Integrated Gravity Sensor
Currently mostly classical sensors are used in applications except for time-keeping in national standards laboratories, where there is no alternative to atomic standards. In addition to frequency standards, cold atom sensors have demonstrated highest precision in the laboratory for gravity, gravity gradients and rotation. The unprecedented sensitivity and practically drift-free operation make atom interferometric sen-sors very interesting for applications, e.g. in oil exploration, inertial navigation or geodesy and sev-eral initiatives have been started towards the realization of a mobile non-laboratory-based sensor, the earliest ones being the ACES/PHARAO initiative for a space atomic clock based on cold atoms and gravity as well as gravity gradient sensors at JPL and Yale/Stanford University (starting around 2000). In particular in gravity and gravity gradient sensing applications Europe has been lagging until the more recent QUANTUS, ICE, FINAQS, SOC and SAI projects, which dem-onstrated the first BEC in microgravity and the first atom interferometer operating in parabolic flights . In addition there is a mobile atom interferometer developed by CNRS-SYRTE and a simplified optical clock under development by UniFi. However, with several 100kg of weight and expert human intervention needed for operation and control the devices realized in these projects are still too large and too delicate to run for commercial applica-tions. iSense targets a significant improvement beyond this already impressive state-of-the-art by the re-alization of a gravity sensor in a commercially interesting form factor with a sensitivity of 10-9 g, comparable to the best laboratory instruments to date and with potential of further sensitivity in-creases in the near future.
Innovative interferometry schemes
Current state-of-the-art atom sensors typically use freely falling cold atoms as a probe, and light-pulse Raman interferometry as an interrogation technique. Such technique has been applied to measure inertial forces and fundamental constants with high precision, better than or compatible with the best classical instruments. The gravitational constant G has been measured with 10-3 precision, with sensitivities of 8*10-8 m/s2 *Hz-1/2 for gravitational acceleration, 3*10-7 s-2 *Hz-1/2 for gravity gradients and 0.6 nrad*Hz-1/2 for rotations, better than or compatible with the best classical instruments. The free fall is a necessary ingredient in these devices to minimize external per-turbations and the associated systematic errors, while the fountain geometry allows increasing the sensitivity-determining interrogation time for a given height of the vacuum chamber. Finite practical heights of typically 0.1-1m pose the main limitation on further increases in sensitivity of fountain-type instruments. Only very recently realized optical clock experiments have so far explored trapped schemes using a magic optical lattice, which remove the interrogation time constraint. iSense will aim at several breakthroughs in developing and optimizing “trapped” schemes for highest precision measurements – removing the need for free-fall distances and allowing for compact local shielding of magnetic fields and other external disturbance. At this stage the main aim is not to beat the fountain laboratory instruments in sensitivity but to realize a similarly sensitive instrument in an application-oriented form factor. However, with the free fall time constraint removed, it is anticipated that the iSense sensor schemes will have a lot of room for future sensitivity improvements. The schemes for force measurement will be evaluated and experimentally investigated, targeting an optimization towards robustness against decoherence and simplicity.
Research progress in the 1st year
Wannier-Stark, Modulation and Levitation interferometry schemes have been implemented and tested. The optimization of the sensitivity and accuracy of these schemes is under way, although a preliminary analysis indicates that the Levitation scheme is unlikely to reach the iSense sensitivity target.