Comparison of spectral linewidths for quantum degenerate bosons and fermions

We observe a dramatic difference in optical line shapes of a He4 Bose-Einstein condensate and a He3 degenerate Fermi gas by measuring the 1557-nm 2 S3-2 S1 magnetic dipole transition (8 Hz natural linewidth) in an optical dipole trap. The 15 kHz FWHM condensate line shape is only broadened by mean field interactions, whereas the degenerate Fermi gas line shape is broadened to 75 kHz FWHM due to the effect of Pauli exclusion on the spatial and momentum distributions. The asymmetric optical line shapes are observed in excellent agreement with line shape models for the quantum degenerate gases. For He4 a triplet-singlet s-wave scattering length a=+50(10)stat(43)systa0 is extracted. The high spectral resolution reveals a doublet in the absorption spectrum of the BEC, and this effect is understood by the presence of a weak optical lattice in which a degeneracy of the lattice recoil and the spectroscopy photon recoil leads to Bragg-like scattering.