Orbital angular momentum-entanglement frequency transducer
  Entanglement is a vital resource for realizing many tasks such as teleportation, secure key distribution, metrology and quantum computations. To effectively build entanglement between different quantum systems and share information between them, a frequency transducer to convert between quantum states of different wavelengths while retaining its quantum features is indispensable. Information encoded in the photons orbital angular momentum OAM degrees of freedom is preferred in harnessing the information carrying capacity of a single photon because of its unlimited dimensions. A quantum transducer, which operates at wavelengths from 1558.3 nm to 525 nm for OAM qubits, OAMpolarization hybrid entangled states, and OAM entangled states, is reported for the first time. Nonclassical properties and entanglements are demonstrated following the conversion process by performing quantum tomography, interference, and Bell inequality measurements. Our results demonstrate the capability to create an entanglement link between different quantum systems operating in photons OAM degrees of freedoms, which will be of great importance in building a high capacity OAM quantum network. 
Zhi-Yuan Zhou, Shi-Long Liu, Yan Li, Dong-Sheng Ding, Wei Zhang, Shuai   Shi, Ming-Xin Dong, Bao-Sen Shi, and Guang-Can Guo
quant-ph physics.optics