Witnessing Quantum Entanglement Using Resonant Inelastic X-ray Scattering

Quantum entanglement is considered to be a central ingredient in our understanding quantum many-body systems as well as an essential resource for cutting edge quantum technologies. However, detecting and quantifying quantum entanglement in real quantum materials is a difficult task due to limited varieties of available experimental observables. Existing protocols are limited to the usage of the inelastic neutron scattering. We developed a new protocol that can utilize the resonant inelastic X-ray scattering for this task, thus enlarging the class of quantum materials whose quantum entanglement is subject to experimental detection. We applied our protocol to iridates dimer systems such as Ba$_3$CeIr$_2$O$_9$, and obtained the first experimental detection of electronic quantum entanglement in a real quantum material with strong correlation, between the electronic orbitals around the two iridium sites. Our protocol provides a new handle for entanglement-related investigations and applications, and can be potentially generalized to other degrees of freedom.