This work presents accurate ab initio investigations of various spectroscopic properties of a few Li-like ions in the presence of a plasma environment within the Debye screening potential. The coupled-cluster theory in the relativistic framework has been employed to compute ionization potentials, excitation energies, electric dipole oscillator strengths, and electric quadrupole transition probabilities of Li-like C${}^{3+}$, N${}^{4+}$, and O${}^{5+}$ ions. The unretarded Breit interaction has been implemented to increase the accuracy of the calculations. The effects of ion density and temperature on the ionization potentials, excitation energies, electric dipole oscillator strengths, and electric quadrupole transition probabilities have been investigated in the plasma environment. It is found that the plasma screening leads to a sharp decrease in the ionization potential as the screening strength increases. With increasing strength, the oscillator strengths associated with 2$s$ ${}^2{S}_{1/2}\to 2p$ ${}^2{P}_{1/2,3/2}$ transitions increase, whereas the transition probabilities associated with 3$d$ ${}^2{D}_{3/2,5/2}\to 2s$ ${}^2{S}_{1/2}$ transitions decrease.

We solve the three-body bound-state problem in three dimensions for mass imbalanced systems of two identical bosons and a third particle in the universal limit where the interactions are assumed to be of zero range. The system displays the Efimov effect and we use the momentum-space wave equation to derive formulas for the scaling factor of the Efimov spectrum for any mass ratio assuming either that two or three of the two-body subsystems have a bound state at zero energy. We consider the single-particle momentum distribution analytically and numerically and analyze the tail of the momentum distribution to obtain the three-body contact parameter. Our findings demonstrate that the functional form of the three-body contact term depends on the mass ratio, and we obtain an analytic expression for this behavior. To exemplify our results, we consider mixtures of lithium with either two caesium or rubidium atoms which are systems of current experimental interest.

A moderately intense, nonresonant, linearly polarized nanosecond laser pulse is used to induce one-dimensional (1D) adiabatic alignment of 1,4 diiodobenzene (C${}_6$H${}_4$I${}_2$), iodobenzene (C${}_6$H${}_5$I), and methyliodide (CH${}_3$I) molecules dissolved in helium nanodroplets. The alignment sharpens as the laser intensity is increased, similar to the behavior in gas phase. For diiodobenzene the highest degree of alignment, $\langle {\cos }^2{\theta }_{2\mathrm{D}}\rangle =0.90$, is essentially identical to the value obtained for isolated molecules in a supersonic beam, whereas the highest degrees of alignment for iodobenzene and for methyliodide in helium droplets fall below the values obtained for isolated molecules. We believe this is due to the deviation from axial recoil in the Coulomb explosion process that probes the alignment of the molecules in the He droplets.

We investigate both theoretically and experimentally the effect of thermal motion of laser-cooled atoms on the coherence of Rabi oscillations induced by an inhomogeneous driving field. The experimental results are in excellent agreement with the derived analytical expressions. For freely falling atoms with negligible collisions, as those used in our experiment, we find that the amplitude of the Rabi oscillations decays with time $t$ as $\exp [-{(t/\tau )}^4]$, where the coherence time $\tau$ drops with increasing temperature and field gradient. We discuss the consequences of these results regarding the fidelity of Rabi rotations of atomic qubits. We also show that the process is equivalent to the loss of coherence of atoms undergoing a Ramsey sequence in the presence of static magnetic field gradients—a common situation in many applications. In addition, our results are relevant for determining the resolution when utilizing atoms as field probes. Using numerical calculations, our model can be easily extended to situations in which the atoms are confined by a potential or to situations where collisions are important.

We investigate the condensation of a two-dimensional homogeneous spin-orbit-coupled boson system at zero temperature. We prove that the condensate is stable although the spin-orbit coupling makes the momentum distribution of depletion more divergent in the infrared limit. The stability of the system depends solely on the infrared behavior of the density waves, while the spin waves play a nonessential role. The condensate fraction is a decreasing function of the ratio of spin-orbit coupling to the square root of the boson density. The peak of the momentum distribution of depletion stretches anisotropically when the strength of spin-orbit coupling increases.

We theoretically investigate how phase-only spatial light modulation can enable controlling and focusing the second-harmonic light generated in transparent nonlinear random structures. The studied structures are composed of domains with random sizes and antiparallel polarization, which accurately model widely used ferroelectric crystals such as strontium barium niobate. Using a first-principles Green-function formalism, we account for the effect that spatial light modulation of the fundamental beam introduces into the second-order nonlinear frequency conversion occurring in the considered class of structures. This approach provides a complete description of the physical origin of the second-harmonic light generation in the system, as well as the optimization of the light intensity in any arbitrary direction. Our numerical results show how the second-harmonic light is influenced by both the disorder in the structure and the boundaries of the crystal. Particularly, we find that the net result from the interplay between disorder and boundary effects is strongly dependent on the dimensions of the crystal and the observation direction. Remarkably, our calculations also show that although in general the maximum possible enhancement of the second-order light is the same as the one corresponding to linear light scattering in turbid media, in the Cerenkov phase matching direction the enhancement can exceed the linear limit. The theoretical analysis presented in this work expands the current understanding of light control in complex media and could contribute to the development of a new class of imaging and focusing techniques based on nonlinear frequency mixing in random optical materials.

A single rubidium atom trapped within a high-finesse optical cavity is an efficient source of single photons. We theoretically and experimentally study single-photon generation using a vacuum stimulated Raman adiabatic passage. We experimentally achieve photon generation efficiencies of up to 34$\%$ and 56$\%$ on the $D_1$ and $D_2$ line, respectively. Output coupling with 89$\%$ results in record-high efficiencies for single photons in one spatiotemporally well-defined propagating mode. We demonstrate that the observed generation efficiencies are constant in a wide range of applied pump laser powers and virtual level detunings. This allows for independent control over the frequency and wave packet envelope of the photons without loss in efficiency. In combination with the long trapping time of the atom in the cavity, our system constitutes a significant advancement toward an on-demand, highly efficient single-photon source for quantum information processing tasks.

We propose a general alternative spectroscopic approach that uses a feedback control of the input probe field parameters to stabilize the medium's response at a fixed constant level while taking the spectra. In this case the extractable spectroscopic information is contained in the variable input parameters. Along this line, we have found a universal and simple way, using the spontaneous emission for feedback control, to dramatically increase the amplitude, contrast, and quality factor of different atomic resonances. Our method, unlike the conventional spectroscopy, does not require an optically dense medium. Theoretical analysis has been experimentally confirmed with spectroscopy of the dark resonance in atomic rubidium vapor. The considerable increase (2 orders) of the resonance amplitude and a threefold decrease of the width have been observed in optically thin medium. As a result, the quality factor of the dark resonance is increased by 2 orders of magnitude and its contrast reaches a record level of 260$\%$. Different schemes, including magneto-optical Hanle spectroscopy and Doppler-free spectroscopy, have also shown an enhanced performance by using the proposed technique.

The one-shot success probability of a noisy classical channel for transmitting one classical bit is the optimal probability with which the bit can be successfully sent via a single use of the channel. Prevedel et al. [Phys. Rev. Lett. 106, 110505 (2011)] recently showed that for a specific channel, this quantity can be increased if the parties using the channel share an entangled quantum state. In this paper, we characterize the optimal entanglement-assisted protocols in terms of the radius of a set of operators associated with the channel. This characterization can be used to construct optimal entanglement-assisted protocols for a given classical channel and to prove the limits of such protocols. As an example, we show that the Prevedel et al. protocol is optimal for two-qubit entanglement. We also prove some tight upper bounds on the improvement that can be obtained from quantum and nonsignaling correlations.

In this article, by treating minimum error state discrimination as a complementarity problem, we obtain the geometric optimality conditions. These can be used as the necessary and sufficient conditions to determine whether every optimal measurement operator can be nonzero. Using these conditions and an inductive approach, we demonstrate a geometric method and the intrinsic polytope for $N$-qubit mixed-state discrimination. When the intrinsic polytope becomes a point, a line segment, or a triangle, the guessing probability, the necessary and sufficient condition for the exact solution, and the optimal measurement are analytically obtained. We apply this result to the problem of discrimination to arbitrary three-qubit mixed states with given a priori probabilities and obtain the complete analytic solution to the guessing probability and optimal measurement.

In the present article we report a comprehensive calculation of total and differential cross sections for electron impact on allene (C${}_3$H${}_4$) molecule. The total cross sections are computed for a wide electron energy range from 0.1 eV to 2 keV. We have employed $R$-matrix code through quantemol-n software for ab initio calculations below 10 eV while intermediate- to high-energy calculations are performed using the spherical complex optical potential formalism. The two methods are found to be consistent at around 3 eV, merging smoothly. The results for both total and differential cross sections are in good agreement with previous results wherever available. We have also observed the presence of a shape resonance at 2.9 eV due to degenerate (${}^2$$B_1$, ${}^2$$B_2$) states. The electronic excitation cross section for $e$-C${}_3$H${}_4$ scattering is reported.

We apply the classical over-the-barrier (COB) model to charge transfer between highly charged ions (HCIs) and targets consisting of thin dielectric films on metals. Distances for the onset of classically allowed above-surface electron capture are obtained as a function of HCI charge state, film thickness, film permittivity, and film-metal electron binding energies. The model describes the crossover between the previously existing COB model for bulk metals and bulk dielectrics as the thickness of a dielectric film on a metal substrate increases through three distinct regimes. For ultrathin films with low permittivity and positive electron affinity, over-the-barrier charge transfer initiates from the metal electrons behind the film and critical distances are greater than those from bare metal targets. This result is consistent and compared with the recent observation of potential emission enhancement above thin C${}_{60}$ films on Au(111) with increasing film thickness [Bodewits, Hoekstra, Kowarik, Dobes, and Aumayr, Phys. Rev. A 84, 042901 (2011)].

We report on experimental investigations of longitudinal collective oscillations in a highly elongated, harmonically trapped two-component Fermi gas with resonantly tuned $s$-wave interactions (“unitary Fermi gas”). We focus on higher-nodal axial modes, which in contrast to the elementary modes have received little attention so far. We show how these modes can be efficiently excited using a resonant local excitation scheme and sensitively analyzed by a Fourier transformation of the detected time evolution of the axial density profile. We study the temperature dependence of the mode frequencies across the superfluid phase transition. The behavior is qualitatively different from the elementary modes, where the mode frequencies are independent of the temperature as long as the gas stays in the hydrodynamic regime. Our results are compared to theoretical predictions based on Landau's two-fluid theory and available experimental knowledge of the equation of state. The comparison shows excellent agreement and thus both represents a sensitive test for the validity of the theoretical approach and provides an independent test of the equation of state. The present results obtained on modes of first-sound character represent benchmarks for the observation of second-sound propagation and corresponding oscillation modes.

We address the important question of how to characterize the normal state and the nature of noncondensed pairs in fermionic superfluids under the influence of a strong effective magnetic field. In ultracold gases, the magnetic field is implemented through rapid rotation or novel artificial field techniques. We consider the near-unitary regime, where noncondensed pairs are likely to be present at temperatures above the condensation temperature $T_c$. We show (based on Gor'kov theory) that these pairs are associated with a precursor of a vortex configuration. Importantly, this nonuniform normal state appears to enable “Bose condensation” in a field which is otherwise problematic due to the effective one-dimensionality of Landau-level dispersion.

We demonstrate a Hong-Ou-Mandel interference between two independent, intrinsically pure, heralded single photons from spontaneous parametric down-conversion (SPDC) at telecommunication wavelength. A visibility of $85.5\pm 8.3\%$ was achieved without using any bandpass filter. Thanks to the group-velocity-matched SPDC and superconducting nanowire single-photon detectors (SNSPDs), the fourfold coincidence counts are one order higher than that in the previous experiments. The combination of bright single-photon sources and SNSPDs is a crucial step for future practical quantum infocommunication systems at telecommunication wavelength.

We study numerically and experimentally the highly nonlinear dynamical regime (far above the primary instability threshold) of a one-dimensional spatially extended feedback system. The spatiotemporal dynamics is very complex and is characterized by the emission of a spatial frequency supercontinuum accompanied by the appearance of abnormally intense localized peaks in transverse patterns. We perform statistical analysis of this highly nonlinear regime in terms of the probability density function (PDF) of the peak intensities rather than usual tools such as correlation functions. We find that the statistics of these peak intensities is described very well by the generalized gamma (GG) probability density function and determine its three parameters which can be used as quantitative indicators of the transition from the weakly to the highly nonlinear regime. Most interestingly, we discover that in the highly nonlinear regime the GG PDF converges to the gamma probability density function with the shape parameter equal to 3/2. This limit corresponds to the Rayleigh probability density function of the peak amplitudes for the oceanic waves. This behavior of the PDF can be an indicator of the universality of the highly nonlinear regime for other processes involving supercontinua and chaos.

We found the self-similar solitary solutions of a recently proposed model for the propagation of pulses in gas-filled hollow-core photonic crystal fibers that includes a plasma induced nonlinearity. As anticipated for a simpler model and using a perturbation analysis, there are indeed stationary solitary waves that accelerate and self-shift to higher frequencies. However, if the plasma nonlinearity strength is large or the pulse amplitudes are small, the solutions have distinguished long tails and decay as they propagate.

We study dipolar relaxation of a chromium Bose-Einstein condensate loaded into a three-dimensional (3D) optical lattice. We observe dipolar relaxation resonances when the magnetic energy released during the inelastic collision matches an excitation towards higher-energy bands. Spectroscopy of these resonances for two orientations of the magnetic field provides a 3D band spectroscopy of the lattice. The narrowest resonance is registered for the lowest excitation energy. Its line shape is sensitive to the on-site interaction energy. We use such sensitivity to probe number squeezing in a Mott insulator.

We study systematically the non-Markovian decoherence dynamics of a dissipative two-level system, i.e., the so-called spin-boson model. It is interesting to find that the decoherence tends to be inhibited with the increase of the coupling strength between the system and the reservoir, which is contrary to the common recognition that a stronger coupling always induces a severer decoherence. This is attributed to the occurrence of a quantum phase transition (QPT). The relationship between this QPT and conventional delocalized-localized QPT is also discussed. Our result suggests a useful control method to overcome the detrimental effects of the reservoir to the system.

Whenever we do not have an informationally complete set of measurements, the estimate of a quantum state cannot be uniquely determined. In this case, among the density matrices compatible with the available data, the one commonly preferred is the one which is the most uncommitted to the missing information. This is the purpose of the maximum entropy estimation (MaxEnt) and the variational quantum tomography (VQT). Here, we propose a variant of VQT and show its relationship with MaxEnt methods in quantum tomographies with an incomplete set of measurements. We prove their equivalence in the case of eigenbasis measurements, and through numerical simulations we stress their similar behavior. Hence, in the modified VQT formulation we have an estimate of a quantum state as unbiased as in MaxEnt and with the benefit that VQT can be more efficiently solved by means of linear semidefinite programs.