Impurities in diamonds can not only give them color, they can also turn diamonds into precise sensors of magnetic field and temperature. When the impurity happens to be a nitrogen atom, the crystal structure is deformed in such a way that a gap or defect, known as a nitrogen vacancy-center (NVC), is created. Electrons that are trapped in the centers show a remarkable coherence with spin states that can be precisely manipulated. If the electron coherence inside nanoscale diamond can be preserved for long enough, not only will we have the perfect qubit for a quantum computer, but the perfect device to reveal the secret lives of neurons.

In bulk diamond, NVCs can store photons, along with the quantum data they might carry, for many milliseconds. New techniques have recently enabled nano-sized diamond structures to be self-assembled by joining them to ring-shaped molecules known as SP1 proteins. The traditional problem with trying to scale down to nanodiamonds has been that they show poor spin coherence — typically in the microsecond range. Now researchers at the University of Cambridge have found a way to protect NVC spins in finely-milled synthetic diamonds, and perform coherence measurements at high resolutions.

There’s no two ways about it, making atomically precise diamond sensors just a few tens of nanometers across can be technically challenging. Perhaps even more so, is building the elaborate set-up needed to interrogate them. Once you have a bunch of these sensors where you want them, the key to extracting the personal vitals of a cell, or for that matter, the hundreds of subcellular organelles within it it, is light.

Nanothermy, or the measurement of minuscule (around 2 millikelvin) temperature changes across even more minuscule spatial (in the range of 200 nanometers) and temporal extents, is possible because the NVCs fluoresce in a temperature-dependent manner. The researchers were able to measure this emitted light using a custom-built scanning confocal microscope. This kind of microscope is able remove all of the light except that emanating from a single plane. Because NVCs are also sensitive to magnetic and electric fields, the researchers were able to operate them as a DC magnetometer. Essentially, they were able to demonstrate the optical detection of magnetic resonance.

It is now known that the temperature landscape inside an actively metabolizing cell is not the same across the entire cell. The activities of organelles, like mitochondria or the centrioles (which anchor the replication machinery), can be correlated with localized cellular hotspots. Even the spikes of neurons have a distinct thermal profile where heat is first absorbed by the cell and then released.

One problem with nanodiamonds has been getting them to stay put once inside the cell. This difficulty has been addressed in cell thermometry by using other methods, including genetically encoded thermosensors. If these new nanodiamonds can be attached to proteins, like the SP1 mentioned above, perhaps the perfect tool to watch and understand cells will be had.