  Flux ratio anomalies in strong gravitationally lensed quasars constitute a
unique way to probe the abundance of non-luminous dark matter haloes, and hence
the nature of dark matter. In this paper we identify double imaged quasars as a
statistically efficient probe of dark matter, since they are 20 times more
abundant than quadruply imaged quasars. Using N-body simulations that include
realistic baryonic feedback, we measure the full distribution of flux ratios in
doubly imaged quasars for cold (CDM) and warm dark matter (WDM) cosmologies.
Through this method, we fold in two key systematics - quasar variability and
line-of-sight structures. We find that WDM cosmologies predict a ~6 per cent
difference in the cumulative distribution functions of flux ratios relative to
CDM, with CDM predicting many more small ratios. Finally, we estimate that ~600
doubly imaged quasars will need to be observed in order to be able to
unambiguously discern between CDM and the two WDM models studied here. Such
sample sizes will be easily within reach of future large scale surveys such as
Euclid. In preparation for this survey data we require discerning the scale of
the uncertainties in modelling lens galaxies and their substructure in
simulations, plus a strong understanding of the selection function of observed
lensed quasars.
