It might sound slightly science fictional or possibly James Bond-ian, but this is what Dmitry Budker actually does: He shoots green lasers at colored diamonds.
Budker is a UC Berkeley physics professor and his specialty is atomic magnetometry. He and his colleagues are bouncing lasers off of diamonds to measure the magnetic properties of materials such as human nerve cells and high-temperature superconductors. Besides being portable and inexpensive, these magnetic sensors are minimally invasive, making them easy to use in the biological sciences. This allows magnetic resonant imaging (MRI) to be performed at sizes a hundred times smaller than the diameter of a human hair, without the need for expensive high-field magnets and cryogenics. At this scale, one could track concentrations of free radicals during immune responses, or the minute electrical currents in the human brain.
To build these sensors, Budker puts synthetic diamonds through what he calls radiation therapy, in which the diamonds are first bombarded with nitrogen and electrons, then heat-treated. The nitrogen and electrons knock out carbon atoms, replacing two side-by-side carbon atoms with a single nitrogen atom, creating what’s called nitrogen-vacancy centers. And a nitrogen-vacancy center has remarkable properties for magnetic sensing.
How does it work?
First, shine a green laser on the diamond. The reflected light will fluoresce red. The intensity of this red fluorescence is monitored. The light also has the effect of aligning the quantum spins of the nitrogen-vacancy centers as if resetting them all to zero.
Now, take a diamond with all its spins zeroed out and apply a microwave magnetic field. As the frequency of this field increases, the nitrogen-vacancy centers absorb more energy. Eventually the field reaches a resonant frequency at which point the center’s spin must take the leap to one of the allowed energy levels greater than zero. When it leaps, it gives off a photon, causing a peak in the intensity of the red fluorescent light. If you know what the resonant frequency is, then you know the center’s magnetic field. You now have a magnetic sensor, a magnetometer.
“A lot of people are trying to apply these centers to detection of neural signals. It’s a very difficult problem because the signals are weak and fast, and the neurons are small. But you can put little pieces of diamond on the cell, and the cell continues to function normally,” Budker explains.
Next steps for Budker include research on nuclear magnetic resonance and MRI at very small spatial dimensions with Professor Alexander Pines in Berkeley’s Department of Chemistry. Budker says that “this diamond field is a field of collaboration between different countries, between different groups. Competitors collaborate with each other, and it’s actually very exciting. That’s the beauty of science.”