Metastable Helium BEC production and numerical simulation of a BEC in a bipartite optical lattice.

Abstract

Bose-Einstein condensations using metastable helium atom provide ideal settings for manipulation of quantum many body systems on single atom basis. Unlike alkali gases, metastable helium BEC is realised in its metastable state rather than the electronic ground state. Owing to the lifetime of more than two hours, helium triplet state can be considered as an effective ground state in experiments. With its considerable internal energy (19.8 eV per atom) single atom detection with high precision is possible when an metastable helium atom hits a multi-channel plate (MCP). This advantage can be employed for single atom resolved measurements of correlation function, which allows addressing quantum phenomena with high spatial and temporal resolution. Furthermore, ultracold atoms in optical lattice potentials serve as an artificial platform to emulate quantum condensed matter phenomena in a periodic landscape ranging from strongly correlated systems, to implementations in information technology. Due to their high controllability, optical lattices can be spatially reconfigurable, which offers possibility to study rich and complex dynamics. In particular, optical lattices with higher band occupation allows investigating physics beyond the tight binding model and mimicking exotic quantum states such as unconventional Bose-Einstein condensates (UBECs) and topological superfluid. This work describes two main projects: (1) The experimental realisation of Bose-Einstein condensation (BEC) of metastable helium atoms using an in-vacuum coil magnetic trap and a crossed beam optical dipole trap. (2) Numerical simulation on interaction induced states in the p-band of a bipartite optical lattice potential. First, we report the realization of metastable helium BEC using an in-vacuum coil magnetic trap and a crossed-beam optical dipole trap. A quadrupole-Ioffe configuration magnetic trap made from in-vacuum hollow copper tubes provides fast switching times while generating traps with a 10-G bias, without compromising optical access. The bias enables in-trap one-dimensional Doppler cooling to be used, which is the only cooling stage between the magneto-optic trap (MOT) and the optical dipole trap. This allows direct transfer to the dipole trap without the need for any additional evaporative cooling in the magnetic trap. The entire experimental sequence takes 3.3 s, with essentially pure BECs observed with 1000000 atoms after evaporative cooling in the dipole trap. Second, we apply the coupled-mode theory to investigate the condensates loaded into the p-band of 2D bipartite optical lattice potential. In the plane-wave basis, we analyse the the linear band-gap structure and the associated Bloch states. The geometry of the first excited state is tuned such that two degenerate energy minima are formed at quasi-momentum of the first Brillouin zone (FBZ). At the two degenerate minima, Bloch states are formed via hybridisation of the s and p-orbital of the individual lattice sites with pi/2 phase delay. We show that these two modes are driven to form 2D vortex array with global orbital angular momentum across the entire lattice. Furthermore, we used the coupled-mode approach to show the existence of a chiral atomic superfluid in the one-dimensional version of the bipartite square optical potential considered in this study. Finally, a full band gap is observed between the ground and the first excited bands, indicating existence of 2D spatially localized states. We extend our study to investigate the localisation of nonlinear matter wave in a 2D bipartite optical lattice. We find that 2D bipartite optical lattice admits unipole and dipole gap solitons. These solitons are associated with the topology of the lattice potential. Moreover, gap solitons with different spatial mobility are bifurcated from the upper edge of the first excited state into the second bandgap under a defocusing nonlinearity.