Quantum cloning of orthogonal qubits
Jaromı́r Fiurášek, S. Iblisdir, Serge Massar, Nicolas J. Cerf · Physical Review A · 2002
An optimal universal cloning transformation is derived that produces M copies of an unknown qubit from a pair of orthogonal qubits. For $M>6,$ the corresponding cloning fidelity is higher than that of the optimal copying of a pair of identical qubits. It is shown that this cloning transformation can be implemented probabilistically via parametric down conversion by feeding the signal and idler modes of a nonlinear crystal with orthogonally polarized photons.