Nanodiamonds with the Optimal Nitrogen Concentration for Quantum Sensing

Abstract

The negatively charged nitrogen-vacancy centre (NV − ) in diamond has been utilized in a wide variety of sensing applications. The centre's long spin coherence and relaxation times (T * 2 , T 2 and T 1 ) at room temperature are crucial to this, as they often limit sensitivity. Using NV − centres in nanodiamonds allows for operations in environments inaccessible to bulk diamond, such as intracellular sensing. Finding NV − centres in diamonds smaller than 100 nm requires the use of fairly high nitrogen concentrations, while to achieve long spin coherence times, researchers have aimed for low nitrogen concentrations. Here we show that both aims can be achieved together by choosing to make the nanodiamonds from a starting material containing 1-10 ppm of nitrogen, a concentration range previously identified for NV − centre sensing with bulk diamond, but not yet tested for nanodiamonds. We report long spin coherence and relaxation times at room temperature for single NV − centres in isotopically-purified polycrystalline ball-milled nanodiamonds. Using a spin-locking pulse sequence, we observe spin coherence times, T 2 , up 786 ± 200 µs. We also measure T * 2 times up to 2.06 ± 0.24 µs and T 1 times up to 2.0 ± 0.4 ms. Producing the diamonds by ball-milling allows for the efficient production of large masses of nanodiamond.

Quantum sensing of electric field distributions of liquid electrolytes with NV-centers in nanodiamonds

“…To study this, we numerically [39,40] simulated FID ξz,ξ ⊥ and included dephasing at rates 1/T * 2 through a Lindblad operator 1/T * 2 S z for T * 2 in the range up to 15 µs (see figure 2(d)). One can resolve ξ ⊥ ± ξ z for nanodiamonds with T * 2 > 10 µs, which is higher than the value of typical nanodiamonds [41,42]. For a nanodiamond with T * 2 ≈ 15 µs it would be possible to distinguish between ξ ⊥ and ξ z and therefore to determine the projection of the electric field onto the symmetry axis of the NV-center.…”