Demonstration of an ultrahigh-sensitivity atom-interferometry absolute gravimeter

We present an ultrahigh-sensitivity gravimeter based on an Rb-87 atom interferometer using stimulated Raman transitions. Compared with our previous work, a two-dimensional magneto-optical trap is added in the new gravimeter to increase the atom number and improve the detection signal-to-noise ratio, and a better optical phase-locked loop system is used to reduce the phase noise of Raman beams. Benefiting from these efforts and the excellent performance of the active vibration isolator, a short-term sensitivity of about 4.2 mu Gal/root Hz (1 mu Gal = 1 x 10(-8) m/s(2)) is reached, which improves the sensitivity by a factor of 2 compared with the former best reported value. By a modulation experiment, we further indicate that the residual vibration noise contribution is about 1.2 mu Gal/root Hz, which implies a possible improvement over the present absolute gravity measurement level by about one order of magnitude. Moreover, we demonstrate a calibration experiment to directly evaluate the sub-mu Gal resolution of our gravimeter.