cell kept inside a magnetic shield. The residual field (measured with a three-axis fluxgate magnetometer) is below 5 mG. The beams have orthogonal linear polarizations and are mixed and separated using PBS’s. The beam powers are about 15 $\mu$W each and adjusted to get good signal-to-noise ratio in the spectrum.

The experiment proceeds as follows. An rf frequency generator whose timebase is referenced to an ovenized quartz clock (uncertainty less than $10^{-8}$) is used to drive the AOM. A computer program is used to set the rf frequency, and the probe signal is measured and recorded. The frequency is changed in steps of 0.1 MHz over a range of 15 MHz to obtain the complete spectrum. A curve fit to the spectrum yields the AOM frequency at the peak center, which is the hyperfine interval.

Error analysis
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Systematic errors can arise due to one of the following reasons.

1.  [*Radiation-pressure effects.*]{} Radiation pressure causes velocity redistribution of the atoms in the vapor cell. In the SAS technique, the opposite Doppler shifts for the counter-propagating beams can result in asymmetry of the observed lineshape. However, with co-propagating beams, the effects are less important because the Doppler shift will be the 