  We report on the observation of quantum interference of a collective single
spin excitation with a spin ensemble of $N_{\text{atom}} =10^5$ atoms.
Detection of a single photon scattered from the atoms creates the single spin
excitation, a Fock state embedded in the collective spin of the ensemble. The
state of the atomic ensemble is then detected by tomography via a quantum
non-demolition measurement of the collective spin. A macroscopic difference of
the order of $\sqrt{N_{\text{atom}}}$ in the marginal distribution of the
collective spin state arises from the interference between the single excited
spin and $N_{\text{atom}}$ atoms. The hybrid discrete-continuous processing of
the collective spin pave the road towards generation of even more exotic states
for quantum information processing, precision measurements and communication.
