Observing bunching in reciprocal space

By a novel reciprocal space measurement, we report a quantitative in situ observation of the bunching effect in a 3D ultracold gas. The quantitative measurement confirms the role of the quantum mechanical exchange symmetry and the Hanbury Brown-Twiss effect in the bunching. The results are in excellent quantitative agreement with the prediction for an ideal Bose gas. Also, the enhanced fluctuations of the bunching effect give a quantitative measure of the increased isothermal compressibility. The experimental difficulties in position space become advantages in reciprocal space.