Quantum Coherence of double-well BEc: a new approach

A new approach is presented to investigate the quantum coherence of Bose gas in a double-well potential by mapping the Bose system to the anisotropic spin system in the prescence of a magnetic field. In this picture, one can easily obtain the change from a coherent ground state to a Fock state in the replusive interaction case, and the change from a coherent state to a Schrödinger cat state in the attractive case. Within the instanton technique, we obtain the tunnel splitting of an attractive Bose gas. And we found that the essential coherence property of BEC leads to the stability of Schrödinger cat states over a wide range of parameters. The phenomena of macroscopic quantum self trapping are also discussed. PACS number(s): 03.75.Fi,