It was noted that circuit-QED had demonstrated a very strong coupling between light and matter, and offered the potential to engineer large quantum devices. Hybrid designs were proposed which coupled large ensembles of atomic and molecular systems to the superconducting resonator. It was shown that one could achieve an effective strong coupling between light and matter for much smaller ensembles (and even a single electronic spin), through the use of an interconnecting quantum system: in the present case a persistent current qubit. Using this interconnect it was shown that one could effectively magnify the coupling strength between the light and matter by over five orders of magnitude g∼7Hz → 100kHz and enter a regime where a single nitrogen-vacancy electronic spin could shift the cavity resonance line by over ∼20 linewidths. With such strong coupling between an individual electronic spin in a nitrogen-vacancy and the light in the resonator, one had the potential build devices where the associated nitrogen-vacancy nuclear spins could be strongly coupled over centimetres via the superconducting bus.

Superconducting Cavity Bus for Single Nitrogen-Vacancy Defect Centers in Diamond. J.Twamley, S.D.Barrett: Physical Review B, 81[24], 241202