Hannover, Universiẗat, Diss., 2000.

Hannover, University, Diss., 2004.

In the 1990¡¦s, the research of quantum information attracts many people¡¦s attention. In this period of time, Shor find a new method to demonstrate that a quantum computer could factor very large numbers super-efficiently. The method also shows that quantum computer has more potential than classical computer. Beside, quantum information contains many different new fields, such as quantum computation, quantum entanglement, quantum searching, etc. We believe the most fundamental physics of the applications of quantum information is quantum entanglement. In order to understand the physical meaning of entanglement, we choose entanglement as the goal of our thesis.

We analyze a coincidence Stern-Gerlach measurement often discussed in connection with the derivation and illustration of Bell's theorem. The treatment is based on our recent analysis of the original Stern-Gerlach experiment (PCCP, 14, 1677‐1684 (2012)), where it is concluded that it is necessary to include a spin relaxation process to account for the experimental observations. We consider two limiting cases of a coincidence measurement using both an analytical and a numerical description. In on limit relaxation effects are neglected. In this case the correlation between the two spins present in the initial state is conserved during the passage through the magnets. However, at exit the z coordinate along the magnetic field gradient is randomly distributed between the two extreme values. In the other limit T2 relaxation is assumed to be fast relative to the time of flight through the magnet. In this case the z coordinate takes one of two possible values as observed in the original Stern‐Gerlach experiment. Due to the presence of a relaxation process involving transfer of angular momentum between particle and magnet the initially entangled spin state changes character leading to a loss of correlation between the two spins. In the original derivations of Bell's theorem based on a coincidence Stern‐Gerlach setup one assumes both a perfect correlation between the spins and only two possible values for the z‐coordinate on exit. According to the present calculations one can satisfy either of these conditions but not both simultaneously.