Towards Direct Observation of Spin Squeezing in Solid State Systems

In the last few years, a tremendous amount of effort has centered on the generation and probing of metrologically useful entanglement in solid-state spin systems. In dense ensembles of two-dimensional spins, the native dipolar interaction can generate spin-squeezed entanglement which can be used to perform metrology beyond the standard quantum limit. This insight has recently led to the first observation of spin squeezing in a strongly-interacting ensemble of Nitrogen-Vacancy centers in diamond. While promising, this observation was achieved using indirect methods that extract the spin projection noise from the NV's relaxation dynamics. To date, the direct observation of spin squeezing or the associated twisting dynamics has yet to be demonstrated. The fundamental challenge arises from the low-fidelity nature of conventional NV fluorescence readout. Here, we demonstrate the high-fidelity, non-destructive readout of spin dynamics in a dense NV system via an ancilla-assisted repetitive readout technique. By mapping the electronic state onto the nitrogen nuclear spin memory, we perform successive quantum non-demolition measurements, allowing us to track the evolution of the many-body spin state while suppressing photon shot noise to the level of quantum projection noise. Starting from a spin coherent state, we observed the emergence of anisotropy in the quantum projection noise under the twisting dynamics induced by the dipolar interactions.