The quantum sensitivity limit of spin-based searches for new fundamental physics

by Alex Sushkov

Thursday 1 May 2025, 16:00 → 18:00 Australia/Sydney

Spin-based searches for new fundamental particles and fields use spin ensembles as transducers that are sensitive to spin interactions with fields beyond the standard model. For such experiments, the standard quantum limit on sensitivity scales with the inverse square root of the number of spins. Consequently, increasing the size of the spin ensemble presents a compelling approach to improving sensitivity. However, this enhancement demands a proportional reduction in other sources of noise. I will present the method that allows us to perform quantum noise-limited magnetic resonance measurements on macroscopic ensembles of 5×10^{21} nuclear spins in a solid. We measure the angular spectral density of the collective spin quantum fluctuations with the amplitude 5×10^{−11} rad/√Hz. The root-mean-squared collective spin quantum fluctuation angle is 9 nano-radians. I will discuss the implications of these results for the Cosmic Axion Spin Precession Experiment (CASPEr) search for ultralight axion-like dark matter.