Observation of dark states in a superconductor diamond quantum hybrid system

We observed a remarkably sharp resonance ($\sim$ 1 MHz) at 2.878 GHz in the spectrum of flux qubit NV-diamond hybrid quantum system under zero external magnetic field. This width is much narrower than that of both the flux-qubit and spin-ensemble. We show this resonance is evidence of a collective dark state in the ensemble which is coherently driven by the superposition of clockwise and counter-clockwise macroscopic persistent super-currents owing in the flux qubit.