Levitated Ferromagnetic Magnetometer with Energy Resolution Well Below ℏ

A quantum limit on the measurement of magnetic fields has been recently pointed out, stating that the so-called energy resolution E_{R} is bounded to E_{R}≳ℏ. This limit indeed holds true for the vast majority of existing quantum magnetometers, including superconducting quantum interference devices and solid state spin and optically pumped atomic magnetometers. However, it can be surpassed by highly correlated spin systems, as recently demonstrated with a single-domain spinor BEC. Here, we show that similar and potentially much better resolution can be achieved with a hard ferromagnet levitated above a superconductor at cryogenic temperature. We demonstrate E_{R}=(0.064±0.010) ℏ and anticipate that E_{R}<10^{-3} ℏ is within reach with near-future improvements. This finding opens the way to new applications in condensed matter, biophysics, and fundamental science. In particular, we propose an experiment to search for axionlike dark matter and project a sensitivity that is orders of magnitude better than in previous searches.