Nanoscale control and imaging of many-body dynamics in a critical dipolar ensemble

Platforms for exploring coherent dynamical phenomena in quantum many-body systems are often limited by the trade-off between scalability and local control. In this work, we integrate strong local magnetic gradients with a dense ensemble of ~10⁴ electron spins in diamond, enabling nanoscale control and Fourier magnetic imaging of their interacting many-body dynamics. Combining fast gradient control with global microwave driving, we measure precession dynamics of spatially-inhomogeneous spin spirals prepared by the gradient field. These observations are explained by a theoretical model based on coherent dipolar spin exchange induced by the spiral texture and Floquet-engineered interactions. Varying the spiral wavevector, we observe signatures of a gapped exchange field that is sensitive to the microscopic spatial extent of the underlying polarization profile that we directly image. These observations reveal a robust physical mechanism that with improved coherence time can be harnessed to generate scalable spin squeezing in three-dimensional dipolar ensembles, providing a realistic path to demonstrating entanglement-enhanced sensing under ambient conditions. More broadly, the nanoscale control techniques developed in this work enable the exploration of macroscopic coherent phenomena in complex quantum many-body systems.