In situ Ramsey interferometry and diffraction echo with an atomic Fermi gas

We report on the observation of Bragg diffraction of a spin-polarized ultracold $^{6}\mathrm{Li}$ Fermi gas and demonstrate a Ramsey-type matter-wave interferometer. Suppression of $s$-wave collisions by the Pauli principle allows for unperturbed dynamics of the atoms in the trap for longer than $2\phantom{\rule{0.3em}{0ex}}\mathrm{s}$. Employing an echo pulse sequence, we observe coherent superpositions of momentum states which do not show any sign of decoherence during the experimentally limited observation time of $100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$. In contrast, significant decoherence is observed in the presence of a cold cloud of $^{87}\mathrm{Rb}$ atoms. The experiment demonstrates the feasibility of high-resolution atomic interferometry with applications for precision measurements in small volumes and near surfaces.