Macroscopic Bell state between a millimeter-sized spin system and a superconducting qubit

Abstract

Entanglement is a fundamental property in quantum mechanics that systems share inseparable quantum correlation regardless of their mutual distances. Owing to the fundamental significance and versatile applications, the generation of quantum entanglement between macroscopic systems has been a focus of current research. Here we report on the deterministic generation and tomography of the macroscopically entangled Bell state in a hybrid quantum system containing a millimeter-sized spin system ($\sim 1\times10^{19}$ atoms) and a micrometer-sized superconducting qubit. The deterministic generation is realized by coupling the macroscopic spin system and the qubit via a microwave cavity. Also, we develop a joint tomography approach to confirming the deterministic generation of the Bell state, which gives a generation fidelity of $0.90\pm0.01$. Our work makes the macroscopic spin system the largest system (in the sense of atom number) capable of generating the maximally entangled quantum state.