We use entropic uncertainty relations to formulate inequalities that witness Einstein-Podolsky-Rosen (EPR)-steering correlations in diverse quantum systems. We then use these inequalities to formulate symmetric EPR-steering inequalities using the mutual information. We explore the differing natures of the correlations captured by one-way and symmetric steering inequalities and examine the possibility of exclusive one-way steerability in two-qubit states. Furthermore, we show that steering inequalities can be extended to generalized positive operator-valued measures, and we also derive hybrid steering inequalities between alternate degrees of freedom.

We introduce a family of criteria to detect quantum non-Gaussian states of a harmonic oscillator, that is, quantum states that cannot be expressed as a convex mixture of Gaussian states. In particular, we prove that for convex mixtures of Gaussian states, the value of the Wigner function at the origin of phase space is bounded from below by a nonzero positive quantity, which is a function only of the average number of excitations (photons) of the state. As a consequence, if this bound is violated, then the quantum state must be quantum non-Gaussian. We show that this criterion can be further generalized by considering additional Gaussian operations on the state under examination. We then apply these criteria to various non-Gaussian states evolving in a noisy Gaussian channel, proving that the bounds are violated for high values of losses, and thus also for states characterized by a positive Wigner function.

We address the problem of how efficiently information can be encoded into and read out reliably from a passive reflective surface that encodes classical data by modulating the amplitude and phase of incident light. We show that nature imposes no fundamental upper limit to the number of bits that can be read per expended probe photon and demonstrate the quantum-information-theoretic trade-offs between the photon efficiency (bits per photon) and the encoding efficiency (bits per pixel) of optical reading. We show that with a coherent-state (ideal laser) source, an on-off (amplitude-modulation) pixel encoding, and shot-noise-limited direct detection (an overly optimistic model for commercial CD and DVD drives), the highest photon efficiency achievable in principle is about 0.5 bits read per transmitted photon. We then show that a coherent-state probe can read unlimited bits per photon when the receiver is allowed to make joint (inseparable) measurements on the reflected light from a large block of phase-modulated memory pixels. Finally, we show an example of a spatially entangled nonclassical light probe and a receiver design—constructible using a single-photon source, beam splitters, and single-photon detectors—that can in principle read any number of error-free bits of information. The probe is a single photon prepared in a uniform coherent superposition of multiple orthogonal spatial modes, i.e., a $W$ state. The code and joint-detection receiver complexity required by a coherent-state transmitter to achieve comparable photon efficiency performance is shown to be much higher in comparison to that required by the $W$-state transceiver, although this advantage rapidly disappears with increasing loss in the system.

The $R$-matrix approach is used to study the electron scattering from ${\text{PO}}_2$ radical at low electron impact energies. The elastic scattering phenomenon is studied in static-exchange, one-state and many-states close-coupling approximation. The elastic differential cross sections, corresponding momentum-transfer cross sections, and collision frequency are calculated in the one-state configuration interaction approximation only. Calculations reveal a stable bound state of ${\text{PO}}_2^{-}$ having symmetry ${}^1{A}_1$, a configuration of $\cdots 8a_1^2,\cdots 2b_1^2,\cdots 5b_2^2,\cdots 1a_2^2$, and vertical electron affinity of $2.94$ eV. The excited state of anion ${\text{PO}}_2^{-}$ having symmetry ${}^3{B}_1$ is also just bound relative to the ground state of ${\text{PO}}_2$ at its equilibrium geometry. The shape, core-excited, and Feshbach resonances are analyzed in different symmetries up to 7 eV. The partial waves up to $l=4$ are used to represent continuum electron. The converged cross sections are obtained for the partial waves having $l$ greater than 4 by applying Born correction. Certain interesting spectroscopic properties of radical are also reported.

We use the nonrelativistic random-phase approximation with exchange to perform calculations of valence-shell photoionization of Ne, Ar, Kr, and Xe from their respective thresholds to photon energy of 200 eV. The energy derivative of the complex phase of the photoionization matrix elements is converted to the photoelectron group delay that can be measured in attosecond streaking or two-photon sideband interference experiments. Comparison with reported time-delay measurements in Ne and Ar at a few selected photon energies is made. Systematic mapping of time delay across a wide range of photon energies in several atomic targets allows to highlight important aspects of fundamental atomic physics that can be probed by attosecond-time-delay measurements.

We calculated photoelectron energy and momentum spectra and the population of high Rydberg states of lithium atoms by intense 785 nm laser pulses at intensities in the over-the-barrier ionization (OBI) regime. The calculated spectra are compared to experiments reported in Schuricke et al. [Phys. Rev. A 83, 023413 (2011)]. It is shown that in the OBI regime, due to strong depletion of the ground state, the photoelectron spectra are generated from the leading edge of the laser pulse only, resulting in spectra that are nearly independent of laser intensities. Analysis of the calculated spectra reveals that total ionization probability is suppressed as the intensity is increased in the OBI regime. The suppression is due to the increase of excitation probability of high Rydberg states in the OBI regime, demonstrating that atoms and molecules are never fully ionized at high intensities. We also conclude that the interference stabilization model is not needed to explain the formation of high Rydberg states, and that there is no evidence that laser-dressed Kramers-Henneberger states play a role in the strong-field ionization of atoms and molecules by infrared lasers in the OBI regime.

We investigate properties of exotic pairing states in a three-dimensional Fermi gas with three-dimensional spin-orbit coupling and an effective Zeeman field. The interplay of spin-orbit coupling, effective Zeeman field, and pairing can lead to first-order phase transitions between different phases, and to interesting nodal superfluid states with gapless surfaces in the momentum space. We then demonstrate that pairing states with zero center-of-mass momentum are unstable against Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states, with a finite center-of-mass momentum opposite to the direction of the effective Zeeman field. Unlike conventional FFLO states, these FFLO states are induced by the coexistence of spin-orbit coupling and Fermi surface deformation, and have intriguing features like first-order transitions between different FFLO states, nodal FFLO states with gapless surfaces in momentum space, and exotic fully gapped FFLO states. With the recent theoretical proposals for realizing three-dimensional spin-orbit coupling in ultracold atomic gases, our work is helpful for future experimental studies and provides valuable information for the general understanding of pairing physics in spin-orbit-coupled fermionic systems.

A mechanism for creating a Newton's cradle (NC) in nonlinear light wave trains under the action of the third-order dispersion (TOD) is demonstrated. The formation of the NC structure plays an important role in the process of fission of higher-order ($N$) solitons in optical fibers. After the splitting of the initial $N$ soliton into a nonuniform chain of fundamental quasisolitons, the tallest one travels along the entire chain, through consecutive collisions with other solitons, and then escapes, while the remaining chain of pulses stays as a bound state, due to the radiation-mediated interaction between them. Increasing the initial soliton's order, $N$, leads to the transmission through, and release of additional solitons with enhanced power, along with the emission of radiation, which may demonstrate a broadband supercontinuum spectrum. The NC dynamical regime remains robust in the presence of extra perturbations, such as the Raman and self-steepening effects, and dispersion terms above the third order. It is demonstrated that essentially the same NC mechanism is induced by the TOD in finite segments of periodic wave trains (in particular, soliton chains). A difference from the mechanical NC is that the TOD-driven pulse passing through the soliton array collects energy and momentum from other solitons. Thus, uniform and nonuniform arrays of nonlinear wave pulses offer an essential extension of the mechanical NC, in which the quasiparticles, unlike mechanical beads, interact inelastically, exchanging energy and generating radiation. Nevertheless, the characteristic phenomenology of NC chains may be clearly identified in these nonlinear-wave settings too.

We investigate cooperative fluorescence in a dilute cloud of strongly driven two-level emitters. Starting from the Heisenberg equations of motion, we compute the first-order scattering corrections to the saturation of the excited-state population and to the resonance-fluorescence spectrum, which both require going beyond the state-of-the-art linear-optics approach to describe collective phenomena. A dipole blockade is observed due to long-range dipole-dipole coupling that vanishes at stronger driving fields. Furthermore, we compute the inelastic component of the light scattered by a cloud of many atoms and find that the Mollow triplet is affected by cooperativity. In a lobe around the forward direction, the inelastic Mollow triplet develops a spectral asymmetry, observable under experimental conditions.

The coefficient matrix is an efficient tool in entanglement classification under stochastic local operation and classical communication. In this work, we take all the ranks of the coefficient matrices into account in the method of entanglement classification, and give the entanglement classification procedure for arbitrary-dimensional multipartite pure states under stochastic local operation and classical communication. As a main application, we study the entanglement classification for quantum systems in the Hilbert space $\mathcal{H}={\mathbb{C}}^2\otimes {\mathbb{C}}^2\otimes {\mathbb{C}}^2\otimes {\mathbb{C}}^d$ in terms of the ranks of the coefficient matrices.

The NIST electron beam ion trap (EBIT) was used to measure the $D_1$(3$s$-3$p_{1/2}$) and $D_2$(3$s$-3$p_{3/2}$) transitions in Na-like ions of xenon, barium, samarium, gadolinium, dysprosium, erbium, tungsten, platinum, and bismuth. The wavelengths are in the range 3–12 nm. Relativistic many-body perturbation theory calculations were carried out for the $D_1$ and $D_2$ lines for every element in the isoelectronic sequence from argon ($Z$ $=$ 18) to uranium ($Z$ $=$ 92), taking into account some higher-order terms in the quantum electrodynamics (QED) expansion. Uncertainties in the calculated values were carefully assessed by considering the uncertainties in the various contributions to the total calculated transition energies. We conclude that at the current level of accuracy, the calculated values can be taken to reliably represent the isoelectronic sequence from $Z$ $=$ 18 to 92. The agreement of theory and experiment for the $D_1$ line of bismuth ($Z$ $=$ 83) provides a test of QED at the level of 0.4$\%$. Our results are also sensitive to retardation effects due to the finite speed of light and to variations in the assumed nuclear size.

We have measured the ratios of doubly to singly charged molecular parent ions of benzene (partially deuterated), naphthalene, anthracene, pentacene, pyrrole, furan, selenophene, and coronene for photon energies ranging from threshold to the carbon $K$ shell. The photon-energy dependence of the ratio curves has been analyzed and compared to each other for the above molecules. We conclude that two—and in some cases three—different photo-double-ionization mechanisms exist for aromatic hydrocarbons. One of the mechanisms is the formation of a two-electron pseudoparticle. This finding may be useful in the quest for understanding high-temperature superconductivity.

The ground state of a two-dimensional, harmonically confined mesoscopic assembly of up to 30 polar molecules is studied by computer simulations. As the strength of the confining trap is increased, clusters evolve from superfluid, to supersolid, to insulating crystals. For strong confinement, the crystalline structure can be predicted based on classical energetics. However, clusters of specific numbers of particles (i.e., $N=12$ and $N=19$) display a nonclassical crystalline structure, stabilized by quantum effects, in an intermediate range of confinement strength. In these cases, coexistence of quantum and classical crystalline configurations is observed at finite temperature.

We study a quantum ladder of interacting fermions with coupled $s$ and $p$ orbitals. Such a model describes dipolar molecules or atoms loaded into a double-well optical lattice, dipole moments being aligned by an external field. The two orbital components have distinct hoppings. The tunneling between them is equivalent to a partial Rashba spin-orbital coupling when the orbital space ($s$, $p$) is identified as spanned by pseudospin 1/2 states. A rich phase diagram, including incommensurate orbital density wave, pair density wave, and other exotic superconducting phases, is proposed with bosonization analysis. In particular, superconductivity is found in the repulsive regime.

Near-field electromagnetic trapping of particles is generally obtained by means of gradient forces. In this paper, we discuss the attractive behavior of curl-spin forces, as well as their potential for near-field electromagnetic trapping and manipulation. It is demonstrated that curl-spin forces enable the trapping of particles operating at their resonant frequency. Such phenomena can be exploited to design more efficient and selective electromagnetic traps, to boost near-field energy exchange systems, and to bring stability to coupled resonant radiators. It also is illustrated how the balance between the gradient, radiation pressure, and curl-spin force components leads to the formation of zero-force rings around their sources, which explicitly demarcate the trapping regions. Analytical and numerical analyses are presented to assess the stability of the trapping mechanism.

We show that open quantum systems of two-level atoms symmetrically coupled to a single-mode photon field can be efficiently simulated by applying a SU(4) group theory to quantum master equations. This is important since many foundational examples in quantum optics fall into this class. We demonstrate the method by finding exact solutions for many-atom open quantum systems such as lasing and steady-state superradiance.

The threshold between classical and nonclassical two-qubit states is drawn at the place when these states can no longer be described by classical correlations, i.e., quantum discord or entanglement appear. However, to check if the correlations are classical (in terms of quantum discord and entanglement) it is sufficient to witness the lack of quantum discord because its zero value implies the lack of entanglement. We explain how the indicator of quantum discord introduced by Girolami and Adesso [Phys. Rev. Lett. 108, 150403 (2012)] can be practically measured in linear-optical systems using standard beam splitters and photon detectors. We study the efficiency of the setup assuming both ideal and real components and show that the efficiency of the proposed implementation is better than the full two-photon quantum tomography. Thus, we demonstrate that a class of experiments previously available on NMR platform can be implemented in optical systems.

The geometric discord, as introduced by Dakić, Vedral, and Brukner [Phys. Rev. Lett. 105, 190502 (2010)], is a significant figure of merit for quantum correlations with wide applications. However, it has a curious drawback in that it may change reversibly by trivially adding a local ancilla, as recently criticized by Piani [Phys. Rev. A 86, 034101 (2012)]. This casts doubt on the information content and usage of geometric discord. In this work, we show that this problem with geometric discord can be remedied simply by starting from the square root of a density operator, rather than the density operator itself, in defining the discord. Moreover, most other basic properties and analytical aspects of the original geometric discord can be carried over to this modified discord. Due to the square-root character, which is reminiscent of probability amplitudes ubiquitous in quantum theory, the modified discord seems to capture quantum correlations in a more intrinsic and informational fashion.

We theoretically investigate a system of two coupled bosonic modes subject to both dissipation and external driving. We show that in the steady state the degree of entanglement between the coupled bosonic modes can be enhanced by dissipation. Nonmonotonic dependence of entanglement on the decay rates is observed when the bosonic modes are asymmetrically coupled to their local baths. This counterintuitive result opens a way to better understand the interplay between noise and coherence in continuous-variable systems driven away from equilibrium.

We show that the Helmholtz free-energy variational principle is the physical principle underlying the ensemble variational theory formulated in seminal papers by Theophilou and by Gross, Oliveira, and Kohn. A method of calculating electronic excitations of atoms and molecules is then proposed, based on the constrained minimization of the free energy. It involves the search for the optimal set of Slater determinant states to describe low electronic excitations and, in a second step, the search for optimal rotations in the space spanned by these states. Boltzmann factors are used as weights of states in the ensemble since for these the free energy achieves a minimum. The proposed method is applied to the Be atom and LiH and BH molecules. The method captures static electron correlation but naturally lacks dynamic correlation. To account for the latter, we describe short-range electron-electron interaction with a density functional, while the long-range part is still expressed by a wave-function method. Using the example of the LiH molecule, we find that the resulting method is able to capture both static and dynamic electron correlations.