UNIVERSAL-NOT GATE AND QUANTUM CLONING

摘要

An arbitrary quantum state cannot be "cloned" perfectly, i.e., 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, i.e., one that flips exactly any input qubit into an orthogonal one. We report an experimental demonstration of the process of cloning an N = 1 input qubit into M = 2 output qubits by a quantum-injected optical parametric amplifier (OPA). By the same apparatus, realization of a universal NOT gate is also demonstrated. The two processes will be found to be universal and optimal, i.e., the measured fidelities of both processes F < 1 will be found to be close to their respective theoretical limits.