Speaker
Description
Erbium ions in crystals combine an optical transition in the telecom band, where optical fibres have their lowest loss, with an electron-spin transition at gigahertz frequencies, where superconducting quantum circuits operate. This makes them a natural candidate for linking superconducting processors to optical quantum networks. In integrated devices, however, erbium coherence is usually limited by the host material. I will present an approach that avoids this limitation: bonding a bulk, coherence-optimised Er:CaWO₄ crystal directly onto a chip. On the optical side, bonding to an electro-optically tuneable thin-film lithium niobate microring resonator yields a collective cooperativity of 6.7 while preserving millisecond optical coherence. On the microwave side, bonding the same crystal to high-quality-factor niobium resonators allows microwave pulses to be stored and retrieved with about 70% efficiency and added noise close to the vacuum level. Along the way, I will introduce the physics of spin ensembles collectively coupled to resonators for a non-specialist audience.