Precision measurements of spin interactions with high density atomic vapors

Polarized atomic vapors can be used to detect fields interacting with a spin. Recent advances have extended the sensitivity of atomic magnetometers to a level favorable for fundamental physics research, and in many cases the sensitivity approaches quantum metrology limits. In this thesis, we present a high density atomic K-He comagnetometer, which features suppressed sensitivity to magnetic fields, but retains sensitivity to anomalous fields that couple differently than a magnetic field to electron and nuclear spins. The comagnetometer was used to measure interactions with a separate optically pumped He nuclear spin source. The He spin precession frequency in the comagnetometer was measured with a resolution of 18 pHz over the course of approximately one month, enabling us to constrain the anomalous spin-spin interaction between neutrons to be less than 2.5 × 10−8 of their magnetic or less than 2 × 10−3 of their gravitational interaction at a length scale of 50 cm. We set new laboratory bounds on the coupling strength of light pseudoscalar, vector and pseudovector particles to neutrons, and we consider the implications of our measurement to recently proposed models for unparticles and Goldstone bosons from spontaneous breaking of Lorentz symmetry. We also describe theoretically and experimentally a quantum non-demolition (QND) measurement of atomic spin in the context of radio frequency magnetometer with hot alkali-metal vapors. Using stroboscopic probe light we demonstrate suppression of the probe back-action on the measured observable, which depends on the probe duty cycle and on the detuning of the probe modulation frequency from twice the alkali Larmor frequency. We study the dependence of spin-projection noise on the polarization for atoms with spin greater than 1/2 and develop a theoretical model that agrees well with the data. Finally, it is shown theoretically that QND measurements can improve the long-term sensitivity of atomic magnetometers with non-linear relaxation. iii