Periodic instanton method and macroscopic quantum tunneling between two   weakly-linked Bose-Einstein condensates
  A new method is used to investigate the tunneling between two weakly-linked Bose-Einstein condensates confined in double-well potential traps. The nonlinear interaction between the atoms in each well contributes to a finite chemical potential, which, with consideration of periodic instantons, leads to a remarkably high tunneling frequency. This result can be used to interpret the newly found Macroscopic Quantum Self Trapping (MQST) effect. Also a new kind of first-order crossover between different regions is predicted. 
Yunbo Zhang and H.J.W. M\"uller-Kirsten (University of Kaiserslautern)