Description du projet - Program description

Introduction

The Matter wave - laser based Interferometer Gravitation Antenna (MIGA) project concerns the construction of a novel infrastructure to study strain tensor of space-time and gravitation. Using a novel approach for strain measurement, based on quantum mechanics, this infrastructure will allow for deeper understanding of the earth's gravity field over a very broad band, from frequencies of less than 1 cycle per second to those in excess of hundreds of hertz. The applications of MIGA extend from monitoring the evolution of the gravitational field to providing a new tool for detecting gravitational waves. By combining geophysics and fundamental physics application in a single infrastructure, MIGA will lead to an unprecedented step in understanding geophysical phenomena and will allow for enhancing existing and future gravitational wave detectors. It will keep France among the leaders in many major fields; fundamental physics, matter wave interferometry, geophysics and gravitational wave astronomy.

This MIGA Instrument will use a novel approach for seismic studies and will improve the sensitivity of strain measurements by combining laser and matter-wave interferometry. It will allow for enhancing our observations of earth strain variation at low frequencies, our understanding of geophysical fluctuations of the gravity field and our knowledge of gravity gradients variations and fluctuations in underground laboratories. Because our combined approach shares many design characteristics with laser-interferometric gravitational wave detectors, MIGA will help pushing the fundamental limits of ground-based gravitational wave (GW) detectors, set by seismic and gravity gradient noise, by embedding matter-wave and laser interferometer together. MIGA can also be used for other basic science objectives. For instance, a precise time-mapping of the fluctuation of gravitational forces can give us limits on the violations of the Lorentz invariance on thus open new windows in our understanding of quantum gravity. The MIGA proposal is triggered by the latest progress in atom interferometry. It will be the first of a new generation of subterranean detectors for geoscience, seismology and fundamental physics.

The Purpose (Mission)

MIGA will be the first generation of large-scale matter-wave seismic sensor. Recent advances in the performance of large area ring lasers have advanced the field of rotational seismometer [Geo 2009].  MIGA will offer performance characteristics competitive with the best large area rings, at significantly smaller installation footprint.  We expect the proposed instrument to compete favorably in performance with existing large area ring laser gyroscopes. The improvements in seismic instrumentation are thought to be relevant for the earthquake prediction/monitoring.

MIGA will enhance the detection of anomalies and variations in gravitational fields [Wu 2009]. To this end, present day technology foresees the use of gravimeters and gravity gradiometers, to measure the gravitational field due to the presence of a mass or mass variation contained in a volume, with a view to gaining an understanding of the shape and density variations of the volume. Information gained by such measurements can be used for example in general surveying [Champolion 2004, Derode 2010], or in the petrochemical industry for analyzing the spatial extent and other properties of underground materials. High performance gravitational sensors will allow detecting smaller gravitational anomalies and thus monitoring the evolution of mass distributions. With levels of 5x10-13 g/Hz1/2, a matter-wave interferometer is capable of resolving 1 cubic-meter of water at distances of 100 m. In the future, MIGA can be extended for identifying local structural traps, stratigraphic traps or fluid movement [Jacob 2008] from surrounding regional geology providing there is a sufficient density contrast, e.g. sufficient sensitivity. It can be applied to any field depending upon the reservoir thickness and size, depth of burial, and the density contrast between the fluids.

In addition to precise gravimetry, the project is built on the ideas around the detection of gravity gradient and gravitational wave monitors in several respects.  In the configurations studied here, performing a differential measurement between two or three atom interferometers run simultaneously using the same laser pulses monitors the effect of seismic or gravitational waves [Dimopulos 2008].  The lasers provide a common “ruler” for comparison of multiple atom interferometers.  The distance between the interferometers can be large because only the light travels over this distance, not the atoms.  In a sense, the atom interferometers are the analogue of the mirrors in a light interferometer and it is the distance between them that determines the size of the signal.

Antenna design 

The MIGA antenna baseline is designed to reach high sensitivity at low frequency. It consists in a chain of three atom interferometers distributed along a single beam path. The laser beam used to drive the matter-wave interferometer laser pulses is also locked to a resonant cavity. Each atom interferometer measures the local accelerations felt by the atoms with respect to the cavity mirrors. A differential measurement between the atom interferometers removes the mirror contribution to the signal and results in gravity gradient, gravity curvature or higher moments readout.

High precision is reached because the atoms are in free fall, and the implementation of atom interferometers with long interrogation time relies on the use of atoms launched along a parabolic trajectory as demonstrated in [Canuel 2006]. Interrogation times of 200 mms will be reached with an “active interrogation area” of diameter 20 cm, thus requiring a beam diameter in the cavity of at least 30 cm (a 10 cm radium would lead to a 4-fold loss in sensitivity). 

In the final implementation, the path length will be about 650 m long, while a 10 m long testbench will be used during the development step. The interrogation laser will be derived from a high power frequency doubled high power telecom source locked on the MIGA arm cavity. Thus, a 2-color cavity, resonant both at 1560 nm and 780 nm will be designed [Bernon 2011].

Short bibliography

[Geo 2009]    Bulletin  of  the  Seismological  Society  of  America,  May  2009  special  issue  Rotational Seismology and Engineering Applications.

[Champolion 2004]    Champollion C., Masson F., Van Baelen J., Walpersdorf A., Chéry J. and Doerflinger E., 2004: GPS monitoring of the tropospheric water vapor distribut ion and variation during the 9 September 2002 torrential precipitation episode in the Cévennes (southern France) J. Geophys. Res., Vol. 109, No. D24.

[Derode 2010]    B. Derode, Y. Guglielmi, F. Cappa, S. Gaffet and T. Monfret (2011) Seismicity and hydromechanical behavior of a fractured porous rock under a high pressure fluid injection - iDUST 2010, DOI : 10.1051/idust/201101003

[Jacob 2008]     Jacob T., R. Bayer, H. Jourde, J. Chery, J. P. Boy, N. Le Moigne, B.Luck, P. Brunet, J. Hinderer. Absolute Gravity monitoring of water storage in a karst aquifer, J. of Hydrology 359, 105– 117

[Dimopulos  2008]    S. Dimopoulos, et al. Phys. Rev. D78, 122002 (2008)


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Philippe BOUYER,
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