Comparison of spectral linewidths for quantum degenerate bosons and fermions

We observe a dramatic difference in optical line shapes of a ^4He Bose-Einstein condensate and a ^3He degenerate Fermi gas by measuring the 1557-nm 2 ^3S-2 ^1S 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 ⁴He a triplet-singlet s-wave scattering length a=+50(10)_stat(43)_syst a₀ 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.