Spin alignment based Rb-87 magnetometry with free spin precession.

Spin alignment based Rb-87 magnetometry with free spin precession has been proposed. A 795-nm linearly polarized laser beam, serving as the pump beam and the probe beam, propagates through a cylindrical Rb-87 enriched atomic vapor cell along the Z axis, with the polarization orientation aligned along the Y axis. Simultaneously, the static magnetic field B0⃗ is applied along the Y axis, and the π/2 pulse radio-frequency (RF) magnetic field BRF⃗ is applied along the Z axis. Given that the laser polarization direction aligns with the static magnetic field, the pump beam exhibits π polarization. This induces a symmetric population distribution of ground-state atoms across the Zeeman sublevels. This leads to the generation of a magnetic quadrupole moment, thereby facilitating the formation of the spin alignment state. The π/2 pulse RF magnetic field aligns the magnetic quadrupole moment with the direction of the RF magnetic field. Upon the cessation of the RF magnetic field, the magnetic quadrupole moment undergoes precession around the static magnetic field. The linearly polarized probe beam, in conjunction with a balanced polarimeter comprising a true zero-order half-wave plate, a Wollaston prism, and a balanced differential photodiode, is employed to measure the free precession decay signal. In this magnetometry system, both the pump power and the RF magnetic field strength have been optimized. The sensitivity of the spin alignment based Rb-87 magnetometry with free spin precession is about 1.7 pT/Hz1/2.