  We demonstrate deterministic generation of two distinct collective
excitations in one atomic ensemble, and we realize the Hong-Ou-Mandel
interference between them. Using Rydberg blockade we create single collective
excitations in two different Zeeman levels, and we use stimulated Raman
transitions to perform a beam-splitter operation between the excited atomic
modes. By converting the atomic excitations into photons, the two-excitation
interference is measured by photon coincidence detection with a visibility of
0.89(6). The Hong-Ou-Mandel interference witnesses an entangled NOON state of
the collective atomic excitations, and we demonstrate its two times enhanced
sensitivity to a magnetic field compared with a single excitation. Our work
implements a minimal instance of Boson sampling and paves the way for further
multi-mode and multi-excitation studies with collective excitations of atomic
ensembles.
