Loss-tolerant quantum enhanced metrology and state engineering via the   reverse Hong-Ou-Mandel effect
  Preparing highly entangled quantum states between remote parties is a major challenge for quantum communications [1-8]. Particularly promising in this context are the N00N states, which are entangled N-photon wavepackets delocalized between two different locations, providing measurement sensitivity limited only by the uncertainty principle [1, 10-15]. However, these states are notoriously vulnerable to losses, making it difficult both to share them between remote locations, and to recombine them to exploit interference effects. Here we address this challenge by utilizing the reverse version of the Hong-Ou-Mandel effect [16] to prepare a high-fidelity two-photon N00N state shared between two parties connected by a lossy optical channel. Furthermore, we demonstrate that the enhanced phase sensitivity can be directly exploited in the two distant locations, and we remotely prepare superpositions of coherent states, known as Schr\"odinger's cat states" [17, 18]. 
Alexander E. Ulanov, Ilya A. Fedorov, Demid Sychev, Philippe Grangier   and Alexander I. Lvovsky
quant-ph