By using photon pairs created in parametric down-conversion, we report on an experiment, which demonstrates that measurement can recover the quantum entanglement of a two-qubit system in a pure dephasing environment. The concurrence of the final state with and without measurement is compared and is analyzed. Furthermore, we verify that recovered states can still violate the Bell inequality, that is, to say, such recovered states exhibit nonlocality. In the context of quantum entanglement, sudden death and rebirth provide clear evidence, which verifies that entanglement dynamics of the system is sensitive not only to its environment, but also to its initial state.

A dynamical map is a map that takes one density operator to another. Such a map can be written in an operator-sum representation (OSR) by using a spectral decomposition. The method of construction applies to general maps, which need not be completely positive. The OSR is not unique; there is freedom to choose a different set of operators in the OSR, yet still obtain the same map. Here, we show that, whereas the freedom for completely positive maps is unitary, the freedom for maps that are not necessarily completely positive is pseudounitary.

We consider a special form of state discrimination in which after the measurement we are given additional information that may help us identify the state. This task plays a central role in the analysis of quantum cryptographic protocols in the noisy-storage model, where the identity of the state corresponds to a certain bit string, and the additional information is typically a choice of encoding that is initially unknown to the cheating party. We first provide simple optimality conditions for measurements for any such problem and show upper and lower bounds on the success probability. For a certain class of problems, we furthermore provide tight bounds on how useful postmeasurement information can be. In particular, we show that for this class finding the optimal measurement for the task of state discrimination with postmeasurement information does in fact reduce to solving a different problem of state discrimination without such information. However, we show that for the corresponding classical state discrimination problems with postmeasurement information such a reduction is impossible, by relating the success probability to the violation of Bell inequalities. This suggests the usefulness of postmeasurement information as another feature that distinguishes the classical from a quantum world.

Arrays of atoms trapped in optical lattices are appealing as storage media for photons, since motional dephasing of the atoms is eliminated. The regular lattice is also associated with band structure in the dispersion experienced by incident photons. Here we study the influence of this band structure on the efficiency of quantum memories based on electromagnetically induced transparency (EIT) and on Raman absorption. We observe a number of interesting effects, such as both reduced and superluminal group velocities, enhanced atom-photon coupling, and anomalous transmission. These effects are ultimately deleterious to the memory efficiency, but they are easily avoided by tuning the optical fields away from the band edges.

The operation of atomic clocks is generally carried out at room temperature, whereas the definition of the second refers to the clock transition in an atom at absolute zero. This implies that the clock transition frequency should be corrected in practice for the effect of finite temperature, of which the leading contributor is the blackbody radiation (BBR) shift. Experimental measurements of the BBR shifts are difficult. In this work, we have calculated the blackbody radiation shift of the ground-state hyperfine microwave transition in ${}^{87}\mathrm{Rb}$ using the relativistic all-order method and carried out a detailed evaluation of the accuracy of our final value. Particular care is taken to accurately account for the contributions from highly excited states. Our predicted value for the Stark coefficient, $k_S=-1.240(4)\times {10}^{-10}\hspace{0.5em}\mathrm{Hz}/(\mathrm{V}/\mathrm{m}{)}^2$, is three times more accurate than the previous calculation [E. J. Angstman, V. A. Dzuba, and V. V. Flambaum, Phys. Rev. A 74, 023405 (2006)].

We report on the extension of the recently formulated relativistic convergent close-coupling (RCCC) method to accommodate two-electron and quasi-two-electron targets. We apply the theory to electron scattering from mercury and obtain differential and integrated cross sections for elastic and inelastic scattering. We compared with previous nonrelativistic convergent close-coupling (CCC) calculations and for a number of transitions obtained significantly better agreement with the experiment. The RCCC method is able to resolve structure in the integrated cross sections for the energy regime in the vicinity of the excitation thresholds for the $(6s6p){\hspace{0.5em}}^3{P}_{0,1,2}$ states. These cross sections are associated with the formation of negative ion (${\mathrm{Hg}}^{-}$) resonances that could not be resolved with the nonrelativistic CCC method. The RCCC results are compared with the experiment and other relativistic theories.

Many precision measurements (e.g., in spectroscopy, atomic clocks, quantum-information processing, etc.) suffer from systematic errors introduced by the light shift. In our experimental configuration, however, the tensor light shift plays a positive role enabling the observation of spectral features otherwise masked by the cancellation of the transition amplitudes and creating resonances at a frequency unperturbed either by laser power or beam inhomogeneity. These phenomena occur thanks to the special relation between the nonlinear Zeeman and light shift effects. The interplay between these two perturbations is systematically studied and the cancellation of the nonlinear Zeeman effect by the tensor light shift is demonstrated.

We investigate theoretically an original route to achieve Bose-Einstein condensation using dark power-law laser traps. We propose to create such traps with two crossing blue-detuned Laguerre-Gaussian optical beams. Controlling their azimuthal order $\ell$ allows for the exploration of a multitude of power-law trapping situations in one, two, and three dimensions, ranging from the usual harmonic trap to an almost square-well potential, in which a quasihomogeneous Bose gas can be formed. The usual cigar-shaped and disk-shaped Bose-Einstein condensates obtained in a 1D or 2D harmonic trap take the generic form of a “finger” or of a “hockey puck” in such Laguerre-Gaussian traps. In addition, for a fixed atom number, higher transition temperatures are obtained in such configurations when compared with a harmonic trap of the same volume. This effect, which results in a substantial acceleration of the condensation dynamics, requires a better but still reasonable focusing of the Laguerre-Gaussian beams.

Complex Ginzburg-Landau (CGL) models of laser media (with cubic-quintic nonlinearity) do not contain an effective diffusion term, which makes all vortex solitons unstable in these models. Recently, it has been demonstrated that the addition of a two-dimensional periodic potential, which may be induced by a transverse grating in the laser cavity, to the CGL equation stabilizes compound (four-peak) vortices, but the most fundamental “crater-shaped” vortices (CSVs), alias vortex rings, which are essentially squeezed into a single cell of the potential, have not been found before in a stable form. In this work we report on families of stable compact CSVs with vorticity $S=1$ in the CGL model with the external potential of two different types: an axisymmetric parabolic trap and the periodic potential. In both cases, we identify a stability region for the CSVs and for the fundamental solitons ($S=0$). Those CSVs which are unstable in the axisymmetric potential break up into robust dipoles. All the vortices with $S=2$ are unstable, splitting into tripoles. Stability regions for the dipoles and tripoles are identified, too. The periodic potential cannot stabilize CSVs with $S⩾2$ either; instead, families of stable compact square-shaped quadrupoles are found.

We present theoretical calculations for polarization and ellipticity of high-order harmonics from aligned ${\mathrm{N}}_2$, CO${}_2$, and ${\mathrm{O}}_2$ molecules generated by linearly polarized lasers. Within the rescattering model, the two polarization amplitudes of the harmonics are determined by the photo-recombination amplitudes for photons emitted with polarization parallel or perpendicular to the direction of the same returning electron wave packet. Our results show clear species-dependent polarization states, in excellent agreement with experiments. We further note that the measured polarization ellipse of the harmonic furnishes the needed parameters for a “complete” experiment in molecules.

The efficiency of frequency up and down conversion of light in an atomic ensemble, with a diamond level configuration, is analyzed theoretically. The conditions of pump field intensities and detunings required to maximize the conversion as a function of optical thickness of the ensemble are determined. The influence of the probe pulse duration on the conversion efficiency is investigated by numeric solution of the Maxwell-Bloch equations.

We theoretically study the propagation through a resonant absorbing medium of a time-dependent perturbation modulating the amplitude of a continuous wave (cw). Modeling the medium as a two-level system and linearizing the Maxwell-Bloch equations for the perturbation, we establish an exact analytical expression of the transfer function relating the Fourier transforms of the incident and transmitted perturbations. It directly gives the gain and the phase shift undergone in the medium by a harmonic modulation. For the case of a pulse modulation, it enables us to determine the transmission time of the pulse center of mass (group delay), evidencing the relative contributions of the coherent and incoherent (population) relaxations. We show that the group delay has a negative value (fast light) fixed by the coherent effects when the cw intensity is small compared to the saturation intensity and becomes positive (slow light) when this intensity increases, before attaining a maximum that cannot exceed the population relaxation time. The analytical results are completed by numerical determinations of the shape of the transmitted pulses in the different regimes.

We study the delivery of few-cycle soliton-like pulses at 800 nm with gigawatt power or microjoule energy through a hollow-core kagome-lattice photonic crystal fiber over 1 m with preserved temporal and spectral shape. We show that with optimized pressure of the argon filling, 5 fs input pulses are compressed up to 2.5 fs after 20 cm and restore their shape after 1 m propagation.

A theoretical study of fully differential cross sections for the single ionization of ${\mathrm{CH}}_4$ by collisions with ${\mathrm{H}}^{+}$, ${\mathrm{He}}^{2+}$, and ${\mathrm{C}}^{6+}$ ions at energies in the order of MeV/amu is presented. We work in terms of the Born-3DW model, which considers a non-Coulomb central potential for the interaction of the active electron with the molecular core. Results obtained with the Born-3DW model are compared to those obtained with the Born-C3 model, which assumes this potential as purely Coulombic. The anisotropic potential of the ${\mathrm{CH}}_4$ molecule is smoothed through an angular integration, and results are averaged over all the possible orientations of the target molecule. Results for the lesser bound molecular orbitals (1T and 2A${}_1$) are presented and discussed for different projectile momentum transfers for the coplanar geometry.

We investigate the localized nonlinear matter waves in the two-component Bose-Einstein condensates with time- and space-modulated nonlinearities analytically and numerically. The similarity transformations are developed to solve the coupled Gross-Pitaevskii equations and two families of explicitly exact solutions are derived. Our results show that not only the attractive spatiotemporal inhomogeneous interactions but the repulsive ones support novel localized nonlinear matter waves in two-component Bose-Einstein condensates. The dynamics of these matter waves, including the breathing solitons, quasibreathing solitons, resonant solitons, and moving solitons, is discussed. We confirm the stability of the exact solutions by adding various initial stochastic noise and study the general cases of the interaction parameters numerically. We also provide the experimental parameters to produce these phenomena in future experiments.

Under the conditions of vectorial diffraction, an increase in refraction at the extremities of the lens rotates the incident polarization state which transfers energy from the initial state to the orthogonal transverse field and the longitudinal field, which is known as depolarization. Since the field is a vectorial field containing three polarization components, the theory for the degree of coherence is first extended to incorporate cross-correlation effects within these vectorial components which are calculated through a coherency matrix. The use of this matrix provides an insight into interesting correlation effects between copropagating vectorial fields such as the coupled modes (linear polarized modes) of the supercontinuum generated by a photonic crystal fiber. An investigation is presented on the coherence times for the supercontinuum field generated by cross coupling into the photonic crystal fiber. The coherence times under cross-coupling conditions show that the degree of coherence of the two coupled modes from the fiber are different, which is due to the differences in phase. For a supercontinuum with a linear polarization state, the coherence times along the $x$, $y$, and $z$ axes are different, with the most significant change occurring along the optical axis ($z$) where the coherence time changes by an order of magnitude when the numerical aperture is increased from 0.1 to 1.

We present an experimental study of electromagnetically induced transparency (EIT) in a $\Lambda$-type molecular lithium system. Copropagating beam geometry is utilized in order to minimize the residual Doppler width. A coupling laser power dependent study of the EIT feature is carried out. Our findings have been complemented by theoretical studies of open systems that trace the presence of EIT starting from the density-matrix equations. Numerical simulations have been performed and are in good agreement with the experimental results.

There are a number of mechanisms via which ions and electrons interact in a plasma, but by far one of the most important is through dielectronic recombination (DR). This is a resonant process through which an ion can capture a free electron and decrease its charge by one unit. Cross sections or strengths for this process are of vital importance in the modeling of hot plasmas and hence any advance in measurement or calculation procedures for obtaining DR data is of great importance. The work presented here examines the underlying basis of a newly developed method based on leaky mode ion extraction from an electron beam ion trap (EBIT), for relatively quickly obtaining DR strengths. The development attains to use the ratio of the leaked ions in adjacent charge states, i.e., the initial and final charge states of a DR process, to obtain the strengths preassuming that this ratio is the same as that of the ions remaining inside the EBIT trap. This work shows this assumption to be false. Hence DR strengths measured using leaked ions may be subject to error. But this work also reveals that the difference between this ratio and the one for trapped ions is insensitive to most of the experimental conditions, which means reliable DR strengths can still be obtained by leaky mode ion extraction if proper corrections are applied. An example is shown in this paper where after correction of some experimental results for DR strengths, obtained using the leaky mode, excellent agreement with corresponding theoretical calculations is obtained.

The quantum critical coupling conditions, where single-photon transmission goes to zero, for systems of a coupled ring resonator and atom in a one-dimensional single-mode waveguide are discussed. The quantum critical coupling conditions could be achieved even with nonvanishing intrinsic losses of the resonator and the atom, with intermode backscattering between the whispering-gallery modes of the ring resonator, and with time-reversal symmetry breaking. The energy transfer efficiency from the waveguide to the resonator and the atom is also discussed.

We exploit a novel approximation scheme to obtain a new and compact formula for the parameters underlying coherent-state control of the evolution of a pair of entangled two-level systems. It is appropriate for long times and for relatively strong external quantum control via coherent-state irradiation. We take account of both discrete-state and continuous-variable degrees of freedom. The formula predicts the relative heights of entanglement revivals and their timing and duration.