The European Physical Journal D最新文献

With the quantum state diffusion measurement theory (QSD), the measurement problem in liquid nuclear magnetic resonance (NMR) quantum computers was addressed and then it was shown that due to stochastical fluctuations, the measured magnetic moment value is comparable to noise for long times. Therefore, we suggested that the measurement time should be short to distinguish signal from noise.

QSD in NMR quantum computers.

Variational expressions for functionals yielding the second-order and fourth-order corrections to the nondegenerate eigenenergies of a Hamiltonian subject to a given perturbation have been derived. Discrete basis sets consisting of the suitably chosen eigenfunctions of the unperturbed Hamiltonian along with a nonlinear variational parameter can be used to evaluate the stationary functional quite accurately. The method has been applied conveniently to calculate the dipole hyperpolarizabilities of the hydrogenic atoms interacting with screened Coulomb potentials (SCP) quite accurately. Three SCPs are considered, namely the static screened Coulomb potential, the exponential cosine screened Coulomb potential and the generalized exponential cosine screened Coulomb potential. It is found that a basis set consisting of 20 eigenfunctions of the p-states of the hydrogenic atoms yields stationary functional corresponding to the second-order correction to the energy, whereas a basis set consisting of 20 d-state eigenfunctions and 19 s-state eigenfunctions of the hydrogenic atoms produces stationary functional corresponding to the fourth-order correction to the energy. Moreover, the stationary values of the functionals are highly accurate (up to 14th place of decimal) and converge rapidly with the increase in the number of terms in the basis sets.

When the space surrounding an atom or molecule is restricted, its electron density is modified compared to its unrestricted counterpart, which further alters its physical and chemical properties. To model the effect of space restriction, or confinement, on atoms and molecules, often the hard-boundary condition is employed to solve the corresponding Schrödinger equation. In the present work, we analyze the modifications in a chemical bond when the space surrounding the molecule is restricted such that some part of the space is forbidden to the electrons. To this end, we choose the simplest hydrogen molecular ion H(_{2}^{+}), in the presence of an external hard-sphere simulating the restriction of space available to the electrons due to Pauli exclusion by an atom close to the molecule. The hard-sphere is moved along the molecular axis and the axis transverse to it to study the changes in the ground-state energy, stretching frequency, kinetic, and potential energies of an H(_{2}^{+}) molecule. For this purpose, we employ a variational approach using a suitably constructed energy estimator that goes beyond the Born–Oppenheimer approximation. We find that as the sphere moves closer to the molecule along the molecular axis, the ground-state energy and the vibrational frequencies get enhanced. On the other hand, for the same case, the potential energy decreases, and the kinetic energy increases. When the sphere moves along the axis transverse to the molecular axis, the changes in the energy and vibrational frequencies show similar trends; however, the magnitude of the changes is significantly smaller than in the corresponding molecular axis case. For the transverse case, both potential and kinetic energies get enhanced marginally. These results demonstrate that the axial approach of the hard surface is more efficient in inducing a bond breaking of the H(_{2}^{+}) molecular ion. The shift of the electron bond by the external sphere explains these results.

In this work, we consider simple systems that are governed by Hamiltonians with time periodicity. Our analysis is mainly focused on the density matrix approach and aims to solve the von Neumann equation of motion from which one can extract the state of the system when the system is in a pure state. We start our analysis with the standard Rabi-oscillation problem. We consider a density matrix corresponding to the entire model system and solve the von Neumann equation of motion. We have then made use of the Lewis-Reisenfeld invariant approach and arrived at the exact same result, which implies that the density matrix of the system can indeed be identified with the Lewis invariant. Then, we consider a two-level system with a constant magnetic field in the z-direction and a time-dependent magnetic field in the x-direction. We solve the von Neumann equation of motion for this system and calculate the various coherence measures, and plot them to investigate the time dependence and reliability of different coherence measures.

Correspondence between the density operator and the Lewis invariant operator of the system

The elastic scattering and resonant charge transfer differential and integral cross sections in (textrm{H}(1s) + mathrm{H^+}) collisions are computed for the center-of-mass energy range of (10^{-10}-10) eV. Fully quantal and semiclassical approaches are utilized in these calculations. The reliability of the semiclassical approximation for very low collision energies is discussed. The results are compared with available data from the literature.

The nonlinear dynamics of plasma waves in multi-species magnetized systems is of great importance for understanding energy transport and particle interactions in both astrophysical and laboratory environments. This study investigates heavy ion-acoustic kinetic Alfvén solitary waves (HIAKASWs) in a magnetized plasma composed of inertial thermal heavy ions and non-extensive electron–positron pairs. Using the reductive perturbation method, we derive both the Korteweg–de Vries (K-DV) and modified Korteweg–de Vries (MK-DV) equations, which capture the balance between dispersion and nonlinearity, and analyze their soliton solutions under different plasma conditions. The electron and positron populations are described by Tsallis non-extensive statistics, allowing exploration of deviations from Maxwellian behavior. The analysis reveals that the K-DV equation admits both compressive and rarefactive solitary structures, while the higher-order MK-DV formulation supports only compressive modes. Parametric investigations demonstrate that heavy ion thermal pressure and inertia play a dominant role in shaping wave amplitude and width compared to electron and positron effects. The results highlight the influence of non-extensivity, magnetic field strength, and propagation angle on solitary wave characteristics, with implications for plasma dynamics in astrophysical and laboratory environments.

The plots highlight the effects of (beta ), (gamma ), and heavy ion density on dispersion, nonlinearity, and electrostatic potential of K-DV and MK-DV solitons, relevant to astrophysical and laboratory plasmas.

The paper proposes a chemical model for describing of the thermophysical and transport properties of dense plasmas. In this model, plasma consists of weakly interacting electrons, ions, atoms, molecules and molecular ions, and two- and threefold ionized atoms. Corrections for charge–charge and charge–atom interactions were analyzed. The Debye approximation in the Grand canonical ensemble is used to take into account the charge–charge interaction. We showed that the interaction between charges plays a significant role in the calculation of the plasma properties. The caloric and thermal equations of state and composition of the lead plasma were calculated. The calculation results obtained from the suggested model have demonstrated satisfactory agreement with the experimental data on the equation of state and the electrical resistivity measured recently for a dense plasma of lead.