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

TL;DR

This paper demonstrates the deterministic creation and verification of a Bell state between a macroscopic spin system with approximately 10^19 atoms and a superconducting qubit, marking the largest such entanglement achieved.

Contribution

It introduces a method to generate and verify macroscopic entanglement between a millimeter-sized spin system and a superconducting qubit, expanding the scale of entangled quantum systems.

Findings

Achieved a Bell state generation fidelity of 0.90±0.01.

Demonstrated entanglement between a system with ~10^19 atoms and a superconducting qubit.

Developed a joint tomography approach for state verification.

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 {\it 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 {\it largest} system (in the sense of atom number) capable of generating the maximally entangled quantum state.