No abstract

Nanodiamonds containing negatively charged nitrogen vacancy centers (NV − ) have applications as localized sensors in biological materials and have been proposed as a platform to probe the macroscopic limits of spatial superposition and the quantum nature of gravity. A key requirement for these applications is to obtain nanodiamonds containing NV − with long spin coherence times. Using milling to fabricate nanodiamonds processes the full 3D volume of the bulk material at once, unlike etching pillars, but has, up to now, limited NV − spin coherence times. Here, we use natural isotopic abundance nanodiamonds produced by Si 3 N 4 ball milling of chemical vapor deposition grown bulk diamond with an average single substitutional nitrogen concentration of 121 ppb. We show that the electron spin coherence times of NV − centers in these nanodiamonds can exceed 400 μs at room temperature with dynamical decoupling. Scanning electron microscopy provides images of the specific nanodiamonds containing NV − for which a spin coherence time was measured.

We present characterization of a lock-in amplifier based on a field programmable gate array capable of demodulation at up to 50 MHz. The system exhibits 90 nV/ √ Hz of input noise at an optimum demodulation frequency of 500 kHz. The passband has a full-width half-maximum of 2.6 kHz for modulation frequencies above 100 kHz.Our code is open source and operates on a commercially available platform.