Back in 1896, a young physicist called Pieter Zeeman was fired for carrying out an experiment against the specific wishes of his laboratory supervisor. Despite the consequences, the experiment led to a remarkable discovery that changed Zeeman’s life.
The experiment involved measuring the light emitted by elements placed in a powerful magnetic field. When he did this, Zeeman discovered that the spectral lines were split by the field. In 1902, he was awarded the Nobel Prize in physics for this discovery which is now known as the Zeeman effect.
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It is particularly useful for measuring magnetic fields at a distance. For example, astrophysicists use it to map variations in the magnetic field on the sun. But it can also be used to measure fields on a much smaller scale. In theory, the effect could be used to observe the influence of a magnetic field on a single atom.
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While they have not got quite this far, Thomas Wolf at the University of Stuttgart in Germany and a few pals, have come pretty close. These guys have used the spectra from nitrogen atoms embedded in diamond to build perhaps the most sensitive magnetometer ever made. They say their new device could soon be capable of measuring the magnetic field associated with protons.
First, some background about magnetometers. In recent years, physicists have made increasingly sensitive magnetometers using a variety of different techniques. One problem they all come up against is that magnetic fields decay very quickly with distance, as 1/r^3.
That means the size of the sensor has an important impact on what it can detect, since magnetic field can change significantly throughout the volume of the sensor. So an important task is to make magnetometers as small as possible.
That’s where diamond comes in. Diamond is a three-dimensional crystal made of carbon. However, when a carbon atom in the structure is replaced with nitrogen, this produces an additional unbound electron.
When this electron is excited with laser light, it then fluoresces at a frequency that depends on its environment. A magnetic field in particular can change this frequency, via the Zeeman effect, making nitrogen defects in diamond a promising type of magnetometer.
Of course, addressing a single atom in such a structure and recording its fluorescence accurately is a tricky business. So Wolf and co use an entire ensemble of nitrogen defects in a volume of diamond occupying just a fraction of a cubic millimetre. They estimate that this contains several billion nitrogen atoms.
Although a centre of this size is many orders of magnitude larger than an individual atom, it produces a fluorescent signal that is much easier to measure. That makes the device practical. Even at this size, the magnetometer is one of the smallest ever made.
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To find out how sensitive, Wolf and co put the device through its paces, carefully eliminating noise at every step. The results are impressive. The team eventually measured a field strength of only 100 femtoTesla. That’s comparable with the most sensitive magnetometers on the planet. And they think they can do even better with relatively straightforward improvements that should increase the sensitivity by two orders of magnitude.
But here’s the thing: what’s unique about this device is that it is both small and sensitive, a combination that has never been achieved before. That makes this device a kind of record breaker. It can measure magnetic field strengths in tiny volumes that have never been accessible before. In other words, it opens up magnetic field strength detection on an entirely new scale using a solid state device that works at room temperature.
One goal in this area is to measure the magnetic fields of protons in water. The sensitivity of this device looks to make this possible. “This value itself allows for detection of proton spins in a microscopically resolvable volume in less than one second,” says Wolf and co.
Magnetometers are used in a wide range of applications, ranging from mineral exploration and archaeology to weapon systems positioning and heartbeat monitors. So a robust, highly sensitive solid-state device that works at room temperature is likely to come in handy. Zeeman would have been impressed.
