Experimental Demonstration of Universal Quantum Cloning in a Circuit

No-cloning theorem forbids perfect cloning of an unknown quantum state. A universal quantum cloning machine (UQCM), which can produce two copies of any input qubit with the optimal fidelity, is of fundamental interest and has applications in quantum information processing. Previous implementations of UQCMs either are probabilistic or do not constitute true cloning of individual qubits. We here report on the first deterministic experimental demonstration of the UQCM for genuinely individual qubits in a superconducting circuit, where five Xmon qubits are coupled to a bus resonator. The high controllability of the system dynamics, enabled by the frequency tunability of each qubit, allows for deterministic implementation of all gate operations necessary for approximately cloning an arbitrary input state. The measured fidelities of the clones are input-state-independent and close to the optimal value 5/6. We further confirm the existence of quantum entanglement among the two clones and the original qubits, which distinguishes a quantum cloning machine from a classical counterpart but has not been demonstrated yet.