  We analyze time evolution of charge and spin states in a quantum dot coupled
to an electric reservoir. Utilizing high-speed single-electron detection, we
focus on dynamics induced by the first-order tunneling. We find that there is a
difference between the spin and the charge relaxation: the former appears
slower than the latter. The difference depends on the Fermi occupation factor
and the spin relaxation becomes slower when the energy level of the quantum dot
is lowered. We explain this behavior by a theory which includes the first-order
tunneling processes. We conduct detailed comparison of the experiment and the
theory with changing the energy of the quantum dot levels, and the theory can
reproduce the experimental results.
