On-chip coherent conversion of photonic quantum signals between   different degrees of freedom
  In the quantum world, a single particle can have various degrees of freedom to encode quantum information. Controlling multiple degrees of freedom simultaneously is necessary to describe a particle fully and, therefore, to use it more efficiently. Here we introduce the transverse waveguide-mode degree of freedom to quantum photonic integrated circuits, and demonstrate the coherent conversion of a photonic quantum state between path, polarization and transverse waveguide-mode degrees of freedom on a single chip. The preservation of quantum coherence in these conversion processes is proven by single-photon and two-photon quantum interference using a fibre beam splitter or on-chip beam splitters. These results provide us with the ability to control and convert multiple degrees of freedom of photons for quantum photonic integrated circuit-based quantum information process. 
Lan-Tian Feng, Ming Zhang, Zhi-Yuan Zhou, Ming Li, Xiao Xiong, Le Yu,   Bao-Sen Shi, Guo-Ping Guo, Dao-Xin Dai, Xi-Feng Ren ...
quant-ph physics.optics