Annalen der Physik

A simplified Heisenberg spin model is studied in order to examine the idea of decoherence in closed quantum systems. For this purpose, we present a quantifiable definition to quantum coherence Ξ, and discuss in some detail a general coherence theory and its elementary results. It appears that coherence is an important measure that helps to understand quantum properties of a system, e.g., the decoherence time can be derived from the coherence function Ξ(t), but not from the entropy dynamics. Moreover, the concept of decoherence time is applicable in closed and finite systems.

Tunneling is an outstanding physical process. The observation that particles surmount a high barrier in spite of the fact that they don't have the necessary energy cannot be explained by classical physics. However, this so called tunneling became allowed by quantum mechanics. Recent experimental and theoretical data confirmed the conjecture that the tunneling process is characterized by a universal tunneling time presumably independent of the kind of field.

In this paper we extend the electrodynamic equations proposed earlier for the charge density and charge current to describe also the space and time dependence of the spin density and spin current. We also provide a `geometric' interpretation of the difference between type I and type II superconductors, discuss how superconductors manage to conserve angular momentum, discuss the relationship between our model and Slater's seminal work on superconductivity, and discuss the magnitude of the expected novel effects for elemental and other superconductors.

We consider a chain of free electrons with periodically switched dimerization and study the entanglement entropy of a segment with the remainder of the system. We show that it evolves in a stepwise manner towards a value proportional to the length of the segment and displays in general slow oscillations. For particular quench periods and full dimerization an explicit solution is given. Relations to equilibrium lattice models are pointed out.

In this paper we study the interaction between a two-level atom and a quantized single-mode field, namely, the Jaynes-Cummings model (JCM). For this system we prove that the revival-collapse phenomenon exhibited in the atomic inversion of the standard JCM can be numerically (naturally) manifested in the evolution of the squeezing factor of the three-photon (standard) JCM provided that the initial photon-number distribution of the radiation field has a smooth envelope.