We explore the behavior in time of the energy exchange between a system of interest and its environment, together with its relationship to the non-Markovianity of the system dynamics. In order to evaluate the energy exchange we rely on the full counting statistics formalism, which we use to evaluate the first moment of its probability distribution. We focus in particular on the energy backflow from environment to system, to which we associate a suitable condition and quantifier, which enables us to draw a connection with a recently introduced notion of non-Markovianity based on information backflow. This quantifier is then studied in detail in the case of the spin-boson model, described within a second-order time-convolutionless approximation, observing that non-Markovianity allows for the observation of energy backflow. This analysis allows us to identify the parameters region in which energy backflow is higher.

Interactions between different bound states in bosonic systems can lead to pair creation. We study this process in detail by solving the Klein-Gordon equation on space-time grids in the framework of time-dependent quantum field theory. By choosing specific external field configurations, two bound states can become pseudodegenerate, which is commonly referred to as the Schiff-Snyder-Weinberg effect. These pseudodegenerate bound states, which have complex energy eigenvalues, are related to the pseudo-Hermiticity of the Klein-Gordon Hamiltonian. In this work, the influence of the Schiff-Snyder-Weinberg effect on pair production is studied. A generalized Schiff-Snyder-Weinberg effect, where several pairs of pseudodegenerate states appear, is found in combined electric and magnetic fields. The generalized Schiff-Snyder-Weinberg effect likewise triggers pair creation. The particle number in these situations obeys an exponential growth law in time enhancing the creation of bosons, which cannot be found in fermionic systems.

Quantum state separation is a probabilistic map that transforms a given set of pure states into another set of more distinguishable ones. Here we investigate such a map acting onto uniparametric families of symmetric linearly dependent or independent quantum states. We obtained analytical solutions for the success probability of the maps—which is shown to be optimal—as well as explicit constructions in terms of positive operator valued measures. Our results can be used for state discrimination strategies interpolating continuously between minimum-error and unambiguous (or maximum-confidence) discrimination, which, in turn, have many applications in quantum information protocols. As an example, we show that quantum teleportation through a nonmaximally entangled quantum channel can be accomplished with higher probability than the one provided by unambiguous (or maximum-confidence) discrimination and with higher fidelity than the one achievable by minimum-error discrimination. Finally, an optical network is proposed for implementing parametric state separation.

We introduce a computational method developed for study of long-range molecular Rydberg states of such systems that can be approximated by two electrons in a model potential of the atomic cores. Only diatomic molecules are considered. The method is based on a two-electron $R$-matrix approach inside a sphere centered on one of the atoms. The wave function is then connected to a Coulomb region outside the sphere via a multichannel version of the Coulomb Green's function. This approach is put into a test by its application to a study of Rydberg states of the hydrogen molecule for internuclear distances $R$ from 20 to 400 bohrs and energies corresponding to $n$ from 3 to 22. The results are compared with previous quantum chemical calculations (lower quantum numbers $n$) and computations based on contact-potential models (higher quantum numbers $n$).

We have recorded, near 4.3 $\mu m$, transmission spectra of pure $\mathrm{C}{\mathrm{O}}_2$ gas inserted between the windows of an extremely thin absorption cell. This was done for three pressures using a Fourier transform spectrometer and five optical paths between 0.17 and 1.15 $\mu m$. For these conditions, the line broadening induced by molecule-surface collisions can be studied under “clean” confinement conditions, i.e., between two parallel well-polished crystal surfaces separated by a known distance. This is in opposition with previous investigations using porous materials which involve pores of unknown dimensions with corrugated inner surfaces of ill-defined shapes. The analysis of the spectra shows that the line broadening due to the collisions of the molecules with the cell windows is independent of the optical transition and inversely proportional to the confinement length. Furthermore, the measured values are quantitatively reproduced if one assumes that a single $\mathrm{C}{\mathrm{O}}_2$-surface collision is sufficient to interrupt the rotating-dipole coherence. This gives a proof, here for the $\mathrm{C}{\mathrm{O}}_2$-sapphire system, of an assumption proposed many years ago and opens promising perspectives for the optical probing of porous materials.

We study the vortex lattice dynamics in the presence of a single impurity as well as random impurities or disorder. We show that in the presence of a single impurity, the vortex lattice gets distorted and the distortion depends on the position of the single impurity with respect to the positions of the vortices in the impurity-free Abrikosov vortex lattice and also on the strength of the impurity potential. We then show that a type of giant hole with hidden vortices inside it can be created in the vortex lattice by a cluster of impurities. In the presence of random impurity potential or disorder, the vortex lattice melts. We show that the vortex lattice also melts in the presence of the pseudorandom potential generated by the superposition of two optical lattices. The absence of long-range order in the melted vortex lattice is demonstrated from the structure factor profile and the histogram of the distances between each pair of vortices.

The polarization direction of an electromagnetic field changes and eventually reaches a steady state when propagating through a birefringent material with off-axis absorption or gain. The steady state orientation direction depends on the magnitude of the absorption (gain) and the phase retardation rate. The change in the polarization direction is experimentally demonstrated in weakly doped $(0.05\%){\text{Pr}}^{3+}:{\text{Y}}_2{\text{SiO}}_5$ crystals, where the light polarization, if initially aligned along the most strongly absorbing principal axis, gradually switches to a much less absorbing polarization state during the propagation. This means that the absorption coefficient $\alpha$ in birefringent materials generally varies with length. This is important for, e.g., laser crystal gain media, highly absorbing and narrow band spectral filters and quantum memories.

Control of the emerging beams obtained from the spontaneous splitting of an optical vortex soliton (OVS) due to the azimuthal modulation instability is demonstrated. The procedure adopted consisted of adding a control Gaussian beam, propagating collinearly with the OVS, and adjusting the beams' relative positions, radius, and intensities. Rotation of the emerging beams in the transverse plane and energy transfer between them were obtained using a control beam with smaller intensity than the vortex beam. The numerical simulations based on a modified nonlinear Schrödinger equation, including saturable nonlinearity and three-photon absorption, are in excellent agreement with the experimental results.

It is known that the Fresnel wave surfaces of transparent biaxial media have four singular points, located on two special directions. We show that, in more general media, the number of singularities can exceed 4. In fact, a highly symmetric linear material is proposed whose Fresnel surface exhibits 16 singular points. Because for every linear material the dispersion equation is quartic, we conclude that 16 is the maximum number of isolated singularities. The identity of Fresnel and Kummer surfaces, which holds true for media with a certain symmetry (zero skewon piece), provides an elegant interpretation of the results. We describe a metamaterial realization for our linear medium with 16 singular points. It is found that an appropriate combination of metal bars, split-ring resonators, and magnetized particles can generate the correct permittivity, permeability, and magnetoelectric moduli. Lastly, we discuss the arrangement of the singularities in terms of Kummer's ${16}_6$ configuration of points and planes. An investigation parallel to ours, but in linear elasticity, is suggested for future research.

We present a method to study the semiclassical dynamics of the Jaynes-Cummings dimer model, describing two coupled cavities, each containing a two-level system (qubit). We develop a Fock-space WKB approach in the polariton basis where each site is treated exactly while the intersite polariton hopping is treated semiclassically. We show that the self-trapped states can be viewed as Fock-space localized states. We find that this picture yields the correct critical value of interaction strength at which the delocalization-localization transition occurs. Moreover, the validity of our WKB approach is supported by showing that the quantum spectrum can be derived from a set of Bohr-Sommerfeld quantization conditions and by confirming that the quantum eigenstates are consistent with the classical orbital motion in the polariton band picture. The underlying idea of our method is quite general and can be applied to other interacting spin-boson models.

The formation of silver cluster structures at submicrometer spatial scales under the irradiation by high-power femtosecond laser pulses with high repetition rate was observed in various glasses containing silver ions. In order to account for the formation of these structures in metal-doped glasses, we present a theoretical model for the organization of noble metallic clusters induced by a train of femtosecond laser pulses. The model includes photoionization and laser heating of the sample, diffusion, kinetic reactions, and dissociation of metallic species. This model was applied to reproduce the formation of cluster structures in silver-doped phosphate glass. The parameters of the silver structures were obtained numerically under various incident pulse intensities and number of pulses. Numerical modeling shows that the involved microscopic physical and chemical processes naturally lead to the emergence of a silver cluster organization, together with charge migration and subsequent trapping giving rise to a strong static electric field buried in the irradiated area as experimentally observed. Based on this modeling, a theoretical basis is provided for the design of new metallic cluster structures with nanoscale size.

Conventionally, unknown quantum states are characterized using quantum-state tomography based on strong or weak measurements carried out on an ensemble of identically prepared systems. By contrast, the use of protective measurements offers the possibility of determining quantum states from a series of weak, long measurements performed on a single system. Because the fidelity of a protectively measured quantum state is determined by the amount of state disturbance incurred during each protective measurement, it is crucial that the initial quantum state of the system is disturbed as little as possible. Here we show how to systematically minimize the state disturbance in the course of a protective measurement, thus enabling the maximization of the fidelity of the quantum-state measurement. Our approach is based on a careful tuning of the time dependence of the measurement interaction and is shown to be dramatically more effective in reducing the state disturbance than the previously considered strategy of weakening the measurement strength and increasing the measurement time. We describe a method for designing the measurement interaction such that the state disturbance exhibits polynomial decay to arbitrary order in the inverse measurement time $1/T$. We also show how one can achieve even faster, subexponential decay, and we find that it represents the smallest possible state disturbance in a protective measurement. In this way, our results show how to optimally measure the state of a single quantum system using protective measurements.

A classical-kind phase-space formalism is developed to address the tiny intrinsic dynamical deviation from what is predicted by Wilczek–Zee theorem during quantum adiabatic evolution on degeneracy levels. In this formalism, the Hilbert space and the aggregate of degenerate eigenstates become the classical-kind phase space and a high-dimensional subspace in the phase space, respectively. Compared with the previous analogous study by a different method, the current result is qualitatively different in that the first-order deviation derived here is always perpendicular to the degeneracy subspace. A tripod-scheme Hamiltonian with two degenerate dark states is employed to illustrate the adiabatic deviation with degeneracy levels.

BosonSampling is an intermediate model of quantum computation where linear-optical networks are used to solve sampling problems expected to be hard for classical computers. Since these devices are not expected to be universal for quantum computation, it remains an open question of whether any error-correction techniques can be applied to them, and thus it is important to investigate how robust the model is under natural experimental imperfections, such as losses and imperfect control of parameters. Here, we investigate the complexity of BosonSampling under photon losses, more specifically, the case where an unknown subset of the photons is randomly lost at the sources. We show that if $k$ out of $n$ photons are lost, then we cannot sample classically from a distribution that is $1/n^{\mathrm{\Theta }\left(k\right)}$ close (in total variation distance) to the ideal distribution, unless a ${\text{BPP}}^{\text{NP}}$ machine can estimate the permanents of Gaussian matrices in $n^{O\left(k\right)}$ time. In particular, if $k$ is constant, this implies that simulating lossy BosonSampling is hard for a classical computer, under exactly the same complexity assumption used for the original lossless case.

We show that it is possible to hide information perfectly within a photograph. The proposed protocol works by selecting each pixel value from two images that differ only by shot noise. Pixel values are never modified, but only selected, making the resulting stego image provably indistinguishable from an untampered image, and the protocol provably secure. We demonstrate that a perfect steganographic protocol is also a perfectly secure cryptographic protocol, and therefore has at least the same requirements: a truly random key as long as the message. In our system, we use a second image as the key, satisfying length requirements, and the randomness is provided by the naturally occurring quantum noise which is dominant in images taken with modern sensors. We conclude that, given a photograph, it is impossible to tell whether it contains any hidden information.

The Casimir-Polder (CP) force acting on a $\mathsf{V}$-type three-level atom which is initially prepared in two different kinds of superposition states, i.e., subradiant and superradiant states, is investigated. The influence of quantum interference on force evolution due to two-dipole transitions is analyzed in detail. It is found that the orientation of the atomic dipole moment has significant influence on the Casimir-Polder force and consequently its evolution. For the ideal degenerate $\mathsf{V}$-type atom with two parallel dipoles, quantum interference leads to population trapping as well as the cancellation of the CP force when the atom is prepared initially in a subradiant state. However, the result changes when we consider the practical Zeeman $\mathsf{V}$-type atom whose two dipole moments are perpendicular to each other. Since quantum interference in such an atom must occur in an anisotropic environment, it is possible to trap atomic population and enhance the CP force simultaneously by preparing the atom initially in sub-radiant states. In principle, our results can be found in an arbitrary anisotropic environment, and here we describe a structure containing left-handed materials to highlight our findings.

Charge-exchange collisions of slow $\mathrm{Ne}^{10+}$ ions with He, Ne, and Ar targets were studied with simultaneous x-ray and cold-target recoil-ion-momentum spectroscopy proving the contribution of several mechanisms to the radiative stabilization of apparent (4,4) doubly excited states for He and Ne targets and of (5,6) states for Ar. In particular, the stabilization efficiency of the mechanism of dynamic auto-transfer to Rydberg states is confirmed. Moreover, we present evidence for direct radiative decays of (4,4) states populated in collisions with He, which is an experimental indication of the population of so-called unnatural-parity states in such collisions. These mechanisms lead to the emission of x-rays that have considerably higher energies than those predicted by current spectral models and may explain recent observations of anomalously large x-ray emission from Rydberg levels.

We investigate the center-of-mass momentum distribution $n_{\mathit{Q}}$ of Cooper pairs and their first-order correlation function $g_1\left(r\right)$ in a strongly interacting two-dimensional Fermi gas. Recently, the Berezinskii-Kosterlitz-Thouless (BKT) transition was reported in a two-dimensional ${}^6\mathrm{Li}$ Fermi gas, based on (1) the observations of anomalous enhancement of $n_{\mathit{Q}=\mathit{0}}$ [Ries et al., Phys. Rev. Lett. 114, 230401 (2015)] , as well as (2) a power-law behavior of $g_1\left(r\right)$ [Murthy et al., Phys. Rev. Lett. 115, 010401 (2015)]. However, including pairing fluctuations within a $T$-matrix approximation (TMA), we show that these results can still be explained as strong-coupling properties of a normal-state two-dimensional Fermi gas. Our results indicate the importance of further experimental observations, to definitely confirm the realization of the BKT transition in this system. Since the BKT transition has been realized in a two-dimensional ultracold Bose gas, our results would be useful for the achievement of this quasi-long-range order in an ultracold Fermi gas.

We propose and realize a radial spin Hall effect (SHE) of light by using a dielectric metasurface. The metasurface with radially varying optical axes introduces a Pancharatnam–Berry (PB) geometrical phase to the incident light. The spatial gradient of PB phase accounts for a shift in the momentum space and thus leads the light to split radially into two concentric rays with opposite spin in the position space, which is called a radial SHE. Further experiments verify that the magnitude of the splitting increases with the rotation rate of the optical-axis orientation and the propagation distance, thereby allowing for macroscopic observation of the SHE. We also find that the phase of the incident light influences the profiles of the two split rays, while the polarization determines their intensities. The results provide methods to tune the SHE of light by engineering metasurfaces and modulating the incident light, and this radial SHE may be extrapolated to other physical systems.

We report on the observation of robust self-trapping of vortex beams propagating in a uniform condensed medium featuring local saturable self-focusing nonlinearity. Optical vortices with topological charge $m=1$, that remain self-trapped over approximately five Rayleigh lengths, are excited in carbon disulfide using a helical light beam at 532 nm and intensities from 8 to $10\mathrm{GW}/\mathrm{c}{\mathrm{m}}^2$. At larger intensities, the vortex beams lose their stability, spontaneously breaking into two fragments. Numerical simulations based on the nonlinear Schrödinger equation, including the three-photon absorption and nonpolynomial saturation of the refractive nonlinearity, demonstrate close agreement with the experimental findings.