Cavity quantum electrodynamics with a tunable spin-inverted ensemble

Ensembles of emitters coupled to cavities are an essential system for studies of fundamental quantum phenomena and have been extensively explored for quantum information applications. Rare-earth ions (REIs) doped in solids offer a promising platform for these studies due to their narrow inhomogeneities and stable transitions in both microwave and optical domains at cryogenic temperatures. Among the REIs, ytterbium-171 is of special interest because of its strong dipole moment and simple hyperfine structure.

In this work, we present an on-chip platform for studying quantum cavity electrodynamics using a planar superconducting microwave resonator fabricated on a 171Yb3+: YVO4 substrate. Due to the resonant coupling between the spin ensemble and the resonator, we observe novel quantum dynamics enabled by controllable initialization of the excited-state spin system. By tuning the spin initialization in the spin-down state, we observe collectively-induced transparency resulting from strong coupling between the spin ensemble and the resonator field. Conversely, when a controllable number of spins is initialized in the spin-up state, we observe synchronized coherent emission in both microwave and optical fields with emission linewidths down to 100 Hz, driven by periodic superradiance. The emission strength and the periodicity will be discussed in the talk.