Quantum interference of a single spin excitation with a macroscopic atomic ensemble

We report on the observation of quantum interference of a collective single spin excitation with a spin ensemble of N-a approximate to 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 a quantum nondemolition measurement of the collective spin. A macroscopic difference of the order of v Na in the marginal distribution of the collective spin state arises from the interference between the single excited spin and Na atoms. These hybrid discrete-continuous manipulation and measurement procedures of collective spin states in an atomic ensemble pave the road towards generation of even more exotic ensemble states for quantum information processing, precision measurements, and communication.