Imagine a carousel carrying not children but tiny magnetic needles inside diamond dust. Usually these needles point randomly, and their signal is lost in noise. But if you tilt the carousel at a special angle and shine a green laser on it, the needles start aligning in one direction. That is exactly what physicists did with diamond particles, spinning them at 54.74° to the magnetic field.
The secret lies in diamond defects called NV centers. These are spots where a nitrogen atom sits next to a vacancy in the crystal lattice. Under light, such a defect becomes polarized, meaning its electron transitions to a state with a specific spin direction. This polarization then transfers to neighboring carbon-13 nuclei. Previously, this transfer required microwave pulses, which are hard to generate and control. Now, spinning the sample replaces microwaves.
In the experiment, two types of diamond particles were used: submicron ones with 20% carbon-13 and commercial micron-sized ones with natural abundance of 1.1%. They were placed in an NMR spectrometer rotor, spun at frequencies up to 6 kHz, and illuminated with a 532 nm laser. Maximum signal enhancement reached 411-fold for the natural sample at 1 kHz. For the enriched sample, enhancement was 280-fold at 2 kHz. Steady-state polarization reached 0.23% and 0.12%, respectively, almost an order of magnitude higher than previous results for micro- and nanodiamonds.
The enhancement as a function of spinning frequency was resonant: the signal first rises, peaks, then falls. This is because at too-high spinning rates, NV centers do not have time to transfer polarization to nuclei. A three-spin model (NV center, single nitrogen P1, and carbon-13 nucleus) reproduced the observed pattern. It showed that level crossings arise specifically from rotation, not from magnetic field changes.
Now that diamond dust can accumulate nuclear polarization without microwaves, a path opens to ultra-sensitive NMR at room temperature. The immediate goal is to transfer polarization from carbon-13 inside the diamond to external nuclei, such as protons on the surface. Estimates indicate that particles smaller than 30 nanometers are needed for this. Then the carousel could not only align its own needles but also pass that order to neighboring molecules.
The angle of 54.74° between the rotation axis and the magnetic field is called the magic angle because it nullifies dipole-dipole interactions between nuclei. It is this angle that made more than 99.9% of NV centers pass through the required level crossing points twice per rotor revolution.