shift would cause a systematic error in the measurement. Though we had done experiments to verify that this error was less than 2 kHz, we wanted to repeat the measurements with another technique that was not at all susceptible to errors arising from lock-point uncertainty. As we will see below, the co-propagating technique achieves precisely this. In addition, by measuring the entire spectrum and looking at the symmetry of the line shape, we can be sure that other sources of error are not significant. The current set of measurements in $^{85}$Rb and $^{133}$Cs, although having slightly smaller precision, are consistent with our earlier work.

Spectroscopy on the $D_2$ line of rubidium
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The schematic for the first set of experiments is shown in Fig. \[schema\]. The pump and probe beams are derived from two home-built frequency-stabilized diode laser systems [@BRW01] operating on the 780 nm $D_2$ line of Rb. The linewidth of the lasers after stabilization is of the order of 1 MHz. The output beams are elliptical with $1/e^2$ size of $2 \times 4$ mm and powers of around 10 $\mu$W each. Part of the probe laser is sent into a Rb SAS cell and the laser is locked 