The quest to measure tiny and enormous values of common physics variables continues; here, the Zeeman effect and NMR techniques are used to create a sub-picotesla magnetometer at a nitrogen-vacancy defect site in a diamond lattice.
The use of sensors for basic parameters is straightforward in concept, the reality is that there are many challenges in their actual implementation, including stimulation (in some cases), sensing, and analog interfacing. Sometimes the problem is the access to the variable being sensed, such as when trying to measure the temperature and pressure conditions in an internal-combustion engine cylinder as goes thought its ignition stage. Other times, the challenge is because you at trying to measure an extremely low or high value, such as fractional-degree temperatures near absolute zero or the temperature of a fusion reaction at millions of degrees centigrade. That’s why I am always interested the clever and innovative techniques that scientists and engineers develop for measuring values at either end of the possible range.
A recent example which I first read about in Laser Focus World shows the extremes to which these experts go to measure that which has not been measured before, or could only be roughly assessed. A team of researchers at the University of Stuttgart (Germany) and the University of Tsukuba (Japan) have taken advantage of lattice defects in diamonds to measure magnetic fields in the sub-picotesla range, see here. Even better, the sensor is tiny and so can be placed close to the field of interest, which is critical because magnetic fields in general fall off as the third power of distance. Also fortunate, it does not rely on a superconducting environment as the less-sensitive SQUID (superconducting quantum interference device) magnetic-field sensor does.
The diamond-based magnetometer exploits a defect in the diamond's lattice, which is occupied by a nitrogen atom; the Zeeman effect is then used to observe the magnetic field at that point
(Source: American Physical Society)
There isn’t room here to fully explain what they have done (see the references below, for more), but in brief, they have combined the technique nuclear magnetic resonance (NMR) spectroscopy with the Zeeman effect, but for defects in a diamond crystal lattice. When a carbon atom in the lattice is missing, it is replaced by a nitrogen atom, called a nitrogen vacancy. The spin and energy of the sublevels of this atom shift when in a magnetic field, in proportion to the field's intensity. By using optical and microwave spectroscopy these spin shifts can be observed and measured—hence, an atomic-size, super-sensitive magnetic field sensor.
The researchers note that this sensor could go on the tip of a probe such as an AFM (atomic force microscope), or a thin diamond disk with defects could be used to “map” a larger field. Since the sensor is both tiny sand sensitive, it can get close into the area of interest. In short, it's impressive and amazing.
Of course, this simple summary explanation doesn’t reveal more than a tiny portion of what it takes to make this happen. It’s a convergence of technologies building on historical instrumentation advances, plus a foundation of very deep principles of physics, that enables this sort of clever and brilliant idea to be conceived and implemented. Plus, it takes lots of sensors, signal conditioning, amplification, and processing control to make this idea into a reality. It’s ironic that the smaller the point of interest, the larger the system must be to trap, sense, control, and assess it—think of the Large Hadron Collider, as just one example.
Have you ever had a sensor/sensing problem which required an advance in basic technology to address and solve? Were you involved in developing that solution, or the associated circuitry? How do you test the accuracy of such extreme sensor designs, whether physically tiny or extremely large, or super-sensitive, or for very large values?