Realization of collective strong coupling with ion Coulomb crystals in an optical cavity

Abstract

The experimental realization of strong coupling between a Coulomb crystal—made of several hundred ions—and the light field in an optical cavity could provide a route to efficient light–matter interfaces. Cavity quantum electrodynamics (CQED) focuses on understanding the interactions between matter and the electromagnetic field in cavities at the quantum level1,2. In the past years, CQED has attracted attention3,4,5,6,7,8,9 especially owing to its importance for the field of quantum information10. At present, photons are the best carriers of quantum information between physically separated sites11,12 and quantum-information processing using stationary qubits10 is most promising, with the furthest advances having been made with trapped ions13,14,15. The implementation of complex quantum-information-processing networks11,12 hence requires devices to efficiently couple photons and stationary qubits. Here, we present the first CQED experiments demonstrating that the collective strong-coupling regime2 can be reached in the interaction between a solid in the form of an ion Coulomb crystal16 and an optical field. The obtained coherence times are in the millisecond range and indicate that Coulomb crystals positioned inside optical cavities are promising for realizing a variety of quantum-information devices, including quantum repeaters12 and quantum memories for light17,18. Moreover, cavity optomechanics19 using Coulomb crystals might enable the exploration of similar phenomena investigated using more traditional solids, such as micro-mechanical oscillators20.