Abstract
No-cloning theorem forbids perfect cloning of an unknown quantum state. A universal quantum cloning machine (UQCM), capable of producing two copies of any input qubit with the optimal fidelity, is of fundamental interest and has applications in quantum information processing. This is enabled by delicately tailored nonclassical correlations between the input qubit and the copying qubits, which distinguish the UQCM from a classical counterpart, but whose experimental demonstrations are still lacking. We here implement the UQCM in a superconducting circuit and investigate these correlations. The measured entanglements well agree with our theoretical prediction that they are independent of the input state and thus constitute a universal quantum behavior of the UQCM that was not previously revealed. Another feature of our experiment is the realization of deterministic and individual cloning, in contrast to previously demonstrated UQCMs, which either were probabilistic or did not constitute true cloning of individual qubits.
Highlights
An unknown quantum state cannot be cloned perfectly due to the linearity associated with the unitary transformation of quantum mechanics
We indicate the universality of the implemented universal quantum cloning machine (UQCM) and quantitatively characterize the entanglement between the input qubit and each of the copy qubits
We find that the degree of the entanglement between each output clone and the original input qubit, quantified by concurrence[31], is 2/3 for an ideal UQCM, which is independent of the input state
Summary
An unknown quantum state cannot be cloned perfectly due to the linearity associated with the unitary transformation of quantum mechanics. Besides the limitation of ensemble aspect or probabilistic nature, previous experiments did not reveal the nonclassical correlations between the original input qubit and the copying qubits These correlations enable the information carried by the input state to be imprinted on the clones with the optimal fidelity and represent the most fundamental difference between the UQCM and a classical cloning machine. Quantitative characterization of these correlations is important for revealing the genuine quantum behavior of the UQCM, which is closely related to the universality and optimality of the copying operation. The entanglement between the two copy qubits is measured
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