Optimal quantum machines by linear and non-linear optics

Since manipulations of qubits are constrained by the quantum mechanical rules, several classical information tasks can not be perfectly extended to the quantum world. The more relevant limitations in quantum information processing are the impossibility to perfectly clone any unknown qubit and to map it in its orthogonal state. Even if these two processes are unrealizable in their exact forms, they can be optimally approximated by the so-called universal quantum machines: the optimal quantum cloning machine (UOQCM) and the universal-NOT (U-NOT) gate. Investigation of these optimal transformations is important since it reveals bounds on optimal manipulations of information with quantum systems. The 1->1 U-NOT gate and the 1->2 UOQCM have been experimentally demonstrated by adopting the process of stimulated emission into an optical parametric amplifier and by slightly modifying the quantum state teleportation protocol. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim