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. [ABSTRACT FROM AUTHOR]