Recoil frequency measurement with ppb-level uncertainty by Li 6 atom interferometer

We report a first realization of cold atom interferometers with the lightest alkali metal Li6 atoms and precisely measure the recoil frequency. The atoms are cooled to approach the half of the photon-recoil limit by using a narrow 2S→3P ultraviolet transition. A "magic"magnetically insensitive state, a high-efficiency state preparation, and a frequency-insensitive Raman coupling are developed to overcome the challenges from the half-integer spin and hyperfine interaction of atoms. A conjugated Ramsey-Bordé interferometer with crossed Raman beams is realized with a coherence time longer than 2.3 ms. The four sets of interferometers developed using geometric relations greatly eliminate the error from the angle between Raman beams. The measured recoil frequency is ωr=2π×73672.789(36) Hz, representing the most precise measurement of the recoil frequency of Li6 by using the atom interferometer so far. The realized Li6 cold atom interferometer is not only an effective supplement to the existing interferometers, but also provides a good candidate in precision measurement owing to its low mass and high recoil frequency.