Manipulating complex hybrid entanglement and testing multipartite Bell inequalities in a superconducting circuit

Multipartite entangled states are crucial not only for fundamental interest, but also for important applications of quantum-based technologies, such as quantum communication, quantum metrology and quantum simulation. Quantum correlations in multipartite observables are direct consequences of entanglement and can be used to characterize the nonclassical nature of the entangled states. As a signature of quantum correlation, the violation of Bell inequalities has not been demonstrated in multipartite hybrid quantum systems involving both discrete and continuous variables. Here we generate a five-partite entangled state with three superconducting transmon qubits and two photonic qubits, each encoded in the mesoscopic field of a microwave cavity. We reveal the quantum correlations among these distinct elements by joint Wigner tomography of the two cavity fields conditional on the detection of the qubits and by test of a five-partite Bell inequality. The measured Bell signal is 8.381±0.038, not only surpassing the classical bound of 4 allowed by local realism, but also above the quantum bound of 8 for four-partite entanglement, demonstrating the genuine five-partite entanglement in a hybrid quantum system.
Reference:
[1] Y. Ma, X. Pan et al. Phys. Rev. Lett. 125.180503 (2020)