Expansion dynamics of a shell-shaped Bose-Einstein condensate*

Bose-Einstein condensates (BECs) confined on shell-shaped surfaces have been proposed as a platform for exploring many nontrivial quantum phenomena on curved spaces. However, as the shell-shaped trapping potential generated with the conventional radio frequency dressing method is very sensitive to gravity, so far experimental studies of shell BECs can only be performed in micro-gravity environments. Here, we overcome this difficulty and create a shell BEC in presence of Earth's gravity with immiscible double species BECs of sodium and rubidium atoms. After eliminating the displacement between the centers of mass of the two BECs with a magic wavelength optical dipole trap, the interspecies repulsive interaction ensures the formation of a closed shell of sodium atoms with its center filled by rubidium atoms. Releasing the double BEC together from the trap, we observe explosion of the shell accompanied by energy transfer from the inner BEC to the shell BEC. With the inner BEC removed, we obtain a hollow shell BEC which shows self-interference as a manifestation of implosion. At the same time, we can tune the interaction between the double BEC system and further explore the rich dynamics of the shell double BEC system.