Entangling Erbium spins with microwave photons

Future quantum networking applications will rely on optical fibers, which presents a challenge to integrate superconducting quantum processors that operate at microwave frequencies. Erbium ions in solid-state naturally bridge these two regimes through their fine structure and Zeeman splitting, while also enabling quantum information storage for up to tens of milliseconds. We propose to entangle an ensemble of Er³⁺ spins in CaWO₄ to microwave photons as a step towards an efficient optical-microwave entanglement generation scheme. For this, we use a low-loss superconducting resonator and a quantum-limited amplifier to adapt the Refocusing of Amplified Spontaneous Emission (RASE) to microwave frequencies. In this scheme, an initially inverted Erbium spin ensemble first emits Amplified Spontaneous Emission (ASE) which is entangled with the spins. A subsequent pulse triggers the emission of a Rephased ASE (RASE) field, entangled with the previous ASE. In this talk, we will present our progress towards implementing the RASE protocol at microwave frequencies and quantifying the resulting entanglement with the Duan-Simon separability criterion. This work paves the way for hybrid quantum architectures for communication and information processing.