  Quantum critical matter has already been studied in many systems, including
cold atomic gases. We report the observation of a universal behaviour of
ultracold quantum critical Bose gases in a one-dimensional optical lattice. In
the quantum critical region above the Berezinskii-Kosterlitz-Thouless
transition, the relative phase fluctuations between neighboring subcondensates
and spatial phase fluctuations of quasi-2D subcondensates coexist. We study the
density probability distribution function when both these two phase
fluctuations are considered. A universal exponential density probability
distribution is demonstrated experimentally, which agrees well with a simple
theoretical model by considering these two phase fluctuations.
