Imaging the Mott Insulator Shells using Atomic Clock Shifts
  Microwave spectroscopy was used to probe the superfluid-Mott Insulator transition of a Bose-Einstein condensate in a 3D optical lattice. Using density dependent transition frequency shifts we were able to spectroscopically distinguish sites with different occupation numbers, and to directly image sites with occupation number n=1 to n=5 revealing the shell structure of the Mott Insulator phase. We use this spectroscopy to determine the onsite interaction and lifetime for individual shells. 
Gretchen K. Campbell, Jongchul Mun, Micah Boyd, Patrick Medley, Aaron   E. Leanhardt, Luis Marcassa, David E. Pritchard, and...