Quantum interference of electromagnetic fields from remote quantum   memories
  We observe quantum, Hong-Ou-Mandel, interference of fields produced by two remote atomic memories. High-visibility interference is obtained by utilizing the finite atomic memory time in four-photon delayed coincidence measurements. Interference of fields from remote atomic memories is a crucial element in protocols for scalable generation of multi-node remote qubit entanglement. 
T. Chaneliere, D. N. Matsukevich, S. D. Jenkins, S.-Y. Lan, R. Zhao,   T. A. B. Kennedy, and A. Kuzmich