  We realize a single-band 2D Bose-Hubbard system with Rb atoms in an optical
lattice and measure the condensate fraction as a function of lattice depth,
crossing from the superfluid to the Mott-insulating phase. We quantitatively
identify the location of the superfluid to normal transition by observing when
the condensed fraction vanishes. Our measurement agrees with recent quantum
Monte Carlo calculations for a finite-sized 2D system to within experimental
uncertainty.
