Hong-Ou-Mandel Interference between Two Deterministic Collective   Excitations in an Atomic Ensemble
  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. 
Jun Li, Ming-Ti Zhou, Bo Jing, Xu-Jie Wang, Sheng-Jun Yang, Xiao   Jiang, Klaus M{\o}lmer, Xiao-Hui Bao, and Jian-Wei Pan
quant-ph