We report the spectral features of a phase-shifted parity and time ($\mathcal{PT}$) symmetric fiber Bragg grating and demonstrate its functionality as a demultiplexer in the unbroken $\mathcal{PT}$-symmetric regime. The length of the proposed system is of the order of millimeters and the lasing spectra in the broken $\mathcal{PT}$-symmetric regime can be easily tuned in terms of intensity, bandwidth, and wavelength by varying the magnitude of the phase shift in the middle of the structure. Surprisingly, the multimodal lasing spectra are suppressed by virtue of judiciously selected phase and the gain-loss value. Also, it is possible to obtain sidelobe-less spectra in the broken $\mathcal{PT}$-symmetric regime without a need for an apodization profile, which is a traditional way to tame the unwanted sidelobes. The system is found to show narrow-band single-mode lasing behavior for a wide range of phase-shift values for given values of gain and loss. Moreover, we report the intensity-tunable reflection and transmission characteristics in the unbroken regime via variation in gain and loss. At the exceptional point, the system shows unidirectional wave transport phenomenon independent of the presence of phase shift in the middle of the grating. For the right light-incidence direction, the system exhibits zero reflection wavelengths within the stopband at the exceptional point. We also investigate the role of multiple phase shifts placed at fixed locations along the length of the FBG and the variations in the spectra when the phase-shift and gain-loss values are tuned. In the broken $\mathcal{PT}$-symmetric regime, the presence of multiple phase shifts aids in controlling the number of reflectivity peaks besides controlling their magnitude. The advantage of the proposed model is that it exhibits multifunctional capabilities like demultiplexing, filtering, and lasing in a short length of the grating depending on the different operating regimes.

The method of the laser pulse self-compression in the process of noise-seeded Raman scattering in a plasma created by a counterpropagating ionized pulse is considered. The sharpness of the ionization front ensures the formation of an output pulse close to the $\pi$-pulse solution with a sharp leading edge. The dependence of the gain increment on the transverse wave number leads to the formation of a smooth phase front of the output pulse. The energy efficiency in the output pulse reaches 28% of the total pump energy. A study of the influence of quasistatic inhomogeneities showed only a slight slowdown in the process, even in a plasma with 8% density modulation.

We find a different mechanism for suppression of decay in an open one-dimensional lattice system, which originates from a dark Floquet state, a sink state to which the system is asymptotically driven, whose overall probability is determined only by the parameters of the periodic driving field. The zero-quasienergy of dark Floquet state is shown to be not a real zero, but a vanishingly small negative imaginary number which will cause undesirable physical effect in long-time evolution of quantum states, being extremely different from the conservative counterpart. Another intriguing finding is that the value of the system's effective decay, determined by the size of the nonzero imaginary part of the dark-Floquet-state-related quasienergy, depends not on how many localized lossy sites there are but on which one of the lossy sites is nearest to the driven site. Thus, for specially designed local dissipation, by controlling the driving parameters, it is possible for us to drive the system to a dark Floquet state with a much lower level of overall probability loss as compared to the undriven case and with good stability over longer evolution time. These results are applicable to the multisite lattice system with an odd number of sites and may be significant for long-time control of decay in a vast family of multistate physical systems with localized dissipation.

We examine the advantages that quantum strategies afford in communication-limited games. Inspired by the card game blackjack, we focus on cooperative, two-party sequential games in which a single classical bit of communication is allowed from the player who moves first to the player who moves second. Within this setting, we explore the usage of quantum entanglement between the players and find analytic and numerical conditions for quantum advantage over classical strategies. Using these conditions, we study a family of blackjack-type games with varying numbers of card types, and find a range of parameters where quantum advantage is achieved.

Using electron spectroscopy technique, we measure the absolute double-differential cross sections (DDCSs) of electrons emitted in collisions of 200-keV protons on He, ${\mathrm{CH}}_4$, and ${\mathrm{O}}_2$ and 5.5-MeV/u bare C ions colliding on ${\mathrm{O}}_2$. The emitted electrons are measured in the energy range from 1 to 400 eV for He and ${\mathrm{CH}}_4$ targets and up to 600 eV for ${\mathrm{O}}_2$ to include the $K\text{−}LL$ Auger line of oxygen. The electrons are detected over different emission angles varying from ${20}^{\circ }$ in the extreme forward direction to ${160}^{\circ }$ in the backward direction. The single-differential cross section (SDCS) and total cross section are deduced from the measured DDCSs spectra for all the targets. The DDCS and SDCS are compared with the state-of-the-art continuum distorted wave–eikonal initial state (CDW-EIS) theoretical model. The CDW-EIS model provides excellent agreement with the oxygen data at MeV energy, whereas the deviation in the case of keV energy is substantial, in spite of having nearly the same perturbation strength. The forward-backward angular asymmetry shows a saturation effect in the case of keV energy protons but no such signature is observed for the high-energy collision. A systematic analysis reveals that the asymmetry at low electron energy is sensitive to the associated atomic or molecular structure and is in close agreement with the theory.

In this study, the recurrent fluorescence of naphthalene ions was investigated using an electrostatic ion beam trap. The number of photons from the recurrent fluorescence on the $D_2\to D_0$ transition was measured as a function of time, after the production of naphthalene ions by electron impact. The emission of photons was observed over a time range of approximately 10 ms. The variation in the number of photons for such a long time range could be explained by conducting simulations using previously reported theoretical rate constants of both the fragmentation and the recurrent fluorescence of vibrationally hot naphthalene ions.

We present fully ab initio calculations of second-overtone rovibrational line shapes of carbon monoxide perturbed by argon. The quantum mechanical scattering problem between CO and Ar is solved numerically for two different ab initio interaction potentials. We use the generalized Hess method to determine spectroscopic cross sections which describe the effect of collisions on each spectral line. Using these cross sections, we determine the line-shape parameters that we use to generate the Hartmann-Tran and speed-dependent billiard ball profiles. We compare the generated line shapes with high-quality experimental line profiles of five lines measured at five pressures between 0.01 and 1 atm. A subpercent agreement over the entire pressure range is obtained. Calculations for the P(9) line are used to inspect the effects of the two interaction potentials. The discrepancies for both the considered interaction potentials and the experiment are explained within the described theoretical framework. The presented results are the most accurate collisional line-shape calculations for a system with collision dynamics representative of atmospherically relevant species.

The dissociation dynamics of LiH molecules induced by two overlapping femtosecond pulses is investigated based on the time-dependent quantum wave packet method. The first pulse induces population transfer from the ground state $X{}^1{\mathrm{\Sigma }}^{+}$ to the excited state $A{}^1{\mathrm{\Sigma }}^{+}$ which is coupled with the repulsive state $B{}^1\mathrm{\Pi }$ and the state $X{}^1{\mathrm{\Sigma }}^{+}$ by the second pulse. The two products $\mathrm{Li}\left(2p\right)$ and $\mathrm{Li}\left(2s\right)$ can be obtained via the ladder and $\mathrm{\Lambda }$ transitions, respectively, and the branching ratio of the products can be controlled by varying the second pulse frequency $\hslash {\omega }_2$. The choice of the intermediate state can affect the dissociation probability and angular distribution of fragments. As the second pulse intensity is increased, more products occur in other directions besides the directions $\theta =0^{\circ }$ and ${180}^{\circ }$. The delay time of two pulses also has influence on the angular distribution.

We theoretically examine the vortex states of a gas of trapped quasi-two-dimensional ultracold bosons subject to a density-dependent gauge potential, realizing an effective nonlinear rotation of the atomic condensate, which we also show is within the reach of current experimental techniques with ultracold atom experiments. The nonlinear rotation has a twofold effect; as well as distorting the shape of the condensate it also leads to an inhomogeneous vorticity resulting in unique morphological and topological states, including ring vortex arrangements that do not follow the standard Abrikosov result. The dynamics of trapped vortices are also explored, which differ from the case of rigid-body rotation due to the absence of a global laboratory reference frame.

We study the magnetic phases of a nonequilibrium spin chain, where coherent interactions between neighboring lattice sites compete with alternating gain and loss processes. This competition between coherent and incoherent dynamics induces transitions between magnetically aligned and highly mixed phases, across which the system changes from a low to an infinite temperature state. We show that the origin of these transitions can be traced back to the dynamical effect of parity–time-reversal symmetry breaking, which has no counterpart in the theory of equilibrium phase transitions. This mechanism also results in very atypical features and we find first-order transitions without phase coexistence and mixed-order transitions which do not break the underlying $\mathrm{U}\left(1\right)$ symmetry, even in the appropriate thermodynamic limit. Thus, despite its simplicity, the current model considerably extends the phenomenology of nonequilibrium phase transitions beyond that commonly assumed for driven-dissipative spins and related systems.

Open quantum walks (OQWs) describe a quantum walker on an underlying graph whose dynamics is purely driven by dissipation and decoherence. Mathematically, they are formulated as completely positive trace preserving (CPTP) maps on the space of density matrices for the walker on the graph. Any microscopically derived OQW must include the possibility of remaining on the same site on the graph when the map is applied. We extend the CPTP map to describe a lazy OQW. We derive a central limit theorem for lazy OQWs on a $d$-dimensional lattice, where the distribution converges to a Gaussian. We show that the properties of this Gaussian computed using conventional methods agree with the general formulas derived from our central limit theorem.

We derive a general quantum-mechanical transition amplitude in the prior version of the three-body boundary-corrected continuum-intermediate-state (BCIS-3B) method for single charge exchange in collisions of heavy nuclei with atomic hydrogenlike targets. The obtained expression is valid for all the transitions from the initial ground to any final hydrogenlike bound states $(1s\to nlm),$ where $\{n,l,m\}$ is the usual triple of the quantum numbers (principal, angular, magnetic). The outcome is semianalytical: a judicious combination of an analytical calculation (as far as feasible) and the subsequent numerical computations (quadrature for integrals that could not be reduced to closed formulas). The final results for the $T$-matrix elements are a compact double integration computed by numerical quadratures over real variables in finite intervals. The ensuing total cross section is a single numerical quadrature of the squared absolute value of this transition amplitude integrated over the magnitude of the transverse momentum transfer vector. This can provide detailed information on the cross sections for each individual subshell $\{n,l,m\}$ of the newly formed hydrogenlike atom or ion comprised of a projectile nucleus and the captured electron. Using these general expressions, we computed a large number of state-selective and state-summed cross sections (both differential and total) for electron capture in the $p\text{−}\mathrm{H}$, $\alpha \text{−}\mathrm{H}$, and $p\text{−}\mathrm{He}$ collisions at intermediate and high impact energies. The energy dependence of the cross sections for the individual shells and their subshells with the fixed quantum numbers $n$, $\{n,l\}$, and $\{n,l,m\}$ are reported via detailed tables and figures. Also, differential cross sections (state selective and summed) are computed for the $p\text{−}\mathrm{H}$ collisions at 60, 125, and 5000 keV. All the present results from the BCIS-3B method are found to be in very good agreement with the existing experimental data on differential and total cross sections, both state selective (whenever available) and state summed. The provided cross-section database can find important applications in several investigative fields ranging from plasma physics, astrophysics, and heavy-ion transport physics through fusion research and technology to hadron therapy.

X-ray-production cross sections for $M_3, M_4$, and $M_5$ subshells of Pt and Au by electron impact were experimentally determined at incident energies ranging between 2.8 and 28 keV. To this purpose, Pt and Au thick targets were irradiated by an electron beam in a field-emission-gun scanning electron microscope, and the x-ray-emission spectra were recorded with an energy-dispersive spectrometer. The x-ray-production cross sections were obtained as a result of the spectral processing performed through a careful parameter optimization routine previously developed, which involves an analytical function for the prediction of the experimental spectra, on the basis of ionization depth distribution functions. The results are compared with the scarce data available in the literature.

The photoionization of an atom in the presence of a neighboring atom of a different species is studied. The latter first undergoes resonant photoexcitation, leading to an autoionizing state of the diatomic system. Afterwards, the excitation energy is transferred radiationlessly via a two-center Auger process to the other atom, causing its ionization. Assuming a fixed internuclear distance, it has been predicted theoretically that this indirect ionization pathway can strongly dominate over the direct photoionization. Here, we extend the theory of resonant two-center photoionization by including the nuclear motion in van der Waals molecules. An analytical formula is derived reflecting the influence of molecular vibrational dynamics on the relative strength of the two-center channel. For the specific example of Li-He dimers, we show that the two-center autoionizing resonances are split by the nuclear motion into multiplets, with the resonance lines reaching a comparable level of enhancement over direct photoionization as is obtained in a model based on spatially fixed nuclei.

We study static-field ionization of He-like ions with nuclear charge number in the range of $2\le Z\le 36$. Both the tunneling and over-the-barrier regimes are considered. We calculate the ionization rates by three approximate methods: a fitting formula based on the Perelemov-Popov-Terent'ev (PPT) formula, a perturbative expansion in powers of $1/Z$, and a single-active electron approximation, and compare them with reference ionization rates computed by the multiconfiguration time-dependent Hartree-Fock (MCTDHF) approach. The relative deviation of the rates computed by the PPT-based fitting formula and the third-order perturbation theory from the rates computed by the MCTDHF approach is found to be around 10%. We discuss quantitatively the contribution from the exchange interaction during the ionization by comparing the single-active electron rates and the reference rates in which multielectron effects are included. We find that a single-active electron approximation, where the exchange interaction is neglected, results in the overestimation of the ionization rates by 30% for He and 2% for He-like Kr, showing that the magnitude of the effect of the exchange interaction scales approximately as $1/Z$.

We consider a mixture of a Bose-Einstein condensate, with a paired Fermi superfluid, confined in a ring potential. We start with the ground state of the two clouds, identifying the boundary between the regimes of their phase separation and phase coexistence. We then turn to the rotational response of the system. In the phase-separated regime, we have center-of-mass excitation. When the two species coexist, the spectrum has a rich structure, consisting of continuous and discontinuous phase transitions. Furthermore, for a reasonably large population imbalance, it develops a clear quasiperiodic behavior, in addition to the one due to the periodic boundary conditions. It is then favorable for the one component to reside in a plane-wave state, with a homogeneous density distribution, and the problem resembles that of a single-component system.

The main objective of this work is to present a theoretical proposal for an implementation of the ($2+1$)-dimensional Dirac equation in classical gravitational and electromagnetic backgrounds, in a two-dimensional waveguide array. For this, a framework for achieving site-dependent effective coupling constants in two-dimensional waveguide arrays is developed. Implementability of the Dirac equation under the proposed scheme puts minor demands on gauge and spacetime backgrounds; however, a wide array of physical spacetimes, such as all vacuum and static solutions, prove to be implementable. As an interesting and instructive example, we discuss a tabletop realization of the gravitational Aharonov-Bohm effect: After devising a thought experiment in which signatures of the gravitational Aharonov-Bohm effect could be detected, we briefly discuss how the analog of such a setting can in principle be implemented using the proposed waveguide setup.

We consider a moving refractive index perturbation in an optical medium as an optical analog to waves under the influence of gravity. We describe the dielectric medium by the Lagrangian of the Hopfield model. We supplement the field theory in curved spacetime for this model to solve the scattering problem for all modes and frequencies analytically. Because of dispersion, the kinematic scenario of the field modes may contain optical event horizons for some frequencies. We calculate the spectra of spontaneous emission in the frame comoving with the perturbation and in the laboratory frame. We also calculate the spectrally resolved photon number correlations in either frame. The emitted multimode field comes in different types depending on the presence of horizons. We show that these types are robust against changes in the system parameters and thus are genuine features of optical and nonoptical analogues. These methods and findings pave the way to new observations of analog gravity in dispersive systems.