Creating nitrogen–vacancy ensembles in diamond for coupling with flux qubit**Project supported in part by the National Natural Science Foundation of China (Grant Nos. 91321208, 11574386, 11374344, and 11574380), the National Basic Research Program of China (Grant Nos. 2014CB921401 and 2016YFA0300601), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB07010300).

Abstract

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.