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Physical Review A

Demonstration of an ultrahigh-sensitivity atom-interferometry absolute gravimeter

Zhong-Kun Hu, Bu-Liang Sun, Xiao‐Chun Duan, Min-Kang Zhou, Lele Chen, Su Zhan, Qiaozhen Zhang, Jun Luo

Published Oct 8, 2013• 403 citations
Publisher

Abstract

We present an ultrahigh-sensitivity gravimeter based on an ${}^{87}$Rb 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 $\ensuremath{\mu}\text{Gal}/\sqrt{\text{Hz}}$ ($1\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{Gal}=1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ $\text{m}/{\text{s}}^{2}$) 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 $\ensuremath{\mu}\text{Gal}/\sqrt{\text{Hz}}$, 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-$\ensuremath{\mu}$Gal resolution of our gravimeter.

DOI
10.1103/physreva.88.043610

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We present an ultrahigh-sensitivity gravimeter based on an 87{}^{87}87Rb 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 \ensuremathμGal/Hz\ensuremath{\mu}\text{Gal}/\sqrt{\text{Hz}}\ensuremathμGal/Hz​ (1\ensuremathμGal=1\ifmmode×\else\texttimes\fi10\ensuremath−81\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{Gal}=1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}1\ensuremathμGal=1\ifmmode×\else\texttimes\fi10\ensuremath−8 m/s2\text{m}/{\text{s}}^{2}m/s2) 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 \ensuremathμGal/Hz\ensuremath{\mu}\text{Gal}/\sqrt{\text{Hz}}\ensuremathμGal/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-\ensuremathμ\ensuremath{\mu}\ensuremathμGal resolution of our gravimeter.
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Summary form only given. Atom interferometry based on cold atoms and light beamsplitters is a mature technique, which allows for the realization of highly sensitive inertial sensors. In the most advanced inertial sensors based on atom interferometry, atomic wavepackets are manipulated by light beamsplitters using two-photon Raman transitions. The interferometer phase shift, which depends on the acceleration and the rotation of the experimental setup with respect to the inertial reference frame defined by the atoms in free fall, is derived from the relative displacement of the atoms with respect to the beamsplitter laser's equiphases. Wavefront aberrations thus induce parasitic phase shifts, which bias the measurement, and which are linked to the residual ballistic motion of the atoms across the laser beam profile during their free fall and cancels out at zero atomic temperature. The wavefront distortions of the lasers beamsplitters is an important limit in the accuracy of atom interferometers and in particular, the dominating contribution in best cold atom gravimeters (CAG) uncertainty budget. This work clears the way towards unprecedented accuracies for absolute gravimetry, below the 1 μGal level. The method proposed can be applied to any atomic sensor based on light beamsplitters, which are inevitably affected by distortions of the lasers wavefronts. The improved control of systematics it provides will have significant impact in high precision measurements with atom interferometry, with important applications to geodesy, fundamental physics tests and to the development of highest grade inertial sensors.
We demonstrate a novel scheme for Raman-pulse and Bragg-pulse atom interferometry based on the 5S−6P5\mathrm{S} - 6\mathrm{P}5S−6P blue transitions of 87^{87}87Rb that provides an increase by a factor ∼2\sim 2∼2 of the interferometer phase due to accelerations with respect to the commonly used infrared transition at 780 nm. A narrow-linewidth laser system generating more than 1 W of light in the 420-422 nm range was developed for this purpose. Used as a cold-atom gravity gradiometer, our Raman interferometer attains a stability to differential acceleration measurements of 1×10−81\times10^{-8}1×10−8 ggg at 1 s and 2×10−102\times 10^{-10}2×10−10 ggg after 2000 s of integration time. When operated on first-order Bragg transitions, the interferometer shows a stability of 6×10−86\times10^{-8}6×10−8 g at 1 s, averaging to 1×10−91\times10^{-9}1×10−9 g after 2000 s of integration time. The instrument sensitivity, currently limited by the noise due to spontaneous emission, can be further improved by increasing the laser power and the detuning from the atomic resonance. The present scheme is attractive for high-precision experiments as, in particular, for the determination of the Newtonian gravitational constant.
For most atom interferometers, the vibration isolation unit is applied to reduce vibration noise. In our experiment, instead of isolation, the vibration signals are monitored, and combining with the sensitive function, the compensation phase shift for the atom interferometer is obtained. We focus on the correction over a wide spectrum rather than on "monochromatic" frequencies. The sensitivity of the atom gravimeter can be upgraded by a factor of more than two. Furthermore, we demonstrate that the atom interferometer can still produce a good measurement result without passive vibration isolation in extremely noisy environments by using vibration compensation.