  The ability to apply GHz frequencies to control the quantum state of a single
$P$ atom is an essential requirement for the fast gate pulsing needed for qubit
control in donor based silicon quantum computation. Here we demonstrate this
with nanosecond accuracy in an all epitaxial single atom transistor by applying
excitation signals at frequencies up to $\approx$ 13 GHz to heavily phosphorous
doped silicon leads. These measurements allow the differentiation between the
excited states of the single atom and the density of states in the one
dimensional leads. Our pulse spectroscopy experiments confirm the presence of
an excited state at an energy $\approx$ 9 meV consistent with the first excited
state of a single $P$ donor in silicon. The relaxation rate of this first
excited state to ground is estimated to be larger than 2.5 GHz, consistent with
theoretical predictions. These results represent a systematic investigation of
how an atomically precise single atom transistor device behaves under rf
excitations.
