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Conference on Precision Electromagnetic Measurements

Progress of an atomic gravimeter at KRISS

Sang-Bum Lee, Taeg Yong Kwon, Sang Eon Park, Myoung-Sun Heo, Hyun-Gue Hong, Chang Yong Park, Won-Kyu Lee, Dai-Hyuk Yu

Published Jul 1, 2016• 0 citations
Publisher

Abstract

This paper introduces an atomic gravimeter under development at KRISS for precise measurement of gravitational acceleration. The gravimeter based on atom interferometer uses two counter-propagating Raman lasers of π/2-π-π/2 pulse sequence and laser cooled87Rb atoms in free fall. Atomic interference fringes caused by Raman lasers were observed at various time intervals between the three Raman pulses. The frequency sweep rate α which compensates Doppler shift due to gravitational acceleration g was obtained from the interference fringes. In the presentation, the current status and recent results of the KRISS atomic gravimeter will be discussed.

DOI
10.1109/cpem.2016.7540578

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This paper introduces an atomic gravimeter under development at KRISS for precise measurement of gravitational acceleration. The gravimeter based on atom interferometer uses two counter-propagating Raman lasers of π/2-π-π/2 pulse sequence and laser cooled87Rb atoms in free fall. Atomic interference fringes caused by Raman lasers were observed at various time intervals between the three Raman pulses. The frequency sweep rate α which compensates Doppler shift due to gravitational acceleration g was obtained from the interference fringes. In the presentation, the current status and recent results of the KRISS atomic gravimeter will be discussed.
Cold atom free-fall interferometers now realize state-of-the-art precision inertial sensors, opening perspectives towards embedded devices for applications and fundamental physics tests on earth and in space. The acceleration of gravity can be measured in a stimulated Raman transition interferometer where the acceleration under gravity of a cloud of freely falling atoms is sensed by three Raman pulses allowing to split, reflect and interfere atomic wave packets [1]. The gravimeter developped at SYRTE allows to measure the acceleration of gravity at state-of-the-art performances compared to the best classical absolute gravimeters, with a sensitivity of 2×10−8g over 1s and an exactitude of 5×10−9 g [2]. This exactitude is currently limited by wavefront aberrations of the lasers, sampled by residual transverse trajectories of the molasse-cooled cloud over the free fall. Unbalanced transverse trajectories will also bias the measurement due to Coriolis effect [3]. We will investigate the use of a colder atom source to reduce these systematic effects, by implementing Bose-Einstein condensation of the initial cloud, allowing both better position control and better focusing of our source.
We demonstrate a precision measurement of local gravity acceleration g in Wuhan by a compact cold atom interferometer. The atom interferometer is in vertical Mach—Zehnder configuration realized using a π/2 - π - π/2 Raman pulse sequence. Cold atoms were prepared in a magneto-optical trap, launched upward to form an atom fountain, and then coherently manipulated to interfere by stimulated Raman transition. Population signal vs Raman laser phase was recorded as interference fringes, and the local gravity was deduced from the interference signal. We have obtained a resolution of 7 × 10 −9 g after an integration time of 236s under the best vibrational environment conditions. The absolute g value was derived from the chirp rate with a difference of 1.5 × 10 −7 g compared to the gravity reference value. The tidal phenomenon was observed by continuously monitoring the local gravity over 123 h.
Cold-atom interferometer based on stimulated Raman transition is interesting for both fundamental researches and precision measurements. Measuring g, the free fall acceleration, plays an important role in these applications. We describe the theory of atom interferometry, including the evolution of the atomic wavepacket in gravity field, the laser-atom interaction, and the interference of the atomic waves. In this article, we introduce the procedure of measuring g using atom interferometer, the experimental setup, and the tendency of the future development. The works of the domestic and international groups for g measurement are concluded. Furthermore, the noise limits and dominant systematic effects in atom gravimeters are analyzed. The suppressions of the background vibration, the Raman phase noise, and the evaluation of the main systematic effects, are introduced. Finally, the other applications using the atom interferometers for precision measurement are presented, such as in measuring the Newton’s gravitational constant, in testing the weak equivalence principle and the other fundamental applications in physics studies.
Atom interferometry gravimeter with stimulated Raman transitions is realized by atom matter wave, it is now widely used in gravity measurements. We are now building a feasible system of cold atom gravimeter, it is based on the atom interferometry technology by coherently manipulating the cold atoms in a fountain with specific Raman lasers, the cold atom wave packet is splitted, combined, and then re-splitted in the process. Then the atomic wave packet will acquire different phase because of the different evolution path. The precise acceleration can be deduced through the precision measurement of atomic interference fringes phase, and this will be a high precision standard of acceleration. At present, the preparation of Raman laser and the precise control of the laser Frequency have been finished, and they have been proved to meet the requirements of the experiment.
We present measurements on Earth's gravitational acceleration (g) using an in-house developed cold atom gravimeter (CAG) in an atomic fountain geometry. In the setup, the laser cooled 87Rb^{87}Rb87Rb atoms are launched vertically up in the fountain geometry and Doppler sensitive two-photon Raman pulse atom interferometry is applied to detect the gravitational acceleration experienced by the atoms. Using our gravimeter setup, we have measured the local value of 'g' in our laboratory with sensitivity of 621 μμμGal for integration time of 1350 s.