Macroscopic magnetic resonance spectroscopy at the quantum limit

Nuclear magnetic resonance (NMR) metrology has found applications ranging from medical imaging to precision tests of fundamental physics. Typically, quantum effects are observed and used in microscopic systems with exquisite control over the spins. For sensing and metrological purposes, however, having a larger spin ensemble comes with clear benefits, but there is a tension between the sensitivity you gain and the fact that quantum phenomena usually rapidly lose out to classical noise in larger systems. In this talk, I will present a novel superconducting resonant NMR receiver that can not only detect quantum mechanical fluctuations in a macroscopic spin ensemble but is limited by them. Using 5×10^{21 19}F spins in PTFE, we demonstrate non-equilibrium spin state preparation, single-shot measurements of subsequent ultra-slow thermalization dynamics, and achieve a measurement of the collective spin quantum fluctuation angular variance at 8×10⁻¹⁷ radians squared. With this detector, we bring the tools of quantum sensing into the macroscopic regime thus enabling truly non-invasive magnetic resonance spectroscopy and precision searches for new fundamental physics.