Creating nitrogen–vacancy ensembles in diamond for coupling with flux qubit

Hybrid quantum system of negatively charged nitrogen-vacancy (NV^-) centers in diamond and superconducting qubits provide the possibility to extend the performances of both systems. In this work, we numerically simulate the coupling strength between NV^- ensembles and superconducting flux qubits and obtain a lower bound of 10¹⁶ cm^-3 for NV^- concentration to achieve a sufficiently strong coupling of 10 MHz when the gap between NV-ensemble and flux qubit is 0. Moreover, we create NV^- ensembles in different types of diamonds by ₁₄N⁺ and ₁₂C⁺ ion implantation, electron irradiation, and high temperature annealing. We obtain an NV^- concentration of 1.05×10¹⁶ cm^-3 in the diamond with 1-ppm nitrogen impurity, which is expected to have a long coherence time for the low nitrogen impurity concentration. This shows a step toward performance improvement of flux qubit-NV^- hybrid system.