The pump–probe signal. (a) Atomic spin is first polarized by a 50 ms pump laser pulse and then goes through free precession in a 9 nT bias field. The polarization rotation angles of the detection beam at vapor cell temperatures of (b) 85 °C and (c) 150 °C. The red lines are fitted lines with the Bloch equation. From the fit, the precession frequencies are 63 Hz and 42 Hz, and the relaxation rates are 54 s⁻¹and 77 s⁻¹in panels (b) and (c), respectively.