A single-step high-pressure industrial process replaces complex irradiation methods, producing quantum-grade nanodiamonds with stable spins and long lifetimes for scalable sensing and imaging.
(Nanowerk Spotlight) Inside a diamond, a missing carbon atom next to a nitrogen atom creates a tiny pocket that can trap an electron. That imperfection, known as a nitrogen vacancy center, glows when light hits it and can reveal magnetic and temperature changes at the atomic scale. The idea has transformed how scientists imagine nanoscale sensing and imaging. Yet making enough of these diamonds in nanoparticle form has remained one of the most persistent practical barriers in quantum technology.
Traditional production depends on particle irradiation with high-energy beams followed by repeated heating stages. Each step adds cost, time, and damage to the crystal structure. The smaller the diamond, the harder it becomes to preserve its structure during processing. Sub-hundred-nanometer particles, essential for biological imaging, often degrade or clump together when heated. The process can take weeks to produce a few grams of usable fluorescent material.
Meanwhile, quantum measurement tools, microscopy, and surface chemistry have all advanced. Researchers can now control and read single spins with precision. If nanodiamonds could be made in large quantities without losing spin quality, they could serve as stable probes inside cells and as building blocks for compact quantum sensors.
The approach, called pressure-temperature qubits or PTQ, yields kilogram-scale batches of particles with strong optical contrast and stable spin behavior. The difference in production rate is striking. The paper notes that a single press can create in one week what would otherwise take decades using the conventional irradiation sequence.
Production of luminescent NDs. The scheme depicts the steps of standardized procedures for irradiation and annealing of nanodiamonds (NDs) or microdiamonds (MDs) to produce NV centers in diamond particles (top two rows) and our pressure & temperature qubits (PTQ) method for NV and H3 creation. Note that PTQ is a single-step process with yields greatly surpassing those of current approaches. The inset shows a photograph of untreated Starting NDs (dark powder) that can be PTQ processed per press in a single workday (1250 carats, or 250 g) and the PTQ oxidized NDs (white powder) that can be produced in a single press run (22.5 carats, or 4.5 g). A 15-cm ruler is shown for scale. (Image: Reprinted from DOI:10.1002/adfm.202520907, CC BY) (click on image to enlarge)
The PTQ process subjects diamond particles about fifty-nanometers wide to pressures near seven gigapascals and temperatures around one thousand seven hundred degrees Celsius for four minutes. Under these conditions, atoms in the diamond lattice can move just enough to create the right kind of defects without destroying the structure. Nitrogen atoms already present in the starting material combine with newly formed vacancies to make nitrogen vacancy centers, while some defects rearrange into another color center known as H3. Both defects emit bright, stable light, and both are useful in imaging and sensing.
To stop the particles from fusing into larger clumps, the researchers mixed the diamond powder with sodium chloride. The salt applies semi-hydraulic pressure as the mixture heats and then melts into a protective fluid that keeps particle surfaces from turning into non-fluorescent carbon. After the treatment, the salt can simply be washed away with water, leaving clean, fluorescent nanodiamonds.
Conventional methods depend on electron irradiation followed by several heating stages. Each step introduces strain and lattice defects that reduce the memory of the quantum spins. The PTQ approach replaces this chain of steps with a single treatment that creates and stabilizes the defects at once.
The author note that nanodiamonds typically show spin relaxation and coherence times about ten times shorter than those in large crystals. The new process extends those times while simplifying production.
Photoluminescence measurements show why. The particles exhibit both the neutral charge state, written NV0, and the negative charge state, written NV minus. The negative state acts as a controllable qubit that can be prepared and read using light. The ratio of NV minus to NV0 emission is a simple measure of charge stability.
PTQ particles showed a ratio more than twice as high as that of a commercial comparator made by irradiation and annealing, with 5.37 compared with 2.57 in thin layers and 3.21 compared with 1.47 in single-particle tests. Their fluorescence lifetime was longer as well, about twenty-four nanoseconds on average versus about nineteen and a half nanoseconds for the comparator, indicating that the defects lose less energy through non-radiative pathways.
Spin measurements reveal further advantages. Using optically detected magnetic resonance, a method that reads spin transitions through light, the team found smaller resonance splitting in PTQ particles, suggesting less internal strain. The Rabi contrast, which measures how strongly fluorescence responds when microwaves flip the spin state, was about five times higher in PTQ particles than in the comparator.
Average contrast exceeded fifteen percent, and one particle reached forty percent. Higher contrast means faster and more precise spin control, which directly improves sensing accuracy.
The spin relaxation time T1, which describes how long a prepared spin state persists before randomizing, reached about one millisecond under low laser power, roughly five times longer than in the comparator. This extension doubles the sensitivity of relaxometry, a sensing technique that detects magnetic noise from nearby molecules or ions. The coherence time T2, which measures how long a superposition of spin states remains stable, stayed similar between samples because both had comparable concentrations of substitutional nitrogen.
Microscopy and spectroscopy support these findings. Transmission electron images show that particle size and shape remain consistent, confirming that sodium chloride prevents sintering. X-ray diffraction verifies the cubic diamond structure, and Raman spectra show almost no sp2 carbon, the graphitic form that quenches fluorescence. Surface analysis indicates more sp3 carbon bonds and cleaner surfaces than in the comparator powder, helping charge stability.
Electron paramagnetic resonance shows that the total amount of nitrogen impurities, known as P1 centers, remains about ten parts per million in all samples, typical for this particle size. While the PTQ particles have fewer nitrogen vacancy centers than the comparator, they retain brightness sufficient for single-particle sensing.
Nuclear magnetic resonance measurements show that the carbon-13 spin relaxation time T1 stays near fifty-five seconds, almost the same as in the starting material and more than double that of the comparator. That stability is valuable for experiments that transfer spin polarization from electrons to nuclei to amplify magnetic signals.
The material’s biological safety was also tested. Cell assays across four human cancer lines and normal fibroblasts showed no measurable toxicity up to concentrations of one hundred micrograms per milliliter. Confocal microscopy revealed strong fluorescence inside HeLa cells in both the red emission of nitrogen vacancy centers and the green emission of H3 centers.
The overlap of these signals shows that both defects are present in the same particles, allowing dual-color imaging. According to the paper, this marks the first observation of H3 fluorescence inside cells using such small particles, which could simplify multi-channel bioimaging and sensing.
The process itself is simple to scale. Mixing fifty-nanometer diamond powder with sodium chloride in a three-to-one ratio, loading it into a metal capsule, pressing and heating it for four minutes, and washing away the salt yields fluorescent nanodiamonds ready for surface oxidation. Each press can process about two hundred-fifty grams per workday, producing more than one kilogram of material per week. Achieving that quantity with traditional irradiation and annealing would take decades of accumulated beam time.
The study identifies room for optimization. The PTQ process currently generates fewer total defects than irradiation, so brightness can still be increased. Higher temperatures tend to favor H3 formation, while slightly lower temperatures favor nitrogen vacancy creation, suggesting that careful control of temperature could tune the balance between brightness and spin quality. Coherence times measured by electron paramagnetic resonance remain shorter in PTQ particles, though the authors note that optical methods probe only active centers.
If widely adopted, the method could reshape how nanodiamond sensors and imaging probes are made. The millisecond-scale spin lifetime and strong optical contrast improve precision in magnetic and temperature sensing. Dual-color emission from a single particle opens new possibilities for bioimaging. The preserved carbon-13 relaxation time supports efforts to boost nuclear magnetic resonance signals.
Most importantly, the process provides abundant, consistent material. Laboratories can begin experiments with ready-made quantum-grade nanodiamonds instead of preparing limited batches through complex irradiation.
This work shows that a common industrial press, combined with a simple salt additive, can transform ordinary diamond powder into quantum-grade nanodiamonds in minutes. It aligns the speed of industrial diamond manufacturing with the precision required for quantum sensing, bringing a once-rare material closer to practical use.
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Petr Cigler (Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences)
, 0000-0003-0283-647X corresponding author
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