The complex scaling method is applied to study the resonances of a Dirac particle in a Morse potential. The applicability of the method is demonstrated with the results compared with the available data. It is shown that the present calculations in the nonrelativistic limit are in excellent agreement with the nonrelativistic calculations. Further, the dependence of the resonant parameters on the shape of the potential is checked with the sensitivity to the potential parameters analyzed. By comparing the energies and widths of the pseudospin doublets, well pseudospin symmetry is discovered in the present model. The relationship between the pseudospin symmetry and the shape of the potential is investigated by changing the Morse potential shaped by the dissociation energy, the equilibrium intermolecular distance, and the positive number controlling the decay length of the potential.

Torsion-rotation transitions in molecules exhibiting hindered internal rotation possess enhanced sensitivities to a variation of the proton-to-electron mass ratio. This enhancement occurs due to a cancellation of energies associated with the torsional and rotational degrees of freedom of the molecule. This effect occurs generally in every internal rotor molecule, but is exceptionally large in methanol. In this paper, we calculate the sensitivity coefficients of methyl mercaptan, which is the thiol analog of methanol. The obtained sensitivity coefficients in this molecule range from $K_{\mu }=-14.8$ to $+12.2$ for transitions with a lower-level excitation energy below 10 cm${}^{-1}$.

Aiming to remedy the incorrect asymptotic behavior of conventional semilocal exchange-correlation (XC) density functionals for finite systems, we propose an asymptotic correction scheme, wherein an exchange density functional whose functional derivative has the correct $(-1/r)$ asymptote can be directly added to any semilocal density functional. In contrast to semilocal approximations, our resulting exchange kernel in reciprocal space exhibits the desirable singularity of the type $O(-1/q^2)$ as $q\to 0$, which is a necessary feature for describing the excitonic effects in nonmetallic solids. By applying this scheme to a popular semilocal density functional, PBE [Perdew, Burke, and Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996)], the predictions of the properties that are sensitive to the asymptote are significantly improved, while the predictions of the properties that are insensitive to the asymptote remain essentially the same as PBE. Relative to the popular model XC potential scheme, our scheme is significantly superior for ground-state energies and related properties. In addition, without loss of accuracy, two closely related schemes are developed for the efficient treatment of large systems.

We report KLL dielectronic recombination (DR) resonance strength for hydrogenlike krypton ${\mathrm{Kr}}^{35+}$ measured with an electron-beam ion trap. X rays emitted from both DR and radiative recombination (RR) are observed as a function of electron-beam energy over the KLL resonances. The DR resonance strength is obtained by normalizing the DR x-ray counts to theoretical RR cross sections. The experimental strength is shown to agree with theoretical strength calculated using the flexible atomic code.

We demonstrate that an exotically chiral $f$-wave topological superfluid can be induced in cold-fermionic-atom triangular optical lattices through a laser-field-generated effective non-Abelian gauge field, controllable Zeeman fields, and $s$-wave Feshbach resonance. We find that the chiral $f$-wave topological superfluid is characterized by three gapless Majorana edge states located on the boundary of the system. More interestingly, these Majorana edge states degenerate into one Majorana fermion bound to each vortex in the superfluid. Our proposal enlarges the topological superfluid family and specifies a unique experimentally controllable system to study Majorana fermion physics.

We propose to use a time-dependent imaginary potential to describe quantum mechanical tunneling through time-varying potential barriers. We use Gamow solutions for stationary tunneling problems to justify our choice of potential, and we apply our method to describe tunneling of a mesoscopic quantum variable: the phase change across a Josephson junction. The Josephson junction phase variable behaves as the position coordinate of a particle moving in a tilted washboard potential, and our general solution to the motion in such a potential with a time-dependent tilt reproduces a number of features associated with voltage switching in Josephson junctions. Apart from applications as artificial atoms in quantum information studies, the Josephson junction may serve as an electric field sensitive detector, and our studies provide a detailed understanding of how the voltage switching dynamics couples to the electric field amplitude.

We study how catlike superpositions of generalized Heisenberg algebras (GHA) nonlinear coherent states behave under dissipative decoherence. Two cases are presented: the infinite square well potential and systems whose spectra, given by infinite strictly increasing sequences of nonnegative real numbers, can be considered perturbations of the harmonic oscillator. The decoherence effect caused by the interaction of these systems with a thermal bath is analyzed: from their fidelity behavior we see that a region always exists in the parameter space where the quantum coherence is better preserved as compared to the harmonic oscillator. Moreover, we show that the qualitative behavior of GHA systems under the studied mechanism of decoherence can be inferred from the algebraic structure via the analysis of their GHA characteristic functions.

Two rectangular models described by the one-dimensional Schrödinger equation with sharply localized potentials are suggested. The potentials have a multilayer thin structure being composed from adjacent barriers and wells. Their peculiar tunneling properties are studied in considerable detail. Particularly, in the zero-range limit when the potentials are squeezed to a single point, sharp peaks with total transmission are observed at certain (positive and negative) quantized values of the potential strength constant forming infinite discrete sets. Beyond these sets, the barrier-well structures behave as a perfectly reflecting wall. The transcendental equations with respect to potential strengths, the solutions of which determine transmission (resonance) sets, are derived. In this regard, both the models are exactly solvable. The energy dependence of an incident particle is shown to reveal a resonance behavior, being completely different from that observed in a typical double-barrier structure.

We introduce the use of entanglement entropy as a tool for studying the amount of information shared between the nodes of quantum complex networks. By considering the ground state of a network of coupled quantum harmonic oscillators, we compute the information that each node has on the rest of the system. We show that the nodes storing the largest amount of information are not the ones with the highest connectivity, but those with intermediate connectivity, thus breaking down the usual hierarchical picture of classical networks. We show both numerically and analytically that the mutual information characterizes the network topology. As a by-product, our results point out that the amount of information available for an external node connecting to a quantum network allows one to determine the network topology.

We investigate the shape resonances of the electron-hydrogen system embedded in Debye plasma environments modeled by screened Coulomb potentials. The complex-coordinate rotation method is employed to calculate the resonance parameters and the Hylleraas-type wave functions are used to represent the correlation effects of the two electrons. The ${}^1{S}^e$ resonance states of H${}^{-}$ above the $N=2$–5 thresholds of H atom are investigated and the resonance parameters for a Debye length ranging from infinity to small values are reported. It is shown that the widths of the shape resonances lying above the H($N=4$,5) thresholds exhibit some interesting behaviors in relatively strong screening situations. The possible reason for such phenomena is discussed.

We analyze low-energy scattering for arbitrary short-range interactions plus an attractive $1/r^6$ tail. We derive the constraints of causality and unitarity and find that the van der Waals length scale dominates over parameters characterizing the short-distance physics of the interaction. This separation of scales suggests a separate universality class for physics characterizing interactions with an attractive $1/r^6$ tail. We argue that a similar universality class exists for any attractive potential $1/r^{\alpha }$ for $\alpha ⩾2$. We also discuss the extension to multichannel systems near a magnetic Feshbach resonance. We discuss the implications for effective field theory with attractive singular power-law tails.

The use of femtosecond pulses produced by x-ray free-electron-laser (XFEL) sources to image the structures of biomolecules involves a competition between the elastic scattering of photons to form a diffraction pattern and the damage initiated by inelastic collisions with the target. Since the electron density of the biomolecule changes rapidly throughout its interaction with a femtosecond XFEL pulse, the diffraction process measured in “diffract and destroy” experiments is, at best, partially coherent. It has been established that a detailed knowledge of these electrodynamical processes may be used to ameliorate the effects of damage in diffractive imaging experiments. It is shown here that, subject to conventional assumptions about the nature of the interactions, it is possible to characterize the extent of electronic damage in biomolecular imaging experiments using XFELs and to use this information transferably across similar systems. We develop a physical model of the interaction of a coherent x-ray pulse with a molecular system that describes the dynamical electronic response of the molecule. The resulting insights open a way forward for the measurement of atomic processes in such systems.

Spin squeezing of collective atomic spins can be achieved conditionally via probing with light and subsequent homodyne detection, as is done in a quantum nondemolition measurement. Recently it has been shown that squeezing can also be created unconditionally by a properly designed dissipative dynamics. We compare the two approaches in a Gaussian description and optimize over all Gaussian light-matter interactions. We find that in the optimal unconditional scheme based on dissipation the level of squeezing scales with optical depth as $d^{-1/2}$. In contrast, the optimal conditional scheme based on measurement of light—which in fact is not a quantum nondemolition measurement—can provide squeezing which scales as $d^{-1}$. Our results apply directly also to the creation of entanglement in the form of nonlocal spin squeezing of two atomic ensembles.

Using coherent states, linear optics, and $N$-photon detection we demonstrate the synthesis of arbitrary interference patterns and establish that neither the shape nor the visibility of $N$-photon interference patterns can be used as a quantum signature in general. Specific examples include saw-curve and rectangle-curve interference patterns and phase super-resolution with period shortening of up to 60 times compared to ordinary interference. The former two have visibility close to 100$\%$ and the latter has visibility in excess of 57$\%$.

Spin squeezed states are a class of entangled states of spins that have practical applications to precision measurements. In recent years spin squeezing with one-axis twisting (OAT) has been demonstrated experimentally with spinor Bose-Einstein condensates (BECs) with more than ${10}^3$ atoms. Although the noise is below the standard quantum limit, the OAT scheme cannot reduce the noise down to the ultimate Heisenberg limit. Here we propose an experimentally feasible scheme based on optimized quantum control to greatly enhance the performance of OAT to approach the Heisenberg limit, requiring only an OAT Hamiltonian and the use of several coherent driving pulses. The scheme is robust against technical noise and can be readily implemented for spinor BECs or trapped ions with current technology.

Superposition in quantum states of large systems plays a crucial role in quantum theory and has important fundamental as well as technological implications, ranging from quantum measurement theory to quantum computers. The power of the potential implications of such states lies in the quantum coherence, as measured by the amount of entanglement and other quantum correlations, between its microscopic and macroscopic sectors. We show that in contrast to other macroscopic superposition states, it is possible to choose the states of the macroscopic sector in a way that the resulting state, which we term as the $H_C$ state, has quantum coherence that is resistant to the twin effects of environmental noise: (a) local decoherence on all the particles and (b) loss of a finite fraction of its particles. The states of the macroscopic sector of the $H_C$ state are macroscopically distinct in terms of their violation of Bell inequality.

A sequence of moments obtained from statistical trials encodes a classical probability distribution. However, it is well known that an incompatible set of moments arises in the quantum scenario, when correlation outcomes associated with measurements on spatially separated entangled states are considered. This feature, viz., the incompatibility of moments with a joint probability distribution, is reflected in the violation of Bell inequalities. Here, we focus on sequential measurements on a single quantum system and investigate if moments and joint probabilities are compatible with each other. By considering sequential measurement of a dichotomic dynamical observable at three different time intervals, we explicitly demonstrate that the moments and the probabilities are inconsistent with each other. Experimental results using a nuclear magnetic resonance system are reported here to corroborate these theoretical observations, viz., the incompatibility of the three-time joint probabilities with those extracted from the moment sequence when sequential measurements on a single-qubit system are considered.

Motivated by link transformations of lattice gauge theory, a method for generating local unitary invariants, especially for a system of qubits, has been pointed out in an earlier work [M. S. Williamson et al., Phys. Rev. A 83, 062308 (2011)]. This paper first points the equivalence of the so constructed transformations to the combined operations of partial transpose and realignment. This allows construction of local unitary invariants of any system, with subsystems of arbitrary dimensions. Some properties of the resulting operators and consequences for pure tripartite higher-dimensional states are briefly discussed.

The formation of superoxide O${}_2^{-}$ anions by collisions of energetic Ar${}^{+}$ ions with oxygen molecules at impact energies of 50–350 keV is reported. Cross sections are about 2 orders of magnitude smaller compared to O${}_2{}^{+}$ ion formation, which is attributed to the larger energy transfer required.