Atomic fountains and clocks
Momentum exchange between light and atoms causes mechanical forces which, in recent years, have been used to slow thermal atoms to speeds on the order of a few photon recoils (~10 cm/sec). At these low speeds, atomic trajectories are significantly altered by gravitational forces. Recently, laser cooled atoms have been launched upwards on vertical trajectories at speeds for which gravity reverses the atom's direction in approximately 30 cm.
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