Scheme for implementing the optimal quantum cloning via long-range off-resonant Raman coupling

We propose a physical scheme to implement the optimal 1 → 2 quantum cloning via long-range off-resonant Raman coupling for distant atoms trapped in separate cavities connected by optical fibers. We show that, if the single-qubit rotation angles are choosen appropriately, the optimal symmetric (asymmetric) universal quantum cloning and optimal symmetric (asymmetric) phase-covariant cloning can be effectively realized. The scheme is insensitive to cavity decay, fiber loss, and atomic spontaneous emission because the effective long-distance interaction induced by long-range off-resonant Raman coupling is mediated by the vacuum fields of the fiber and cavity and because the total system evolves in the decoherence-free subspace in which neither of the subsystems is excited.