Driven Macroscopic Quantum Tunneling of Ultracold Atoms in Engineered Optical Lattices

– Coherent macroscopic tunneling of a Bose-Einstein condensate between two parts of an optical lattice separated by an energy barrier is theoretically investigated. We show that by a pulsewise change of the barrier height, it is possible to switch between tunneling regime and a self-trapped state of the condensate. This property of the system is explained by effectively reducing the dynamics to the nonlinear problem of a particle moving in a double square well potential. The analysis is made for both attractive and repulsive interatomic forces, and it highlights the experimental relevance of our findings. The term Macroscopic Quantum Tunneling (MQT) usually refers to a property of a macroscopic system, or part of it, that performs tunneling through a potential barrier like a single particle. After the first experimental discovery of this phenomenon [1] as a tunneling escape from a metastable state in Josephson Junctions (JJ), there have been a number of similar realizations in completely different physical systems, such as liquid Helium [2] and nanomagnets [3]. Frequently MQT is associated with the tunneling between different states of the system with no reference to a spatial energy barrier [4–7], but there are also several examples where macroscopic objects, such as vortices [8, 9] or fluxons [10] in JJ’s, or magnetic domain walls [11], tunnel through a potential barrier. Recently [12, 13], following earlier theoretical predictions [14], it has been found that a BEC trapped in an harmonic well potential of mesoscopic length behaves like a single JJ: for nonzero initial imbalance of the number of atoms in different wells, Josephson oscillations are present in the system, i.e. the condensate tunnels back and forth through the barrier. The only difference with respect to superconducting JJ’s is that, for large initial imbalance, the condensate is mainly trapped in one of the wells, producing what is called Macroscopic Self-Trapping (MST).