Short answer: A 2020 review says nitrogen-vacancy (NV) diamond ensemble magnetometers have sensitivities orders of magnitude away from theoretical limits, and names spin dephasing time, readout fidelity and diamond material properties as the most promising routes to improve them. A 2014 review says photon shot noise dominates spin projection noise in most reported NV schemes, and a recent four-diamond study reports a lowest shot-noise-limited sensitivity of 380 pT per square root hertz. These are laboratory results.
Current performance and theoretical limits
A 2020 Reviews of Modern Physics review says solid-state spin systems, including nitrogen-vacancy (NV) centers in diamond, are an increasingly favored quantum sensing platform. It also says present NV ensemble devices have sensitivities orders of magnitude away from theoretical limits, which handicaps existing implementations and curtails the envisioned application space. The review analyzes approaches for broadband ensemble-NV-diamond magnetometers and identifies improvements to the spin dephasing time, the readout fidelity, and the host diamond material properties as the most promising avenues.
Noise sources and measurement constraints
A 2014 Reports on Progress in Physics review says the sensitivity of all spin-based magnetometers is fundamentally limited by the quantum noise associated with spin projection. If spin readout is performed optically, photon shot noise adds to it. For NV defects in diamond, the review says readout through spin-selective photoluminescence typically has a detection efficiency of about one in a thousand, mainly limited by the collection efficiency of the detection optics and the imperfect quantum efficiency of the NV radiative transition. It concludes that photon shot noise then dominates spin-projection noise for the vast majority of NV magnetometry schemes reported up to that time, and so it focuses on the photon shot-noise limited sensitivity. It also notes that using the collective response of a large ensemble of NV defects increases the collected signal.
A multi-parameter experimental study
A third paper says systematic studies of NV-diamond magnetometer sensitivity as a function of multiple parameters are sparse. By comparing optically detected magnetic resonance (ODMR) dips for four single-crystal diamonds, the authors explored how the photon shot-noise-limited sensitivity and the experimental magnetic noise floor depended on the density of the NV centers and the methods used to form them. They report a lowest shot-noise-limited sensitivity of 380 pT per square root hertz, significantly affected by ODMR fluorescence, contrast and linewidth. Using dual-resonance modulation in ODMR, they report a lowest magnetic noise floor of 1.14 nT per square root hertz based on power spectral density. They say these values can be improved with an increase in laser power, and that the study will help in selecting NV diamonds when designing magnetometers.
Limits
These are laboratory measurements and reviews. The sensitivity figures come from specific diamonds and setups, and the review itself describes ensemble devices as far from theoretical limits. Nothing here describes a commercial instrument.
FAQ
What limits NV magnetometer sensitivity?
Per the 2014 review, spin projection noise is a fundamental limit, and photon shot noise dominates it in most reported NV schemes because optical readout efficiency is low.
How far are ensemble devices from theory?
The 2020 review says orders of magnitude away from theoretical limits.
What does the review say would help most?
Improvements to the spin dephasing time, the readout fidelity and the host diamond material properties.
What sensitivity did the four-diamond study report?
A lowest shot-noise-limited sensitivity of 380 pT per square root hertz, and a lowest noise floor of 1.14 nT per square root hertz with dual-resonance modulation.
Can laser power change the result?
The authors say these values can be improved with an increase in laser power.
All field notes