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Transportable gravity gradiometer

The recent development of atom interferometers using laser light pulses has provided a sensitive new technique for precision inertial sensing. Unlike traditional inertial sensors employing bulk test masses, atom interferometer-based sensors employ the individual atoms as ideal drag-free test masses. The de Broglie wave associated with each atom is then utilized to perform an interferometric measurement of the local acceleration. Atom interferometers have demonstrated state-of-the-art sensitivity as inertial sensors in the laboratory, and the operation of an atom interferometer-based instrument in space offers even greater sensitivities due to the much longer interaction times available in a microgravity environment [1].

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Figure 1: A transportable atomic gravity gradiometer prototype developed at JPL. (Click to view full image.)

Atomic sensor

Figure 2: Cut-away illustration of a single atomic sensor inside magnetic shields.

JPL‘s quantum gravity gradiometer employs two identical atom interferometers separated by a measurement baseline of 1 m in order to measure the gravity gradient. In a typical measurement sequence, cesium atoms are first captured from a novel cold atom beam source [2] and laser-cooled in two separate ultra-high vacuum magneto-optical traps. Each trap collects up to 109 atoms, and these atoms are then cooled about 1 µK (corresponding to an rms velocity less than 1 cm/s) by detuning the laser frequencies from the atomic resonance after all magnetic fields are switched off. This low kinetic energy allows a large fraction of the ultra-cold atoms to interact with the velocity-sensitive stimulated Raman transitions used in the atom interferometer, and also prevents the ensemble of atoms from expanding outside the detection region during long interactions times. In microgravity applications, this slow thermal expansion of the ultra-cold atom cloud also allows much longer interrogation times than are possible in terrestrial instruments where the large vertical acceleration from Earth’s gravity field causes the atoms to quickly fall from the interrogation region. In the current laboratory instrument, the laser-cooled atoms are launched vertically as in an “atom fountain” in order to obtain measurement times up to one-third of a second. Immediately after launch, the cold atoms undergo a state and velocity selection sequence to select atoms in the magnetic field insensitive (mF = 0) Zeeman sublevel and with a one-dimensional temperature of a 100 nK or less. A vertically aligned pair of phase-locked lasers drive velocity-sensitive stimulated Raman transitions in the free-falling atoms in both fountains simultaneously to realize two atom interferometers with a common reference frame. This common reference frame allows vibrations in the measurement platform as well as laser fluctuations to effectively cancel as common-mode noise. The relative phase shift Δφ between the two atom interferometers gives the gravitational acceleration difference over the measurement baseline Δz, and the linear gravity gradient is calculated from the measured phase as Δφ⁄(2kT2Δz).

AI sensor data

Figure 3: Atom interferometer sensor signals in the gravity gradiometer with T = 165 ms. The phase is updated electronically to generate the interferometer fringes.

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Figure 4: Parametric plot of the same atom interferometer signals illustrating the suppression of common-mode noise. The relative phase between the two interferometers can be determined from the fitted ellipse (red curve).

Publications

  1. J. R. Kellogg, Nan Yu, J. M. Kohel, R. Thompson, D. C. Aveline, E. D’Ambrosio, D. Strekalov and L. Maleki, “Development of a Quantum Gravity Gra- diometer for Gravity Measurement from Space,” Division of Atomic, Molecular, and Optical Physics conference (DAMOP), 2007, Calgary, Alberta, Canada.
  2. N. Yu, J. M. Kohel, J. R. Kellogg, and L. Maleki, “Development of an atom-interferometer gravity gradiometer for gravity measurement from space,” Appl. Phys. B 84, 647 (2006).
  3. J. Ramirez-Serrano, N. Yu, J. M. Kohel, J. R. Kellogg, and L. Maleki, “Multistage two-dimensional magneto-optical trap as a compact cold atom beam source,” Opt. Lett. 31, 682 (2006).
  4. J. R. Kellogg, N. Yu, J. M. Kohel, R. J. Thompson, D. C. Aveline, and L. Maleki, “Longitudinal coherence in cold atom interferometry,” J. Mod. Opt. 54, 2533 (2007).
  5. J. M. Kohel, R. J. Thompson, J. R. Kellogg, D. C. Aveline, and N. Yu, “Development of a transportable quantum gravity gradiometer for gravity field mapping,” Earth Science Technology Conference 2008.
  6. R. J. Thompson, J. M. Kohel, J. R. Kellogg, D. C. Aveline, L. Maleki, and N. Yu, “Development of a transportable gravity gradiometer for ground and space applications,” NASA Science Technology Conference 2007.
  7. J. M. Kohel, N. Yu, J. R. Kellogg, R. J. Thompson, D. C. Aveline, and L. Maleki, “Quantum gravity gradiometer development for space,” Earth Science Technology Conference 2006.
  8. N. Yu, J. M. Kohel, J. R. Kellogg, and L. Maleki, “Towards a space-borne quantum gravity gradiometer: progress in laboratory demonstration,” Earth Science Technology Conference 2005.
  9. L. Maleki, N. Yu and J. Kohel, “Quantum gravity gradiometer for sub-surface imaging,”
    Space 2004 Conference and Exhibit, AIAA-2004-5906 (2004).
  10. N. Yu, J. M. Kohel, J. Ramirez-Serrano, J. R. Kellogg, L. Lim, and L. Maleki, “Progress towards a space-borne quantum gravity gradiometer,” Earth Science Technology Conference 2004.
  11. N. Yu, J. M. Kohel, L. Romans, and L. Maleki, “Quantum gravity gradiometer sensor for earth science applications,” Earth Science Technology Conference 2002.