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.

无克隆定理禁止完全克隆未知的量子态。能够产生具有最佳保真度的任何输入量子位的两个副本的通用量子克隆机（UQCM）具有根本的意义，并已在量子信息处理中得到应用。输入的量子位和复制的量子位之间经过精心定制的非经典相关性使这成为可能，这些相关性将UQCM与经典qubit区别开来，但仍缺乏实验证明。我们在这里在超导电路中实现UQCM，并研究这些相关性。测得的纠缠度与我们的理论预测非常吻合，即它们独立于输入状态，因此构成了UQCM的普遍量子行为，这是先前未揭示的。