Observing Bulk Diamond Spin Coherence in High-Purity Nanodiamonds
The paper presents an investigation into nitrogen-vacancy centres (NVs) in nanodiamonds derived from synthetic high-pressure high-temperature (HPHT) bulk diamonds with less than 50 ppm nitrogen impurity concentration. The research aims to mitigate previously known spin coherence challenges in nanodiamonds, posing a significant barrier for advancing their applications in quantum information science, magnetometry, and thermometry. Historically, bulk diamond NVs have demonstrated prolonged spin coherence times, while nanodiamonds have fallen short, exhibiting coherence times within the range of 1.4 to 10.9 µs.
A central contribution of this study is the observation of spin coherence in nanodiamonds reaching times greater than 60 µs, achieved through universal dynamical decoupling techniques. Calculations reveal this coherence surpasses the typical benchmarks in type Ib bulk diamonds and approaches those observed in impurity-free type IIa diamonds. The authors identify adjacent nitrogen impurities—not surface states—as the primary source of decoherence. Progress is demonstrated through improvements in NV free induction decay time, extending from 440 ns to 1.27 µs under the influence of motional narrowing regimes. Consequently, these high-purity nanodiamonds offer significant potential for quantum technologies by retaining both high sensitivity and nanometre-scale resolution.
Methodologically, the paper applies spin-echo (SE) and dynamical decoupling (DD) protocols to probe NV coherence properties in nanodiamonds. By driving nitrogen impurities into a motional narrowing regime, the study successfully prolongs coherence times. For instance, trials using NV1 extend the spin coherence time to 67 µs with the employment of DD techniques free from pulse error correction—indicative of a quasi-static spin reservoir dominated by paramagnetic impurities within the diamond lattice rather than surface dynamics.
Additionally, the paper explores nanodiamonds' potential as magnetometers, elucidating reservoir spin properties through NV SE measurements synced with radiowave pulses. The analysis reveals distinct spin populations attributable to Jahn-Teller distortions, consistent across varied nitrogen orientations. From the spectra, a nitrogen impurity concentration of less than 36 ppm is inferred, suggesting significant refinements in purity compared to previously examined nanodiamonds.
Furthermore, Ramsey interferometry experiments evaluate NV spin dynamics in free evolution states, detailing coherence and motional averaging efficacy by actively driving nitrogen impurity spins. The research achieves a free evolution spin coherence time of 1.27 µs using the Ramsey Double Resonance (RADOR) technique—an advancement recuperating 75% of type IIa diamond coherence times.
Implications of the study indicate substantive enhancements in NV spin coherence, advocating for future exploration in quantum applications, including DC and AC magnetometry, nanoscale thermometry, and potential spin squeezing within mesoscopic reservoirs. These advancements widen the scope for deploying nanodiamonds in emergent quantum technologies by leveraging prolonged coherence times and reinforced reservoir dynamics.
In conclusion, the researchers present a compelling case for the viability of employing high-purity nanodiamonds in cutting-edge quantum applications. The study addresses critical aspects of NV decoherence, establishing methodological pathways to harness extended coherence times, which may stimulate subsequent progress in the practical deployment of nanodiamond-based quantum devices.