Paul Schechter

Quadruply lensed quasars (“quads”) are used to study several important astronomical problems, one of which, the abundance of LIGO-mass primordial black holes, we discuss at some length. But quads are rare, with only one out of every 3000 quasars lensed into a quad. The alternative schemes used to identify them all have serious shortcomings of either an astrophysical or observational nature. The ground- and space-based surveys of the past decade have nonetheless yielded dozens of new quads.  The surveys of the next decade will produce many hundreds.  We discuss various different search techniques, some of which have until now proven productive and others of which are soon likely to be so.