Further constraints on variation of the fine-structure constant from alkali-doublet QSO absorption lines

Comparison of quasar (QSO) absorption-line spectra with laboratory spectra provides a precise probe for variability of the fine-structure constant, al pha, over cosmological time-scales. We constrain variation in alpha in 21 K eck/HIRES Si IV absorption systems using the alkali-doublet (AD) method in which changes in a are related to changes in the doublet spacing. The preci sion obtained with the AD method has been increased by a factor of 3: Delta alpha/alpha = (-0.5 +/- 1.3) x 10(-5). We also analyse potential systemati c errors in this result. Finally, we compare the AD method with the many-mu ltiplet method, which has achieved an order of magnitude greater precision, and we discuss the future of the AD method.