Spectroscopy of the Mott insulator to Bose glass transition

We present measurements probing the Mott insulator to Bose glass transition in three dimensions using spectroscopic techniques.~Ultracold 87Rb atoms trapped in a cubic optical lattice with independently controlled speckle disorder are used to realize the disordered Bose Hubbard model.~The Mott gap is measured in the clean lattice using Raman spectroscopy.~The addition of disorder causes a phase transition to the Bose glass state, which has a gapless excitation spectrum.~We use this signature to detect the phase transition as the ratio of Hubbard to tunneling energies is varied.