Superfluid to Mott insulator transition in a 3D optical lattice!

We are happy to report the observation of a quantum phase transition from superfluid to Mott-insulator in a three-dimensional cubic optical lattice at the Institute for Molecular Science (IMS).

  Figure 1: Absorption images of the atomic cloud at different lattice depths, taken after 15 ms free expansion.  


   87Rb, 5S1/2, F=1, mF=+1

   We first created a spin-polarized Bose-Einstein Condensate (BEC) of 87Rb atoms after seven seconds of evaporative cooling in a crossed-optical dipole trap with a wavelength of 1064 nm under a magnetic field gradient. The last one second of the evaporation proceeded by lowering the field gradient to perform a runaway evaporation that yielded a nearly pure BEC (5S1/2, F=1, mF=+1). After slightly deepening the trap depth within 100 ms, we loaded the BEC into a three-dimensional cubic optical lattice at a wavelength of 1064 nm by linearly ramping up the power of three lattice beams for 100 ms. After holding lattice beam powers for 20 ms, the atomic samples were subject to absorption imaging from horizontal direction to measure momentum distribution with a time of flight of 15 ms.

   Figure 1 shows typical momentum distribution of atoms at different lattice depths. While we observe a spin-polarized BEC in (a), atoms are transferred into a superfluid phase in (b) where atoms hop between neighboring sites. Due to the phase coherence between atoms in different sites, we see a matter-wave interference in a momentum space. In (c), since the hopping to neighboring sites is suppressed due to deeper potential, the superfluid phase experiences a quantum phase transition to a Mott-insulator phase. Finally in (d), we see clear feature of vanishing phase coherence, where atoms are localized in each site. We also confirm the restoration of BEC when we decrease the lattice depth from Mott insulator, indicating the coherent dynamics of atoms in an optical lattice.

Posted: 25 July 2016
First observation: 26 May 2016

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