Optical realization of universal quantum cloning
  Beyond the no-cloning theorem, the universal symmetric quantum cloning machine was first addressed by Buzek and Hillery. Here, we realized the one-to-two qubits Buzek-Hillery cloning machine with linear optical devices. This method relies on the representation of several qubits by a single photon. We showed that, the fidelities between the two output qubits and the original qubit are both 5/6 (which proved to be the optimal fidelity of one-to-two qubits universal cloner) for arbitrary input pure states. 
Yun-Feng Huang, Wan-Li Li, Chuan-Feng Li, Yong-Sheng Zhang, Yun-Kun   Jiang, and Guang-Can Guo (University of Science and Te...