Gravity Probe Spin

Under conditions where the total angular momentum of a ferromagnet is dominated by its intrinsic spin, the ferromagnet is predicted to behave as a gyroscope [Kimball, Sushkov, and Budker, Phys. Rev. Lett. {\textbf{116}}, 190801 (2016)]. If such a ferromagnetic gyroscope (FG) can be sufficiently isolated from the environment, it has the potential to measure spin-dependent interactions with a sensitivity far surpassing that of other systems [Band, Avishai, and Shnirman, Phys. Rev. Lett. {\textbf{121}}, 160801 (2018)]. The high sensitivity is the result of rapid averaging of quantum noise. We propose to use a mm-scale FG in orbit around the Earth to investigate physics at the intersection between quantum mechanics and general relativity by measuring relativistic frame dragging (the Lense-Thirring effect) with intrinsic spin. The behavior of intrinsic spin in spacetime dragged by a massive rotating body is an experimentally open question, hence the results of such a measurement may have important theoretical consequences.