Renormalization of trace distance and multipartite entanglement close to the quantum phase transitions of one- and two-dimensional spin-chain systems

We investigate the quantum phase transitions of spin systems in one and two dimensions by employing trace distance and multipartite entanglement along with real-space quantum renormalization group method. As illustration examples, a one-dimensional and a two-dimensional $XY$ models are considered. It is shown that the quantum phase transitions of these spin-chain systems can be revealed by the singular behaviors of the first derivatives of renormalized trace distance and multipartite entanglement in the thermodynamics limit. Moreover, we find the renormalized trace distance and multipartite entanglement obey certain universal exponential-type scaling laws in the vicinity of the quantum critical points

Optimal entanglement generation in cavity QED with dissipation

We investigate a two-level atom coupled to a cavity with a strong classical driving field in a dissipative environment and find an analytical expression of the time evolution density matrix for the system. The analytical density operator is then used to study the entanglement between the atom and cavity by considering the competing process between the atom-field interactions and the field-environment interactions. It is shown that there is an optimal interaction time for generating atom-cavity entanglement.Comment: 9 pages, 7 figure

Diffusion and entanglement of a kicked particle in an infinite square well under frequent measurements

We investigate the dynamics of a kicked particle in an infinite square well undergoing frequent measurements of energy. For a large class of periodic kicking force, constant diffusion is found in such a non-KAM system. The influence of phase shift of the kicking potential on the short-time dynamical behavior is discussed. The general asymptotical measurement-assisted diffusion rate is obtained. The entanglement between the particle and the measuring apparatus is investigated. There exist two distinct dynamical behaviors of entanglement. The bipartite entanglement grows with the kicking steps and it gains larger value for the more chaotic system. However, the pairwise entanglement between the system of interest and the partial spins of the measuring apparatus decreases with the kicking steps. The relation between the entanglement and quantum diffusion is also analyzed. PACS numbers: 05.45.Mt, 03.65.TaComment: 7 pages, 5 figures, RevTex4, Accepted by Phys. Rev.