Atom interferometry has proven both a powerful means for probing fundamental physics, and a promisingtechnology for high-precision inertial sensing. However, their performance has been limited by the availableinterrogation time of atoms falling freely in Earth's gravitational eld. Trapped geometries have thus beenexplored as a means to improve the sensitivity of atom interferometers, but attempts to date have su ered fromdecoherence caused by trap inhomogeneities. We have demonstrated a trapped atom interferometer with anunprecedented interrogation time of 20 seconds,1 achieved by trapping the interferometer in the resonant modeof an optical cavity. The cavity is instrumental to this advance, as it provides spatial mode ltering for thetrapping potential. Because the interferometer is held with the arms vertically separated along the gravitationalaxis, a phase shift accumulates due to the gravitational potential energy di erence between the arms. Moreover,this phase accumulates continuously during the hold time, providing an orders-of-magnitude greater immunityto vibrations than previous atom-interferometric gravimeters at the same sensitivity.
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