Probing the quantum state of a guided atom laser pulse

We describe bichromatic superradiant pump-probe spectroscopy as a tomographic probe of the Wigner function of a dispersing particle beam. We employed this technique to characterize the quantum state of an ultracold atomic beam, derived from a Rb87 Bose-Einstein condensate, as it propagated in a 2.5 mm diameter circular waveguide. Our measurements place an upper bound on the longitudinal phase space area occupied by the 3×105 atom beam of 9(1) and a lower bound on the coherence length of L 13(1)μm. These results are consistent with full quantum degeneracy after multiple orbits around the waveguide.