Quantum sensors based on diamond spin systems are on their way to become major tools in material science. While the detection of magnetic fields generated from ferro- or antiferromagnetic structures is routinely done, this is not the case for electric fields. A major part of the work accomplished in SMeL was thus to invent methods to enhance the sensitivity of the quantum sensor for electric fields. It should be noted, that the spin levels of the NV center are not sensitive to electric fields in first order. We have carried out systematic investigations of various experimental conditions to maximize electric field sensitivity. As a result, we were able to achieve record-breaking electric field sensitivities. We were able to apply this technique to nanoscale detection and imaging of electric fields. This is of particular relevance as in most materials, either used in electronics or in biostructures strong local electric fields exist. Those fields are shielded macroscopically but exist on small length scales. In SMeL we were able to detect those fields in model materials and even provide image of local electric fields with nanometer resolution.
To improve electric field sensitivity, the grantee was also investigating quantum systems in other host materials like silicon carbide that, like diamond defects, carries a spin and do show optically detected spin resonance. Those spins can be used for quantum sensing, albeit with less sensitivity due to the lower signal contrast. This is in part, especially for electric field sensing, compensated by a larger susceptibility to electric fields. The grantee has investigated its quantum properties and derived optimum parameters for quantum sensing experiments. These results pave the way for an integrated quantum device based on SiC which is currently explored with leading companies in Europe.
Temperature sensing is an attractive feature of the NV quantum sensor. Its physics is very similar to electric field sensing. However, by dedicated quantum control methods, the impact of temperature changes can be separated from electric field induced level shifts. The beauty of temperature sensing with spin quantum sensors is, that small temperature changes can be measured at very small length scales, e.g. a few nm. This allows to measure temperature gradients in e.g. living cells and this provide insight into biochemical processes in these structures. A major task was to achieve sufficient sensitivity to measure small temperature differences. The applicant was able to demonstrate the so far best temperature sensitivity of a few 10 mK/√Hz.
By improving sample material and developing novel imaging techniques, as well as fabricating we achieved high resolution imaging of nuclear spins using a technique called wide-field nuclear magnetic resonance imaging. In this approach, we combine imaging of diamond defects by a wide field fluorescence microscope with the sensing capabilities of the nitrogen vacancy (NV) center. We were demonstrating a sub 200nm spatial resolution in the detection of a nuclear magnetic resonance (NMR) signal of a thin layer of CaF2. This is the so far highest achieve spatial resolution achieved with this method and is a further step towards a versatile application of the technique.
