Large-area Sagnac atom interferometer with robust phase read out

We report on recent progress on our matter-wave Sagnac interferometer capable of resolving ultra-slow rotations below the μrad s − 1 level with a 1-s measurement time and a repetition rate of 2 Hz. Two Raman interferometers are employed that are susceptible to rotation and acceleration. We demonstrate two read-out schemes exploiting the strict phase correlation of the dual interferometer, the first one locking the interferometer to the mid-fringe position, and the second relying on phase modulation combined with ellipse fitting. In both, the sensitivity to gravity acceleration is employed for controlling the differential interferometer phase without influencing the rotation signal. Furthermore, we discuss errors in the rotation signal arising from atom source instabilities combined with a residual misalignment of the three pulsed light gratings used for atomic diffraction. Monitoring the source position fluctuations allows us to suppress this spurious signal. We achieve stable operation with a sensitivity of 850   nrad s − 1 Hz − 1 / 2 for a 1-s measurement time, and 20   nrad s − 1 after 4000 s of averaging.