Optical realization of universal quantum cloning

Beyond the no-cloning theorem, the universal symmetric quantum cloning machine was first addressed by Bužek and Hillery. Here, we realized the $\overrightarrow{1}$2 qubit Bužek-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 $\frac{5}{6}$ (which proved to be the optimal fidelity of $\overrightarrow{1}$2 qubit universal cloner) for arbitrary input pure states.