Monogamy is a nonclassical property that restricts the sharability of quantum correlation among the constituents of a multipartite quantum system. Quantum correlations may satisfy or violate monogamy for quantum states, which was tested mainly for three-qubit states. Here we establish a sufficient condition for monogamy of arbitrary quantum correlation measures of states of an arbitrary number of parties, using which and further numerical results, we obtain evidence for monogamy of measures such as distillable entanglement and relative entropy of entanglement, which are physically important but mathematically intractable, for almost all quantum states of a moderate number of parties. The result is generic and holds for a large class of quantum correlation measures. Nonetheless, we identify important zero Haar measure classes of pure states that remain nonmonogamous with respect to quantum discord and quantum work deficit, irrespective of the number of qubits.

Boson sampling has been presented as a simplified model for linear optical quantum computing. In the boson-sampling model, Fock states are passed through a linear optics network and sampled via number-resolved photodetection. It has been shown that this sampling problem likely cannot be efficiently classically simulated. This raises the question as to whether there are other quantum states of light for which the equivalent sampling problem is also computationally hard. We present evidence, without using a full complexity proof, that a very broad class of quantum states of light—arbitrary superpositions of two or more coherent states—when evolved via passive linear optics and sampled with number-resolved photodetection, likely implements a classically hard sampling problem.

When manipulating a quantum system $S$, its surrounding system, or environment, $E$ induces unwanted effects. It is mainly due to its vastness and the lack of knowledge about the Hamiltonian $H_{SE}$ that governs the dynamics inside $E$ and the interaction with $S$. The detail of $H_{SE}$ is usually extremely hard to identify, since $E$ can hardly be measured or controlled directly. Nevertheless, here we show that it is possible to probe and control a part of, if not all, the dynamics involving $E$, within the time scale in which its effective dimension can be seen to be finite. That is, we may be able to let a noisy environment work in our favor as a part of quantum computer.

We found a scheme for teleporting probabilistically an unknown pure state with optimal probability and without losing the information of the state to be teleported. Accordingly, without having to have copies of the unknown state, the teleportation process can be repeated as many times as one has available quantum channels. Thus, although the quantum channels have a weak entanglement, teleportation is achievable with a high number of repetitions, whereas for channels with strong entanglement only a small number of repetitions are required to guarantee successful teleportation.

We consider the propagation of two photonic qubits, initially maximally entangled in their orbital angular momenta (OAM), across a weakly turbulent atmosphere. By introducing the phase correlation length of an OAM beam, we show that the photonic entanglement exhibits a universal exponential decay.

Single and double Auger processes following ionization of $4f$ and $5p$ inner shells have been studied using multielectron coincidence spectroscopy. Coincidence technique enables us to resolve state by state all single and double Auger paths with a resolution better than the lifetime broadening. Drastic step-to-step decay lifetime changes are observed and reported as Coster-Kronig transition takes place either in the first $\left(5p\right)$ or in the second $\left(4f\right)$ step of the Auger cascade. Relativistic ab initio theory has been used to predict and interpret the experimental observations.

We derive a hypervirial relation for the positronium ion, a three-body bound state of two electrons and a positron. It relates expectation values of three operators and reconciles two recently published, seemingly inequivalent formulas for the magnetic moment of the ion. As a by-product, the precision of the leading binding correction is improved.

The determination of the proton charge radius by different methods has produced an inconsistency. The most precise value (from the spectroscopy of muonic H) strongly disagrees with three less-accurate values (from the spectroscopy of ordinary hydrogen and deuterium, from relative measurements of the cross section of the elastic electron-proton scattering at MAMI and from evaluation of the world data on absolute measurements of $e\text{−}p$ cross sections). Here, we question the accuracy of the determination of the proton charge radius by means of the spectroscopy of ordinary hydrogen and deuterium and demonstrate that its accuracy was probably overestimated. In particular, we revisit the determination from each relevant transition and find that the results of two optical experiments, which are the most statistically important, are not perfectly consistent. The inconsistency is rather a “tension” between the results than their discrepancy, however, it implies that a more conservative estimation of the uncertainty is needed. With the more realistic estimation of the uncertainty, the results for the proton charge radius from the spectroscopy of ordinary and muonic atoms are rather in fair agreement.

Modifications of Casimir-Polder interactions due to confinement inside a cylindrical cavity and due to curvature in- and outside the cavity are studied. We consider a perfectly conducting cylindrical shell with a single particle (atom or macroscopic sphere) located next to its interior or exterior surface or two atoms placed inside the shell. By employing the scattering approach, we obtain the particle-cavity interaction and the modification of the two-particle interaction due to the cavity. We consider both retardation and thermal effects. While for the atoms a dipole description is sufficient, for the macroscopic sphere we sum (numerically) over many multipole fluctuations to compute the interaction at short separations. In the latter limit we make comparisons to the proximity approximation and a gradient expansion and find agreement. Our results indicate a confinement-induced suppression of the force between atoms. General criteria for suppression and enhancement of Casimir interactions due to confinement are discussed.

We report a joint theoretical-experimental investigation of elastic electron scattering by dimethyl sulfide in the low- and intermediate-energy regions. More specifically, experimental differential, integral, and momentum-transfer cross sections are given in the 30–800 eV and ${10}^{\circ }-{130}^{\circ }$ ranges. Theoretical cross sections are reported in the 1–500 eV interval. The experimental differential cross sections were determined using a crossed electron-beam–molecular-beam geometry, whereas the absolute values of the cross sections were obtained using the relative-flow technique. Theoretically, a complex optical potential was used to represent the collision dynamics, and a single-center expansion method combined with the Padé approximant method was used to solve the scattering equations. Our experimental data are in good agreement with the present calculated data but strongly disagree with those reported in a previous investigation.

Transmission experiments of $7–200$ keV ${\mathrm{H}}_2{}^{+}$ ions through conical multicapillaries with inlet or outlet diameters of 4/2 $\mu \mathrm{m}$ and a length of 30 $\mu \mathrm{m}$ etched in a PC polymer are reported. The yield of the transmitted particles as a function of the capillary tilt angle was measured. The results show that for 200 keV ${\mathrm{H}}_2{}^{+}$ ions the guiding effect disappears but a focusing effect is obtained with a density enhancement factor of 3.5. For $7–50$ keV ${\mathrm{H}}_2{}^{+}$ ions their transmitted particle curves exhibit a pair of shoulder peaks on the left- and right-hand side of the center angle $0^{\circ }.$ As the ion energy increases, the shoulder-peak structure is found to vanish leaving one peak at $0^{\circ }$.

The contribution to electron-impact ionization cross sections from excitations to high-$nl$ shells and a consequent autoionization is investigated. We perform relativistic subconfiguration-average and detailed level-to-level calculations for this process. Ionization cross sections for the ${\text{W}}^{27+}$ ion are presented to illustrate the large influence of the high shells $\left(n\ge 9\right)$ and orbitals $\left(l\ge 4\right)$ in the excitation-autoionization process. The obtained results show that the excitations to the high shells $\left(n\ge 9\right)$ increase cross sections of the indirect ionization process by a factor of 2 compared to the excitations to the lower shells $\left(n\le 8\right)$. The excitations to the shells with orbital quantum number $l=4$ give the largest contribution compared with the other orbital quantum numbers $l$. Radiative damping reduces the cross sections of the indirect process approximately twofold in the case of the level-to-level calculations. Determined data show that the excitation-autoionization process contributes approximately 40 % to the total ionization cross sections.

We study superfluid (SF) states of strongly interacting Bose-Bose mixtures with equal mass and intracomponent interaction in optical lattices both in the presence and absence of a barrier potential. We show that the SF order parameters obey the two-component nonlinear Schrödinger equation (NLSE) with not only cubic but also quintic nonlinearity in the vicinity of the first-order transitions to the Mott insulators with even fillings. In the case of no barrier potential, we analyze solitary-wave solutions of the cubic-quintic NLSE. When the SF state changes from a ground state to a metastable one, a standard dark solitary wave turns into a bubblelike dark solitary wave, which has a nonvanishing density dip and no $\pi$-phase kink even in the case of a standing solitary wave. It is shown that the former and latter solitary waves are dynamically unstable against an out-of-phase fluctuation and an in-phase fluctuation, respectively, and the dynamical instabilities are weakened when one approaches the transition point. We find that the size and the inertial mass of the solitary waves diverge at the first-order transition point. We suggest that the divergence of the inertial mass may be detected through measurement of the relation between the velocity and the phase jump of the solitary wave. In the presence of a barrier potential, we reveal that when the barrier strength exceeds a certain critical value, the SF state that was metastable without the barrier is destabilized towards complete disjunction of the SF. The presence of the critical barrier strength indicates that the strong barrier potential qualitatively changes the criticality near the metastability limit of the SF state. We derive critical behaviors of the density, the compressibility, and the critical current near the metastability limit induced by the barrier. It is also found that the relation between the supercurrent and the phase jump across the barrier exhibits a peculiar behavior, owing to the nontopological nature of the bubblelike solitary wave.

We investigate signatures of the excited-state quantum phase transition in the periodic dynamics of the Lipkin-Meshkov-Glick model and the Tavis-Cummings model. In the thermodynamic limit, expectation values of observables in eigenstates of the system can be calculated using classical trajectories. Motivated by this, we suggest a method based on the time evolution of the finite-size system to find singularities in observables, which arise due to the excited-state quantum phase transition.

We propose a classical optical interferometry scheme that reproduces dispersion-insensitive Hong-Ou-Mandel interferograms. The interferometric scheme is derived from a systematic method based on the time-reversal symmetry of quantum mechanics. The scheme uses a time-reversed version of a Hong-Ou-Mandel interferometer with pairs of orthogonally polarized input laser pulses. We experimentally demonstrate automatic dispersion cancellation using the interferometry. The results show that the interferometer can obtain high-visibility interferograms with high signal conversion efficiency.

While wave-packet solutions for relativistic wave equations are oftentimes thought to be approximate (paraxial), we demonstrate, by employing a null-plane- (light-cone-) variable formalism, that there is a family of such solutions that are exact. A scalar Gaussian wave packet in the transverse plane is generalized so that it acquires a well-defined $z$ component of the orbital angular momentum (OAM), while it may not acquire a typical “doughnut” spatial profile. Such quantum states and beams, in contrast to the Bessel states, may have an azimuthal-angle-dependent probability density and finite uncertainty of the OAM, which is determined by the packet's width. We construct a well-normalized Airy wave packet, which can be interpreted as a one-particle state for a relativistic massive boson, show that its center moves along the same quasiclassical straight path, and, which is more important, spreads with time and distance exactly as a Gaussian wave packet does, in accordance with the uncertainty principle. It is explained that this fact does not contradict the well-known “nonspreading” feature of the Airy beams. While the effective OAM for such states is zero, its uncertainty (or the beam's OAM bandwidth) is found to be finite, and it depends on the packet's parameters. A link between exact solutions for the Klein-Gordon equation in the null-plane-variable formalism and the approximate ones in the usual approach is indicated; generalizations of these states for a boson in the external field of a plane electromagnetic wave are also presented.

The steepest-entropy-ascent quantum thermodynamic (SEAQT) framework is used to model the decoherence that occurs during the state evolution of two different microscopic composite systems. The test cases are a $\text{two}-\text{spin}-\frac{1}{2}-\text{particle}$ composite system and a particle-photon field composite system like that experimentally studied in cavity quantum electrodynamics. The first system is used to study the characteristics of the nonlinear equation of motion of the SEAQT framework when modeling the state evolution of a microscopic composite system with particular interest in the phenomenon of decoherence. The second system is used to compare the numerical predictions of the SEAQT framework with experimental cavity quantum electrodynamic data available in the literature. For the two different numerical cases presented, the time evolution of the density operator of the composite system as well as that of the reduced operators belonging to the two constituents is traced from an initial nonequilibrium state of the composite along its relaxation towards stable equilibrium. Results show for both cases how the initial entanglement and coherence is dissipated during the state relaxation towards a state of stable equilibrium.

The physical parameters governing the dynamics of a light-emitting quantum system can be estimated from the photon counting signal. The information available in the full detection record can be analyzed by means of the distribution of waiting times between detection events. Our theory allows calculation of the asymptotic, long-time behavior of the sensitivity limit, and it applies to emission processes with branching towards different final states accompanied by the emission of distinguishable photons. We illustrate the theory by application to a laser-driven $\Lambda$-type atom.

Envariance, or environment-assisted invariance, is a recently identified symmetry for maximally entangled states in quantum theory with important ramifications for quantum measurement, specifically for understanding Born's rule. We benchmark the degree to which nature respects this symmetry by using entangled photon pairs. Our results show quantum states can be $(99.66\pm 0.04)\%$ envariant as measured using the quantum fidelity, and $(99.963\pm 0.005)\%$ as measured using a modified Bhattacharya coefficient, as compared with a perfectly envariant system which would be 100% in either measure. The deviations can be understood by the less-than-maximal entanglement in our photon pairs.