Contextual realization of the universal quantum cloning machine and of the universal-NOT gate by quantum-injected optical parametric amplification

Abstract

A simultaneous, contextual experimental demonstration of the two processes of cloning an input qubit $|\ensuremath{\Psi}〉$ and of flipping it into the orthogonal qubit $|{\ensuremath{\Psi}}^{\ensuremath{\perp}}〉$ is reported. The adopted experimental apparatus, a quantum-injected optical parametric amplifier is transformed simultaneously into a universal optimal quantum cloning machine and into a universal-NOT quantum-information gate. The two processes, indeed forbidden in their exact form for fundamental quantum limitations, were found to be universal and optimal, i.e., the measured fidelity of both processes $F<1$ was found close to the limit values evaluated by quantum theory. A contextual theoretical and experimental investigation of these processes, which may represent the basic difference between the classical and the quantum worlds, can reveal in a unifying manner the detailed structure of quantum information. It may also enlighten the yet little explored interconnections of fundamental axiomatic properties within the deep structure of quantum mechanics.