Probing Surface Noise with Depth-Calibrated Spins in Diamond
In this paper, Myers et al. investigate the nuclear spin noise at the diamond surface using depth-calibrated nitrogen-vacancy (NV) centers as a tool for sensitive nanoscale magnetic resonance imaging (MRI). This is crucial for applications in quantum sensing and imaging technologies. The NV centers are renowned for their prowess as quantum sensors of magnetic fields capable of detecting small numbers of electronic and nuclear spins within close proximity to the diamond surface.
The primary hindrance to the widespread application of NV centers in spin imaging has been the ability to craft shallow NVs near the surface while retaining long coherence times. Surface-induced decoherence is a significant challenge, with coherence times reduced from 2 ms in bulk NVs to less than 10 µs for few-nm deep NVs, which presents a formidable task to overcome. Understanding and mitigating surface noise is critical for advancing nanoscale MRI and coherent spin coupling applications.
Methodology
The research leverages a combination of nitrogen delta-doping during chemical vapor deposition (CVD) of single-crystal diamond (SCD) and dynamical decoupling (DD) techniques to probe the dominant noise attributed to surface spins. By employing variations in CPMG pulses, the team deciphers the spectral contributions from both surface and bulk environments as a function of NV depth. Depth-calibrated layers enable precise NV creation, eliminating sample-to-sample surface variations and permitting focused studies on surface noise.
To assess NV depths beyond standard microscopy, the team applies optically detected electron spin resonance (ODESR) imaging in a magnetic field gradient, achieving nanometer resolution without assumed models. This setup allows for the distinct measurement of NV coupling frequencies to either surface or bulk noise baths, furnishing insights into the nature and mitigation of spin bath fluctuations.
Results
Key findings reveal that surface noise is predominantly caused by a bath of 2D electronic spins with faster correlation dynamics than the bulk nitrogen spin bath, with a surface bath correlation rate of 200 kHz. Experimental data indicate that coherence times increase with NV depth, implicating faster surface spin fluctuations in decoherence processes for shallower NVs. By mathematically modeling the influence of surface spins as a 2D layer with a uniform distribution, the authors quantify a surface spin density of σ = 0.04 spins/nm², correlating NV depth with noise power. Results demonstrate significant coherence enhancement and sensitivity with appropriate inter-pulse timing in DD protocols, achieving near-detection thresholds for surface proton spins.
Implications and Future Directions
This research advances understanding of spin noise dynamics at the diamond surface, essential for refining NV-based MRI and quantum sensing technologies. The paper suggests practical applications in nanoscale magnetometry and coherent spin coupling setups, with implications for diverse material surfaces.
Further exploration using even shallower NVs (< 5 nm) could uncover variations arising from discrete surface-spin effects and spin clustering. Formation of isolated nitrogen layers could serve as test beds for 2D spin bath analysis. Moreover, experimental setups under varying magnetic fields and temperatures could elucidate the role of spin fluctuations in surface-induced decoherence processes.
The paper provides a foundation for improved noise mitigating strategies and NV depth calibration, enhancing the role of diamonds in advanced quantum applications. The methodology and findings offer direction for future research into the mechanisms of surface decoherence, contributing to the broader field of spin dynamics and quantum sensing.
