WASHINGTON, Jan. 30 — In March 2004, the science and technology directorate of the Central Intelligence Agency called a secret meeting of hundreds of the government's top experts in nuclear intelligence to address a problem that had bedeviled Washington for decades: how to know, with precision, when a country is about to cross the line and gain the ability to build an atomic bomb. The aim of the two-day conference was to reinvigorate the nation's atomic espionage efforts, not with spies on the ground or satellites in space but with a new generation of advanced technologies meant to detect the...