  We prepare a superposition of two motional states by addressing lithium atoms
immersed in a Bose-Einstein condensate of sodium with a species-selective
potential. The evolution of the superposition state is characterized by the
populations of the constituent states as well as their coherence. The latter we
extract employing a novel scheme analogous to the spin-echo technique.
Comparing the results directly to measurements on freely-evolving fermions
allows us to isolate the decoherence effects induced by the bath. In our
system, the decoherence time is close to the maximal possible value since the
decoherence is dominated by population relaxation processes. The measured data
are in good agreement with a theoretical model based on Fermi's golden rule.
