Parallel NV Center Entanglement Generation for Enhanced Quantum Sensing

Entanglement ienables quantum sensors to improve their sensitivity up to the Heisenberg limit. In the NV-center platform, the Heisenberg limit can only be achieved if the NV centers have parallel axes. Nevertheless, parallel NV centers are spectrally indistinguishable and no mechanisms to directly prepare Heisenberg-limit-grade entanglement in such configurations are known to date. In this work we propose for the first time a viable mechanism to prepare entangled states in the double quantum transition of two dipolarly coupled NV centers whose axes are parallel without populating intermediate states. Our approach is based on the NV effective Raman coupling (NV-ERC) protocol and makes use of global addressing of both NV centers with a single monochromatic microwave pulse. Supported by an adiabatic elimination analysis, several mechanisms for the preparation of different entangled states are identified which avoid participation of intermediate states in all cases, thus limiting the impact of additional unwanted noise sources. All proposed states belong to the double quantum transition, sensitive to either transverse electric fields or longitudinal magnetic fields, all with a fourfold improved sensitivity compared to conventional single NV settings.