One of the many interesting features of quantum nonlocality is that the states of a multipartite quantum system cannot always be distinguished as well by local measurements as they can when all quantum measurements are allowed. In this work, we characterize the distinguishability of sets of multipartite quantum states when restricted to separable measurements, those which contain the class of local measurements but nevertheless are free of entanglement between the component systems. We consider two quantities: the separable fidelity, a truly quantum quantity, which measures how well we can “clone” the input state, and the classical probability of success, which simply gives the optimal probability of identifying the state correctly. We obtain lower and upper bounds on the separable fidelity and give several examples in the bipartite and multipartite settings where these bounds are optimal. Moreover the optimal values in these cases can be attained by local measurements. We further show that for distinguishing orthogonal states under separable measurements, a strategy that maximizes the probability of success is also optimal for separable fidelity. We point out that the equality of fidelity and success probability does not depend on an using the optimal strategy, only on the orthogonality of the states. To illustrate this, we present an example where two sets (one consisting of orthogonal states and the other nonorthogonal states) are shown to have the same separable fidelity even though the success probabilities are different.

A model approach is derived for estimates of cross sections and rate coefficients in low-energy inelastic collisions of hydrogen atoms and negative ions with other atoms and positive ions, which are of astrophysical interests. The approach is based on the asymptotic method for electronic molecular structure determination and on the branching probability current method for a nonadiabatic nuclear dynamical treatment. The derived approach is applied to low-energy Al + H and Al${}^{+}$ + H${}^{-}$ inelastic collisions. It is shown that the processes with the largest values of cross sections and rates are the excitation and de-excitation ones between the Al($3d$) and Al($4p$) states in collisions with H, as well as the ion-pair formation and the mutual neutralization processes between these states and the ionic state; the second largest cross sections correspond to the similar processes involving the Al($4s$) state. The mechanisms of the processes are discussed in detail.

We study finite-temperature properties of a two-dimensional superfluid made of ultracold alkali-metal atoms in the BCS-BEC crossover. We investigate the region below the critical temperature $T_{\mathrm{BKT}}$ of the Berezinskii-Kosterlitz-Thouless phase transition where there is quasicondensation by analyzing the effects of phase and amplitude fluctuations of the order parameter. In particular, we calculate the superfluid fraction, the sound velocity, and the quasicondensate fraction as a function of the temperature and of the binding energy of fermionic pairs.

We consider an optomechanical cavity made by two moving mirrors which contain a Kerr-down-conversion nonlinear crystal. We show that the coherent oscillations of the two mechanical oscillators can lead to splitting in the electromagnetically induced transparency (EIT) resonance and the appearance of an absorption peak within the transparency window. In this configuration, the coherent-induced splitting of EIT is similar to driving a hyperfine transition in an atomic $\Lambda$-type three-level system by a radio-frequency or microwave field. Also, we show that the presence of nonlinearity provides an additional flexibility for adjusting the width of the transparency windows. The combination of an additional mechanical mode and the nonlinear crystal suggests new possibilities for adjusting the resonance frequency, the width and the spectral positions of the EIT windows, as well as the enhancement of the absorption peak within the transparency window.

We determine how a system composed of two nonidentical two-level atoms with different resonance frequencies and different damping rates could work as a nanoantenna for controlled mode switching and light routing. We calculate the angular distribution of the emitted field detected in a far-field zone of the system including the direct interatomic interactions and arbitrary linear dimensions of the system. The calculation is carried out in terms of the symmetric and antisymmetric modes of the two-atom system. We find that as long as the atoms are identical, the emission cannot be switched between the symmetric and antisymmetric modes. The switching may occur when the atoms are nonidentical and the emission can then be routed to different modes by changing the relative ratio of the atomic frequencies, or damping rates, or by a proper tuning of the laser frequency to the atomic resonance frequencies. It is shown that in the case of atoms of different resonance frequencies but equal damping rates, the light routing is independent of the frequency of the driving laser field. It depends only on the sign of the detuning between the atomic resonance frequencies. In the case of atoms of different damping rates, the emission can be switched between different modes by changing the laser frequency from the blue to red detuned from the atomic resonance. The effect of the interatomic interactions is also considered, and it is found that in the case of unequal resonance frequencies of the atoms, the interactions slightly modify the visibility of the intensity pattern. The case of unequal damping rates of the atoms is affected rather more drastically, the light routing becoming asymmetric under the dipole-dipole interaction with the enhanced intensities of the modes turned towards the atom of smaller damping rate.

We explore theoretically the bistable behavior of the intracavity photon number in a two-mode cavity optomechanical system, where two cavity modes are coupled to a common mechanical resonator. When the two cavity modes are driven by two pump laser beams, respectively, we find that the optical bistability can be controlled by tuning the power and the frequency of the pump beams. The common interaction with a mechanical mode enables one to control the bistable behavior in one cavity by adjusting the pump laser beam driving another cavity. We also show theoretically that both branches of optical bistability at photon numbers below unity can exist in this two-mode optomechanical system. This phenomenon can find potential applications in a controllable optical switch.

We extend the theory of Kapitza stabilization within the complex domain, i.e., for the case of an imaginary oscillating potential. At a high oscillation frequency, the quasienergy spectrum is found to be entirely real valued; however, a substantial difference with respect to a real potential emerges, that is, the formation of a truly bound state instead of a resonance. The predictions of the Kapitza averaging method and the transition from a complex to an entirely real-valued quasienergy spectrum at high frequencies are confirmed by numerical simulations of the Schrödinger equation for an oscillating Gaussian potential. An application and a physical implementation of the imaginary Kapitza pendulum to the stability of optical resonators with variable reflectivity is discussed.

In order to achieve practical implementations of emerging quantum technologies, it is important to have a firm understanding of the dynamics of realistic quantum open systems. Master equations provide a widely used tool in this regard. In this work we first construct a master equation, valid for weak system-environment coupling, which explicitly takes into account the impact of preparing an initial system state from an equilibrium system-environment state that has system-environment correlations. We then investigate the role of initial system-environment correlations using this master equation for a system consisting of many two-level atoms interacting with a common environment. We show that, in general, due to the initial system-environment correlations before a state preparation, the quantum state of the system can evolve at a faster time scale. Moreover, we also consider different initial state preparations, and demonstrate that the influence of state preparations depends on the initial states prepared. Our results can be of interest to many topics based on quantum open systems where system-environment correlation effects have been neglected before.

We demonstrate that the Wigner function of a pure quantum state is a wave function in a specially tuned Dirac bra-ket formalism and argue that the Wigner function is in fact a probability amplitude for the quantum particle to be at a certain point of the classical phase space. Additionally, we establish that in the classical limit, the Wigner function transforms into a classical Koopman–von Neumann wave function rather than into a classical probability distribution. Since probability amplitude need not be positive, our findings provide an alternative outlook on the Wigner function's negativity.

Resonant scattering of a photon by an electron in the presence of the moderately strong field of the circularly polarized pulsed laser wave is studied theoretically. The approximation used when a laser pulse duration is significantly greater than the characteristic oscillation time. The probability of such process for the moderately strong field $I\sim {10}^{17}$–${10}^{18}\mathrm{W}{\mathrm{cm}}^{-2}$ is calculated. It is demonstrated that the resonant probability can be significantly greater than the probability of the Compton effect in the absence of the external field. The results may be experimentally verified, for example, in the FAIR project.

In the framework of cavity QED, we propose a quantum repeater scheme that uses coherent light and atoms coupled to optical cavities. In contrast to conventional schemes, we exploit solely the cavity QED evolution for the entire quantum repeater scheme and, thus, avoid any explicit execution of quantum logical gates. The entanglement distribution between the repeater nodes is realized with the help of pulses of coherent light interacting with the atom-cavity system in each repeater node. In our previous paper [D. Gonta and P. van Loock, Phys. Rev. A 86, 052312 (2012)], we already proposed a dynamical protocol to purify a bipartite entangled state using the evolution of atomic chains coupled to optical cavities. Here, we incorporate parts of this protocol in our repeater scheme, combining it with dynamical versions of entanglement distribution and swapping.

The embedding of the $n$-qubit space into the $n$-fermion space with $2n$ modes is a widely used method in studying various aspects of these systems. This simple mapping raises a crucial question: Does the embedding preserve the entanglement structure? It is known that the answer is affirmative for $n=2$ and $n=3$. That is, under either local unitary (LU) operations or with respect to stochastic local operations and classical communication (SLOCC), there is a one-to-one correspondence between the two- (or three)-qubit orbits and the two- (or three)-fermion orbits with four (or six) modes. However, these results do not generalize as the mapping from the $n$-qubit orbits to the $n$-fermion orbits with $2n$ modes is no longer surjective for $n>3$. Here we consider the case of $n=4$. We show that, surprisingly, the mapping of orbits from qubits to fermions is injective under SLOCC, and a similar result holds under LU for generic orbits. As a by-product, we obtain a complete answer to the problem of SLOCC equivalence of pure four-qubit states.

We propose a protocol by which entanglement can be extracted repeatedly from a quantum field. In analogy with prior work on entanglement harvesting, we call this protocol entanglement farming. It consists of successively sending pairs of unentangled particles through an optical cavity. Using nonperturbative Gaussian methods, we show that in certain generic circumstances this protocol drives the cavity field towards a nonthermal metastable state. This state of the cavity is such that successive pairs of unentangled particles sent through the cavity will reliably emerge significantly entangled. We calculate thermodynamic aspects of the harvesting process, such as energies and entropies, and also the long-term behavior beyond the few-mode approximation. Significant for possible experimental realizations is the fact that this entangling fixed point state of the cavity is reached largely independently of the initial state in which the cavity was prepared. Our results suggest that reliable entanglement farming on the basis of such a fixed point state should be possible also in various other experimental settings, namely with the to-be-entangled particles replaced with arbitrary qudits and with the cavity replaced with a suitable reservoir system.

We address the issue of dephasing effects in flying polarization qubits propagating through optical fiber by using the method of dynamical decoupling. The control pulses are implemented with half-wave plates suitably placed along the realistic lengths of the single-mode optical fiber. The effects of the finite widths of the wave plates on the polarization rotation are modeled using tailored refractive index profiles inside the wave plates. We show that dynamical decoupling is effective in preserving the input qubit state with the fidelity close to unity when the polarization qubit is subject to the random birefringent noise in the fiber, as well the rotational imperfections (flip-angle errors) due to the finite width of the wave plates.

Electric-field gradients at bismuth nuclei in BiN and BiP molecules were determined from relativistic Dirac-Coulomb calculations by means of the coupled cluster approach with single and double substitutions with perturbative triple excitation corrections CCSD(T) and CCSD-T approaches. These theoretical values were combined with accurate experimental nuclear quadrupole coupling constants in order to obtain the nuclear electric quadrupole moment (NQM) of the ${}^{209}$Bi nucleus. Our best results indicate an NQM of −420(8) mb, which considerably deviates from the previously accepted standard value given by the atomic method −516(15) mb.

A detailed investigation of the $L{\alpha }_{1,2}$ ($L_3\to M_{4,5}$) and $L{\beta }_1$ ($L_2\to M_4$) x-ray satellite and hypersatellite structures in zirconium, molybdenum, and palladium atoms multiply ionized by impact with 278.6-MeV oxygen ions is reported. The x-ray spectra were measured with a high-resolution von Hamos bent crystal spectrometer. For the interpretation of the complex spectral features, relativistic multiconfiguration Dirac-Fock calculations were performed for all multivacancy configurations expected to contribute to the observed spectra. The data analysis clearly demonstrates that the spectra are dominated by structures originating from ($L^{-1}{M}^{-m}{N}^{-n}$) satellite and ($L^{-2}{M}^{-m}{N}^{-n}$) hypersatellite transitions corresponding to the radiative decay of the excited multivacancy configurations. The ionization probabilities of the $L$ and $M$ shell were determined from the data and compared with theoretical predictions from the geometrical model and the semiclassical approximation, using in the latter case both relativistic hydrogenlike and self-consistent Dirac-Hartree-Fock wave functions. The results support the independent electron picture of the multiple ionization. They also show the importance of using relativistic and self-consistent electronic wave functions for the $L$ and $M$ shells.

We report on high-resolution measurements of the photoinduced $L_1{M}_2$ and $L_1{M}_3$ x-ray emission lines of ${}_{69}$Tm, ${}_{70}$Yb, ${}_{71}$Lu, ${}_{73}$Ta, ${}_{74}$W, ${}_{75}$Re, ${}_{77}$Ir, ${}_{81}$Tl, ${}_{83}$Bi, and ${}_{95}$Am. From the linewidths of the measured transitions an accurate set of $M_2$ and $M_3$ level widths is determined assuming for the $L_1$ level widths the values reported by Raboud [P.-A. Raboud $et$ $al.$, Phys. Rev. A 65, 022512 (2002)]. Furthermore, the present experimental $M_{2,3}$ data set is extended to ${}_{80}$Hg, ${}_{90}$Th, and ${}_{92}$U, using former $L_1{M}_{2,3}$ high-resolution x-ray emission spectroscopy measurements performed by our group. A detailed comparison of the $M_2$ and $M_3$ level widths determined in the present work with those recommended by Campbell and Papp [J. L. Campbell and T. Papp, At. Data Nucl. Data Tables 77, 1 (2001)] and other available experimental data as well as theoretical predictions is done. The observed abrupt changes of the $M_{2,3}$ level widths versus atomic number $Z$ can be explained satisfactorily by the cutoffs and onsets of the $M_2{M}_4{N}_1$, respectively $M_3{M}_4{N}_{3,4,5}$ and $M_3{M}_5{N}_{2,3}$ Coster-Kronig transitions deduced from the semiempirical $(Z+1)$ approximation. As a spin-off result of this study, precise $L_1{M}_2$ and $L_1{M}_3$ transition energies are obtained for the investigated elements. A very good agreement with transition energies calculated within the many-body perturbation theory is found.

We theoretically study the quantum dynamics of transverse vibrations of a one-dimensional chain of trapped ions in harmonic potentials interacting via a Reggeon-type cubic nonlinearity that is nonunitary but preserves $\mathcal{P}\mathcal{T}$ symmetry. We propose the notion of quantum fragility for the dissipative structural phase transition that spontaneously breaks the $\mathcal{P}\mathcal{T}$ symmetry. In the quantum fragile regime, the nonlinearity dominates the response to mechanical perturbations and the chain supports neither the ordinary quantum phonons of a Luttinger liquid, nor the supersonic solitons that arise in classical fragile critical points in the absence of fluctuations. Quantum fluctuations, approximately captured within a one-loop renormalization group, give rise to mechanical excitations with a strongly momentum-dependent phonon velocity and dissipative spectral behavior. Observable signatures of the quantum fragile chain in trapped ion systems are discussed.

We show how additional constraints, restricting the spectrum of the optimized pulse or confining the system dynamics, can be used to steer optimization in quantum control towards distinct solutions. Our examples are multiphoton excitation in atoms and vibrational population transfer in molecules. We show that a spectral constraint is most effective in enforcing nonresonant two-photon absorption pathways in atoms and avoiding unnecessarily broad spectra in Raman transitions in molecules. While a constraint restricting the system to stay in an allowed subspace is also capable of identifying nonresonant excitation pathways, it does not avoid spurious peaks in the pulse spectrum. Both constraints are compatible with monotonic convergence but imply different additional numerical costs.

A collinear beam apparatus has been used to study photodetachment of Cs${}^{-}$. Partial cross sections were measured using state-selective detection based on a resonance ionization scheme. Several resonances were observed in the Cs($10{}^{2}S$), Cs($6{}^{2}F$), Cs($6{}^{2}G$), and Cs($6{}^{2}H$) final-state channels below the Cs($10{}^{2}P$${}_{3/2}$) channel opening. A model was developed to account for the interference between overlapping resonances. This model is essentially a generalization of the Fano [U. Fano, Phys. Rev. 124, 1866 (1961)] and Shore [B. W. Shore, Phys. Rev. 171, 43 (1968)] parametrizations for single resonances. Resonance parameters were extracted by fitting the model to the data sets. The openings of the Cs($10{}^{2}S$) and Cs($6{}^{2}F$) channels, where the polarizabilities of the atomic states are large and positive, showed rapid onsets. In the case of the Cs($6{}^{2}G$) and Cs($6{}^{2}H$) channels, on the other hand, the photodetachment cross sections increased slowly with energy. For the Cs($6{}^{2}H$) channel this is the expected behavior, since it is the result of a large and negative polarizability of the $6{}^{2}H$ state. In addition, the excitation of the Cs($6{}^{2}H$) state with respect to the Cs ground state was found to be 28 356.3(2) ${\mathrm{cm}}^{-1}$, in agreement with a previous experiment.