The nitrogen-vacancy (NV) defect in diamond is an optically-active colour centre that shows much promise for all-optical sensing. Its ground state is a spin-triplet that has been investigated as a potential qubit. The interaction of the system with the environment also allows detection of magnetic fields and, through the frequency shift of a microwave-frequency spin-resonance, temperature. Current measurements use pulsed control of the spin state of the NV centre to enact rephasing of the spin, allowing decoupling of the system from environmental noise while retaining sensitivity to the parameters being measured.

In addition, even when embedded in nanodiamond (typically diamond particles smaller than a few hundred nanometers), the NV centre retains the ability to be used as an effective sensor. Diamond is biologically inert, yet can be functionalised through surface chemistry modifications to target structures of interest within cells. Nanodiamond therefore offers an exciting route to all-optical, sub-cellular detection of biological activity.

Many biological processes of interest to a wide range of researchers involve local thermal effects (both endothermic and exothermic); sub cellular temperature sensing offers a powerful way to infer the activity of cells undergoing numerous processes. Furthermore, when performing thermometry with NV centres, there are effective ways to decouple the sensor from sources of noise within the system of interest. Excitingly, this approach enables correlated thermometry / magnetometry with the same setup and samples.

While NV centres are perhaps better known for magnetometry applications, sensitivity to noise-generating processes within cells poses significant engineering challenges that are being addressed by numerous groups, including those in this project proposal. Therefore, we feel that specialising on sub-cellular thermometry will offer a significant benefit to the training of the student in this project – they can quickly apply and develop quantum sensing techniques to biologically relevant questions.

With this project, we propose to build on our existing expertise with NV sensing to further develop the use of nanodiamond as a sub-cellular thermometer for applications in biological imaging. A key focus of the project will be ensuring that the hardware and sample processing requirements with be compatible with the needs for effective biological research. An existing, low-cost widefield NV sensing system will be expanded to allow investigation of biological systems while incorporating state of the art pulsed microwave techniques to improve sensitivity and reject noise. Anticipated systems of interest include mitochondrial activity within cells and the impact of photosynthesis on the local thermal environment within a cell.

This will be done through collaboration with biological researchers from the outset (local researchers in SIPBS and Jones), the development of flexible, comparatively low-cost, hardware that can be integrated with gold-standard biological imaging techniques, and a focus on interdiscplinary training. The latter will allow the successful candidate to develop a broad range of skills beyond those in their primary degree, and will also equip them for effective research in the highly interdisciplinary research that characterises much of the intersection of quantum technologies with the other disciplines we wish to see it embedded in.