the electron-ion recombination of ${\mathrm{W}}^{18+}(\left[\mathrm{Kr}\right]4d^{10}4f^{10})$ forming ${\mathrm{W}}^{17+}$. At low electron-ion collision energies, the merged-beam rate coefficient is dominated by strong, mutually overlapping recombination resonances. In the temperature range where the fractional abundance of ${\mathrm{W}}^{18+}$ is expected to peak in a fusion plasma, the experimentally derived Maxwellian recombination rate coefficient is 5 to 10 times larger than that which is currently recommended for plasma modeling. The complexity of the atomic structure of the open-$4f$ system under study makes the theoretical calculations extremely demanding. Nevertheless, the results of the present Breit-Wigner partitioned dielectronic recombination calculations agree reasonably well with the experimental findings. This also gives confidence in the ability of the theory to generate sufficiently accurate atomic data for the plasma modeling of other complex ions.

We consider the possibilities for producing ultracold mixtures of K and Cs and forming KCs molecules by magnetoassociation. We carry out coupled-channel calculations of the interspecies scattering length for ${}^{39}\mathrm{KCs}$, ${}^{41}\mathrm{KCs}$, and ${}^{40}\mathrm{KCs}$ and characterize Feshbach resonances due to $s$-wave and $d$-wave bound states, with widths ranging from below 1 nG to 5 G. We also calculate the corresponding bound-state energies as a function of magnetic field. We give a general discussion of the combinations of intraspecies and interspecies scattering lengths needed to form low-temperature atomic mixtures and condensates and identify promising strategies for cooling and molecule formation for all three isotopic combinations of K and Cs.

In a recent Letter [H. Bachau, M. Dondera, and V. Florescu, Phys. Rev. Lett. 112, 073001 (2014)] we considered the hydrogen atom in interaction with an electromagnetic field consisting in the coherent superposition of two keV pulses centered around two frequencies ${\omega }_1$ and ${\omega }_2$ that differ by a few atomic units. The analysis of the results obtained from the resolution of the time-dependent Schrödinger equation focused on stimulated Compton scattering (SCS). We have developed in parallel an approach based on perturbation theory and it proved to be an appropriate and useful tool in complement to the nonperturbative approach. In this paper we present a detailed analysis of the electron spectra obtained for $\hslash {\omega }_1=$ 55 a.u. and two values of $\hslash {\omega }_2$, 50 and 54 a.u. Emphasis is put on the case $\hslash {\omega }_2=54$ a.u., where the electron emitted through SCS has the lowest energy, showing in particular that in the vicinity of the ionization threshold the cross section increases as $\hslash {\omega }_1-\hslash {\omega }_2$ decreases. We calculate photoelectron energy and angular distributions at various relative directions of propagation of the pulses. We discuss the limitations of the approximations underlying the numerical and analytical approaches used in this work.

We study optimal quantum control of the dynamics of trapped Bose-Einstein condensates: The targets are to split a condensate, residing initially in a single well, into a double well, without inducing excitation, and to excite a condensate from the ground state to the first-excited state of a single well. The condensate is described in the mean-field approximation of the Gross-Pitaevskii equation. We compare two optimization approaches in terms of their performance and ease of use; namely, gradient-ascent pulse engineering (GRAPE) and Krotov's method. Both approaches are derived from the variational principle but differ in the way the control is updated, additional costs are accounted for, and second-order-derivative information can be included. We find that GRAPE produces smoother control fields and works in a black-box manner, whereas Krotov with a suitably chosen step-size parameter converges faster but can produce sharp features in the control fields.

We report on switching the intensity correlation or anticorrelation by changing the polarization basis for measuring light that has already propagated through the electromagnetically induced transparency (EIT) medium of the $5S_{1/2}(F=2)-5P_{1/2}(F^{\prime }=1)$ transition of ${}^{87}\mathrm{Rb}$ atoms. When a linearly polarized laser beam interacts with the EIT medium in the Hanle configuration, selection of the polarization basis after the optical field has already interacted with the atoms was found to significantly affect the intensity-intensity correlation of the two orthogonal polarization components. We compared the second-order correlation functions of two postselected polarization bases (linear and circular), and the polarization basis effect on the second-order correlation functions as a function of the applied magnetic field was numerically calculated using an eight-level atomic model.

A detailed analysis of the propagation of laser Gaussian beams at critical angles shows under which conditions it is possible to maximize the breaking of symmetry in the angular distribution and for which values of the laser wavelength and beam waist it is possible to find an analytic formula for the maximal angular deviation from the optical path predicted by the Snell law. For beam propagation through $N$ dielectric blocks and for a maximal breaking of symmetry, a closed expression for the Goos-Hänchen shift is obtained. The multiple-peak phenomenon clearly represents additional evidence of the breaking of symmetry in the angular distribution of optical beams. Finally, the laser wavelength and beam-waist conditions to produce focal effects in the outgoing beam are also briefly discussed.

Quantum systems are invariably open, evolving under surrounding influences rather than in isolation. Standard open quantum system methods eliminate all information on the environmental state to yield a tractable description of the system dynamics. By incorporating a collective coordinate of the environment into the system Hamiltonian, we circumvent this limitation. Our theory provides straightforward access to important environmental properties that would otherwise be obscured, allowing us to quantify the evolving system-environment correlations. As a direct result, we show that the generation of robust system-environment correlations that persist into equilibrium (heralded also by the emergence of non-Gaussian environmental states) renders the canonical system steady state almost always incorrect. The resulting equilibrium states deviate markedly from those predicted by standard perturbative techniques and are instead fully characterized by thermal states of the mapped system-collective coordinate Hamiltonian. We outline how noncanonical system states could be investigated experimentally to study deviations from canonical thermodynamics, with direct relevance to molecular and solid-state nanosystems.