Nuclear Spin Oscillator Based on $^3$He to Search for Exotic Spin Coupling

We describe an experimental investigation of a nuclear spin oscillator based on $^3$He nuclei as a possible detector to search for exotic spin couplings. A magnetically shielded vapor cell comprised of an alkali atom mixture ($95\%$ potassium and $5\%$ rubidium) and $^3$He gas is polarized via laser light resonant with the $D_1$ transition in rubidium in the presence of a dc magnetic field. The potassium atoms and $^3$He nuclei are polarized via spin-exchange collisions with the polarized rubidium atoms. The nuclear spins are tipped with a magnetic field applied perpendicular to the dc magnetic field. The resulting Larmor precession of the $^3$He nuclear spins is monitored via Faraday rotation of laser light near resonant with the $D_1$ transition in potassium. The Faraday rotation signal is filtered, amplified, and used to apply a magnetic field in a direction perpendicular to the dc magnetic field, resulting in a self-sustained oscillation of the nuclear spins at a frequency that is directly proportional to the dc magnetic field. We demonstrate a sensitivity to exotic spin couplings that is $\approx 5$ times higher than the alkali atom magnetometers that have been used in the Global Network of Optical Magnetometers to Search for Exotic Physics collaboration.