The T₁ Puzzle: Understanding Low Temperature Spin Relaxation in Dense Ensembles of Nitrogen-Vacancy Centers

Strongly-interacting ensembles of nitrogen-vacancy (NV) centers in diamond represent a promising quantum simulation platform. To date, the majority of such quantum simulations, ranging from observations of emergent hydrodynamics to demonstrations of spin squeezing, have operated under ambient conditions. Exploring the quantum dynamics of interacting NV ensembles at low temperatures promises new windows into the physics of disordered, dipolar systems. In order to access this physics, it is essential to understand the dominant mechanisms governing the exchange of energy between NV spins and their environment in this regime. Prior measurements along this direction have revealed a long-standing puzzle: the NVs spin relaxation (T₁) time exhibits a low-temperature plateaus that cannot be explained by either spin-spin or spin–phonon relaxation. Here, we discuss several candidate mechanisms, including noise from paramagnetic defects, electric field noise from charge hopping, and NV–NV spin diffusion, each exhibiting distinct dependencies on defect density and temperature. To help disentangle these contributions, we present ongoing experiments using diamond samples with fixed nitrogen concentration and systematically varied NV density.