Short answer: A single nitrogen-vacancy center in diamond can act as a magnetic sensor small enough to detect nuclear spins outside the diamond. A 2013 Science paper reports detecting proton NMR signals from an organic sample with a near-surface center, and a 2013 New Journal of Physics paper proposes in theory that such a center could detect the position and spin state of a single nucleus. A 2024 review says single-molecule resonance with these centers is still limited in sensitivity and resolution.
The challenge
The Science abstract opens with the problem: extending nuclear magnetic resonance to nanoscale samples has been a longstanding challenge because conventional detection methods are insensitive. The New Journal of Physics abstract adds that a single nucleus has a small magnetic moment and that environmental noise is unavoidable, which are the key obstacles to detecting a nuclear spin in an individual molecule.
2013: proton NMR from outside the diamond
In the Science paper, the authors used an individual, near-surface nitrogen-vacancy center as a sensor to detect proton NMR in an organic sample located outside the diamond. They combined electron spin echoes with proton spin manipulation. They report that the center senses the nanotesla field fluctuations from the protons, which enabled both time-domain and spectroscopic NMR measurements on the nanometer scale. The abstract does not claim a single-molecule measurement. The detected signal comes from protons in a sample at the nanometer scale.
2013: a theoretical single-molecule proposal
The New Journal of Physics paper is explicitly theoretical. The authors demonstrate theoretically that a single nitrogen-vacancy center could be used to construct a nano-scale single-molecule spectrometer. They say it would be capable of detecting the position and spin state of a single nucleus and could determine the distance and alignment of a nuclear or electron spin pair. They suggest applications in single-molecule spectroscopy in chemistry and biology, for example in determining protein structure or monitoring macromolecular motions. These applications are proposals, not results.
2024: where the field stands
A review in Reviews of Modern Physics describes single-molecule technology as a tool for characterizing structural and dynamic information otherwise hidden in the average over many molecules. It says quantum sensing with nitrogen-vacancy centers is a promising avenue for single-molecule magnetic resonance. The review notes that although significant strides have been made over the past two decades, current approaches still show limitations in magnetic sensitivity, spectral resolution and spatial resolution. It singles out the full reconstruction of three-dimensional positions of nuclear spins within single molecules as a goal not yet attained. The review surveys the techniques and the optimization of their technical parameters.
Limits
The three sources cover different stages. One is an experiment with proton signals from a sample, one is a theory proposal, and one is a review that states what has not been achieved. Together they do not show routine single-molecule structure determination, and the review says the three-dimensional reconstruction of nuclear spin positions in a single molecule is still unattained.
FAQ
What does an NV center sense?
Magnetic fields. In the 2013 Science experiment it sensed nanotesla field fluctuations from protons in a nearby sample.
Did the 2013 experiment detect a single molecule?
The abstract reports nanometer-scale NMR on an organic sample, not a single-molecule measurement.
What did the theory paper propose?
A single NV center as a single-molecule spectrometer that could detect the position and spin state of a single nucleus.
What limits the technique today?
The 2024 review lists magnetic sensitivity, spectral resolution and spatial resolution.
Is this quantum computing?
No. It is quantum sensing, using the same kind of defect that is also studied as a qubit.
All field notes