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High Sensitivity Matter-Wave Gravitation Sensors

Periodic Reporting for period 1 - MWGRAV (High Sensitivity Matter-Wave Gravitation Sensors)

Reporting period: 2016-04-15 to 2018-04-14

Context of the project.
Atom interferometers offer interesting applications in geophysics (gravimetry, gradiometry, Earth rotation rate measurements), inertial sensing (e.g. submarine positioning), metrology (new definition of the kilogram) and fundamental physics (tests of the standard model, tests of general relativity). The field of atom interferometry (AI) now enters a phase where extreme sensitivity levels must be demonstrated, in order to enlarge the potential applications. In particular, several theoretical proposals on the application of AI to gravitational wave (GW) detection have been published in the last years. However, there is an important gap between the current sensitivities of atom interferometers and the requirements for using AI in future GW astronomy facilities. The proposed MSC project aimed at studying new atom and optical interferometry techniques, in order to significantly improve the performances of matter-wave gravitation sensors. On this path, major breakthroughs are also expected in metrology, geophysics and inertial navigation.
Various international groups are developing and operating experiments to improve the performances of matter-wave inertial sensors. In particular, they have started to study the gain in sensitivity associated with the use of new atom optics techniques, such as large momentum transfer (LMT) beam splitters, delta-kick cooling or ultracold atoms. Through this MSC action, the Experienced Researcher was intended to strengthen the leading position of the SYRTE laboratory in the field of AI metrology, by studying advanced AI and optical interferometry techniques and implementing them on a state of the art atomic inertial sensor.

Objectives of the action.
The project was intended to proceed in two steps. First, the Experienced Researcher was intended to push the state of the art of atom interferometer performances by demonstrating a two order of magnitude improvement to the inertial sensitivity of a cold atom gyroscope/accelerometer based at the SYRTE laboratory. In particular, Experienced Researcher would implement a technique to avoid the dead time between consecutive measurements occurring in cold atom sensors, which are currently an important limit of AI to several applications, in particular inertial navigation. Second, she would study the potentially important performance improvement of using an optical cavity to interrogate the atoms in the interferometer. We aimed at improving by more than one order of magnitude the interferometer sensitivity by using multiple photon diffraction to coherently split the atomic waves. The Experienced Researcher would then study the physics of the atom interferometer coupled to the optical cavity, both experimentally and theoretically, which has hardly been investigated by other groups.
One aim of this MSC action was to develop the researcher towards being an independent scientist under the mentorship of her supervisors. The researcher would acquire new scientific and management skills, and learn how to develop her theoretical ideas to experimental realisation and publication. Therefore, a further aim under the host’s guidance would be to develop the researcher’s publication record to be commensurate with her experience upon the completion of the MSC fellowship.
The proposed research was intended to be both of experimental nature by implementing new techniques to improve the sensitivity of the SYRTE cold atom gyroscope/accelerometer, and of theoretical nature by modelling the limits to the sensitivity of the interferometer and the interrogation of the atoms by the optical cavity. Within the Atom Interferometry group at SYRTE, the Experienced Researcher would work on an existing experiment which has been setup and optimized since 2009. This setup would allow implementing and testing various techniques on a platform which represents the state of the art in terms of rotation rate sensitivity levels for cold atom sensors. She would then study
Work Package 1: Pushing the sensitivity of matter-wave inertial sensors
In a first task, we have set up and characterized a large collimated top-hat laser beam and illustrated some of its benefit over Gaussian beams for atom interferometry experiments. This part of project was not mentioned very clearly in the proposal but we realized its importance while working to improve the sensitivity of the interferometer. This work is reported in a publication submitted in August 2018 (preprint available at http://arxiv.org/abs/1808.03355).

Work Package 2: Cavity atom interferometry
In the second, main task of the project, we have set up an optical cavity experiment as a prototype to be implemented on a state of the art cold atom interferometer.
We were interested in studying the experimental realisation of large mode resonating cavity with commercially available optical components, to enhance the atom interferometry. In this part of work package, we realized a compact large mode degenerate optical resonator from scratch, and studied in detail the behaviour of the cavity in terms of transverse misalignment, mode structure, gain of the cavity and effect of aberrations. The achievements in this Work package and the main limitations about this resonators are discussed in the technical report. A publication describing our results is currently in preparation, with the MSC fellow as main author. We achieved to resonate beams of 4 mm waist in a 40 cm long optical resonator featuring two mirrors and an intra cavity lens, with an optical gain of 10.
In the first task, we have impleented for the first time an atom interferometer with a laser beam that features a flat intensity profile in a given region. This allows to improve the contrast of the atom interferometer thanks to the homogeneity of the atom light coupling constant. We think that this solution will be vastly implemented in future atom interferometer designs, for improved signal to noise ratio.

In the second task, we have tackled the difficult problems of realizing a large mode (1 cm waist) optical cavity in a compact (40 cm) design. We set up an optical cavity system and studied in very details the effect of any optics imperfections on the cavity performance. For that puropose we developped a complete numerical model of the cavity, which may be used by other teams. While the result of interfacing the cavity with an atom interferometer has not been obtained, we have performed the pioneering studies that show why it is a difficult problem, and what remains to be studied.
Picture of the optical cavity setup built during this project