Realization Of Universal Quantum Cloning and of the Not Gate by Optical Parametric Amplification

Abstract

An arbitrary quantum state cannot be “cloned” perfectly, that is, reproduced with “fidelity” F = 1 into M > 1 states identical to the original by any conceivable physical device. The main root of this impossibility resides in the linearity of quantum mechanics. A second “quantum impossibility” process, based on the complete positivity character of any quantum operation, forbids the realization of a universal NOT gate that is, one that flips exactly any input qubit into an orthogonal one. A detailed investigation of these results, representing the most fundamental difference between classical and quantum information, can reveal the detailed structure of the latter. We report an experimental demonstration of the process of optimal cloning of N = 1 input qubit into M = 2 output qubits by a quantum-injected optical parametric amplifier (OPA). By the same apparatus the realization of a universal NOT gate is also demonstrated. The two processes will be found to be universal and optimal, that is, the measured fidelity of both processes F < 1 will be found close to the theoretical values.