Quantum Sensors Gain New Control Under Extreme Pressure

A new study reveals how tiny defects in diamond, known as nitrogen-vacancy centers, change their behavior under extreme pressures, offering a path to optimize quantum sensors used in high-pressure research. These sensors, embedded in diamond anvil cells, allow scientists to image stresses and magnetism with sub-micron resolution inside materials subjected to pressures exceeding a million atmospheres. Understanding how pressure affects these quantum defects is crucial for advancing applications in quantum materials, superconductivity, and geophysics, where precise measurements under extreme conditions are needed.

The researchers found that the optical contrast of nitrogen-vacancy centers, a key metric for their sensing capability, depends sensitively on the type of stress applied. Under symmetry-preserving stresses, such as uniaxial stress along the [111] crystal direction, the contrast can be enhanced, while under symmetry-breaking stresses, like uniaxial [100] stress, it can invert, leading to unexpected positive contrast in measurements. This work resolves long-standing puzzles, including why contrast improves in (111)-oriented diamond anvils and why it flips in certain high-pressure regimes, as observed in experiments up to 130 GPa.

To achieve these insights, the team combined first-principles calculations with high-pressure experiments on diamond anvils with three different crystal orientations: (100)-, (110)-, and (111)-cut. They used ab initio simulations to estimate inter-system crossing rates and spin polarization in the ground-state manifold as functions of the stress tensor. This allowed them to develop a microscopic model that characterizes the optically detected magnetic resonance contrast under general stress conditions. The calculations focused on parameters like spin-orbit coupling and Jahn-Teller effects, which influence how the defects interact with light and magnetic fields.

Show that for symmetry-preserving stresses, the optical contrast is mainly determined by the upper inter-system crossing rate, which exhibits a non-trivial trend with pressure. For example, under hydrostatic stress, this rate peaks around 30 GPa due to competition between spin-orbit coupling and vibrational overlap. In contrast, under uniaxial [111] stress, the rate increases monotonically, leading to better contrast. Experimental measurements on (111)-cut anvils with varying hydrostaticity levels confirmed these predictions, showing semi-quantitative agreement with the simulations. For symmetry-breaking stresses, the interplay between stress-induced spin-orbit coupling and Jahn-Teller effects causes non-monotonic changes in lower inter-system crossing rates, ultimately producing contrast inversion, as observed in (100)-cut anvils around 60 GPa.

This research has significant for optimizing quantum sensors in high-pressure environments, enabling more accurate imaging of stress and magnetic fields in materials like superconductors and magnetic minerals. By controlling the local stress environment, scientists can enhance sensor performance, potentially leading to new discoveries in condensed matter physics and earth sciences. also suggest that symmetry-breaking stresses could serve as a novel tuning knob for other solid-state spin defects, broadening the scope of quantum technologies. However, the study acknowledges limitations, such as the need to generalize the computational framework to other environmental conditions like temperature and magnetic fields, and the open of understanding contrast inversion details in different anvil cuts. Future work will refine these models to improve accuracy and explore applications in quantum information and sensing.