  Two-photon processes that involve different sub-levels of the ground state of
an atom, are highly sensitive to depopulation and decoherence within the ground
state. For example, the spectral width of electromagnetically induced
transparency resonances in $\Lambda-$type system, are strongly affected by the
ground state depopulation and decoherence rates. We present a direct
measurement of decay rates between hyperfine and Zeeman sub-levels in the
ground state of $^{87}$Rb vapor. Similar to the relaxation-in-the-dark
technique, pumping lasers are used to pre-align the atomic vapor in a well
defined quantum state. The free propagation of the atomic state is monitored
using a Ramsey-like method. Coherence times in the range 1-10 ms were measured
for room temperature atomic vapor. In the range of the experimental parameters
used in this study, the dominant process inducing Zeeman decoherence is the
spin-exchange collisions between rubidium atoms.
