Global cloning fidelity ($F_{\mathrm{global}}(\varphi )=|⟨\varphi {|}^{\otimes 2}U|\varphi ,0⟩{|}^2$)

For a quantum cloning device with pure state output $U|\varphi ,0⟩$, the global cloning fidelity is its squared quantum fidelity with the desired joint two-copy pure state. It is the probability that a joint projector test for two ideal copies accepts the actual output. It differs from a one-clone marginal fidelity: correlations between the output qubits affect the joint overlap. For a controlled-NOT gate with input $|+⟩|0⟩$, the output is a Bell state and its global cloning fidelity with $|++⟩$ is $1/2$; for input $|0⟩|0⟩$ it is one.