Competition between Spin Echo and Spin Self-Rephasing in a Trapped Atom Interferometer

We perform Ramsey interferometry on an ultracold 87Rb ensemble confined in an optical dipoletrap. We use a \pi-pulse set at the middle of the interferometer to restore the coherence of the spinensemble by canceling out phase inhomogeneities and creating a spin echo in the contrast. However,for high atomic densities, we observe the opposite behavior: the \pi-pulse accelerates the dephasingof the spin ensemble leading to a faster contrast decay of the interferometer. We understand thisphenomenon as a competition between the spin-echo technique and an exchange-interaction drivenspin self-rephasing mechanism based on the identical spin rotation effect. Our experimental data iswell reproduced by a numerical model.