Diamond anvil cell technology has been used for many years to study materials under high pressure. These materials can exhibit exotic phases of matter such as superconductivity with exceptionally high critical temperatures. The main difficulty lies in the challenge to measure magnetic properties at high pressure (> 100 GPa) due to the minute size of the sample that is allowed by the pressurized chamber of the diamond anvil cell.
I will describe the implementation of high-pressure magnetometry based on the optical detected magnetic resonance (ODMR) of nitrogen-vacancy (NV) centers . Due to their electronic spin properties, these atomic-like systems are highly sensitive magnetic probes and their atomic size can allow for sub-micrometer spatial resolution. Using a customized optical microscope, we use the spin dependent luminescence of NV centers to perform a mapping of the magnetic field at the diamond anvil tip. Expulsion of magnetic field lines due to the Meissner effect in a superconductor results in a clear drop of the magnetic field in the close vicinity of the sample, where the NV sensors are located. This direct identification provides an unambiguous diagnosis of superconductivity that does not rely on questionable electrical contacts or indirect probes. This measurement can be performed on any magnetic sample and is compatible with synchrotron X-ray diffraction for structural characterization .
I will discuss how this method can be implemented at extreme pressures above 100 GPa by fabricating pillars on the anvil tip to create a quasi-hydrostatic stress environment for the NV centers . This is quantified using the pressure dependence of the diamond Raman shift, the NV ODMR dependence on applied magnetic field, and NV photoluminescence spectral shift. In these conditions the sensitivity of NV micro-sensing is at 130 GPa almost as if at ambient pressure. This result enables the undisputable detection the Meissner effect of super-hydrides that is currently under stifling debate.
 M. Lesik, T. Plisson, L. Toraille, J. Renaud, F. Occelli, M. Schmidt, O. Salord, A. Delobbe, T. Debuisschert, L. Rondin, P. Loubeyre, J.-F. Roch, Magnetic measurements on micrometer-sized samples under high pressure using designed NV centers, Science 366, 1359–1362(2019).
 L. Toraille, A. Hilberer, T. Plisson, M. Lesik, M. Chipaux, B. Vindolet, C. Pépin, F. Occelli, M. Schmidt, T. Debuisschert, N. Guignot, J.-P. Itie, P. Loubeyre, J.-F. Roch, Combined synchrotron X-ray diffraction and NV diamond magnetic microscopy measurements at high pressures, New J. Phys. 22, 103063 (2020).
 A. Hilberer, L. Toraille, C. Dailledouze, M.-P. Adam, L. Hanlon, G. Weck, M. Schmidt, P. Loubeyre, J.-F. Roch, NV center magnetometry up to 130 GPa as if at ambient pressure, arXiv:2301.05094 (2023).