Diamond, clearly, is synonymous with the gemological trade but in the last 20 years of material science research, diamond is now firmly embedding itself in technological applications. You may think that these are the so called “industrial diamonds” used to coat tools to slice through the toughest of materials – afterall, diamond is the hardest material. But you’d be wrong.
In a previous article, we highlighted the interest in understanding nitrogen in the diamond crystal and how this can be engineered to form the nitrogen vacancy defect. This defect has a “very simple” premise – you shine green light on it and get red light back. However, it is the amount of red light we get back which is of most interest.
To dive deeper, the underlying principle of this red light emission is due to the absorption of energy the green light delivers to the defect; effectively causing an electron to become excited and, as it relaxes, emits a red photon (a technical term for a specific amount of light). However, there is a 30% chance that the electron we excite does not emit a red photon when it relaxes, and this is where the quantum nature starts to enter relative to technical applications.
Electrons have a property known as spin (analogous to the spin of the Earth), we can be either spin-up or spin-down (think of it like the North and South pole of the Earth). Quantum gets weird, and we can also have a spin that is both up and down at the same time. For the NV defect, it is no different, there is a spin associated with it, and depending on which state of the spin we excite depends upon if we get red light or not.
We can also go one further. We can also force the spin state in a process known as optical initialisation, by shining the green light on the defect continuously we force the defect to relax into a state whereby we guarantee the emission of a red photon on the next cycle. But, it is also possible to force the defect to enter the pathway where we are less likely to get a red photon by applying an additional amount of energy during excitation (this time by a microwave source) and by doing so we change the spin state of the defect and reduce our chances of getting a red photon. Now, it’s possible to plot the amount of red light detected as a function of this additional energy, and at a specific energy (for NV, it’s at a microwave frequency of 2.87 GHz) we repeatedly see a reduction in red light. This is known as Optically Detected Magnetic Resonance (ODMR). You may be familiar with magnetic resonance imaging (MRI) and the principle is very much related.
Finally, to complete the understanding of the usefulness of ODMR, we now know that if we apply a magnetic field in the vicinity of the defect, this dip in fluorescence moves and becomes two dips in red light at specific locations as we see a resonance due to either the spin-down or spin-up state! It’s this separation in dips that is known to be linear with the magnetic field strength applied that makes it so useful for quantum sensing applications.
There are many articles published relating to the science and application of NV centres, such as this one, and this one, but a team of researchers at the University of Warwick have recently demonstrated two key developments in employing NV defect-based technology.
The first describes the miniaturisation of all the complex components such that an extremely portable and highly sensitive magnetometer can now be realised. Here, the limit of sensitivity is on the order of 310 pT and to put that in context, it is about 100,000 times smaller than Earth’s own magnetic field strength! Coupled with the portability the team at Warwick are hoping to find an application in the magnetic resonance imaging associated with the heart (magnetocardiography). Read the full article here.
Their next key development is the use of this portable magnetometer to map out defects in steel, demonstrating a plausible real-world application. A copy of this paper is available to read in pre-print format over on arXiv.
If these interest you, be sure to watch the video from Gavin Morley and Matthew Markham talking about the science behind these developments here at the UK Quantum Technology Showcase.
This is a rough pink diamond from the Juina area in Brazil. What do pink diamonds have to do with quantum sensing? A rare type of natural pink diamond is coloured by the NV centre. These NV centres can also be manufactured into the diamond. There are two types of NV centres that have different charge states - neutral and negative. The ability to switch between these two charge states creates a quantum sensor! Image by Karen Smit, diamond courtesy of Graham Pearson.