Echo Spectroscopy and Quantum Stability of Trapped Atoms

We investigate the dephasing of ultra cold ⁸⁵Rb atoms trapped in an optical dipole trap and prepared in a coherent superposition of their two hyperfine ground states by interaction with a microwave pulse. We demonstrate that the dephasing, measured as the Ramsey fringe contrast, can be reversed by stimulating a coherence echo with a π pulse between the two π/2 pulses, in analogy to the photon echo. We also demonstrate that “echo spectroscopy” can be used to study the quantum dynamics in the trap even when more than 10⁶ states are thermally populated and to study the crossover from quantum to classical dynamics.