Modern Physics Letters B

The quantization of the current in a superconducting quantum point contact is reviewed and the critical current is discussed at different temperatures depending on the carrier concentration as well by suggesting a constant potential in the semiconductor and then a Maxwell potential. When the Fermi wavelength is comparable with the constriction width we showed that the critical current has a step-like variation as a function of the constriction width and the carrier concentration.

A new scenario to realize negative refraction with a photonic-resonant vapor material that can exhibit both electric and magnetic responses via multilevel quantum coherence is suggested. Compared with the previous method of artificial composite metamaterial, where the mechanism was considered by means of classical electromagnetic theory and the materials produced have anisotropic millimetre-scale composite structures, the present scheme suggested within the framework of quantum optics can be used to design and fabricate isotropic negatively-refracting materials with atomic-scale microscopic structure units. Such an advantage may lead to a potentially important application in the techniques of superlens and perfect imaging.

In this paper, a scheme is presented to implement the 1→2 universal quantum cloning machine (UQCM) with trapped ions. In this way, we also show that quantum information can be directly transferred from one ion to another. The distinct advantage of the scheme lies in the fact that it does not use the vibrational mode as the data bus. The vibrational mode is only virtually excited, which makes our scheme insensitive to heating, provided the system remains in the Lamb–Dicke regime.

The local structure and the g factors for the orthorhombic Ti³⁺ center in YAP are theoretically studied from the perturbation formulas of the g factors g_x, g_y and g_z for a 3d¹ ion in orthorhombic symmetry based on the cluster approach. The observed large anisotropy of the g factors may be attributed to the significant low-symmetrical (tetragonal and orthorhombic) distortion near the Al³⁺ site occupied by the impurity Ti³⁺. The calculated g factors in this work show reasonable agreement with the experimental data.

Quantum entanglement of anharmonic stretches in molecules H₂O and SO₂ is studied in terms of the reduced-density linear entropy with initial squeezed vacuum states and thermal states. It is shown that the entropy of both kinds of initial states with a small amplitude is quasi-periodic with the period of thermal states being larger than that of squeezed vacuum states for each molecule. The entropy evolution of both kinds of initial states with a larger amplitude is irregular. For each kind of initial state, the frequency of oscillation of the entropy for SO₂ is larger than that for H₂O. That can be useful in quantum computing based on vibrational states in H₂O and SO₂.

The collision of positrons with beryllium and magnesium positive ions is treated for the first time as a three-channel problem with the assumption that the elastic, ground-positronium and excited-positronium formation channels are open. A one-valence-electron model for the targets, based on the Clementi–Roetti Slater basis functions, as well as an improved coupled-static approach allowing for the polarization of the excited positronium, are used for calculating the partial cross-sections of eight partial waves (corresponding to 0≤ℓ≤7, where ℓ is the total angular momentum of the scattering problem considered). The calculations are carried out, in each case, at 19 values of the incident energy lying above the excited positronium formation threshold (i.e. above 16.42 eV in e⁺–Be⁺ scattering and above 13.02 eV in e⁺–Mg⁺ scattering). The total elastic cross-sections of e⁺–Mg⁺ scattering show a peak around the ionization threshold of Mg⁺ (at 14.723 eV) but for e⁺–Be⁺ scattering, display a peak at 90 eV (remember that the ionization threshold of Be⁺ is 18.2 eV). Although the resulting total collisional positronium formation cross-sections are smaller than the elastic ones, their relatively large values should draw the attention of experimental and theoretical physicists to the field of positron–ion collisions.