From Wikipedia:
Nikita Khrushchev, leader of the Soviet Union from 1953 to 1964, said in his posthumously published memoir that he “cannot specifically say what kind of help the Rosenbergs provided us” but that he learned from Joseph Stalin and Vyacheslav Molotov that they “had provided very significant help in accelerating the production of our atomic bomb.”[54]
Boris V. Brokhovich, the engineer who later became director of Chelyabinsk-40, the plutonium production reactor and extraction facility that the Soviet Union used to create its first bomb material, alleged that Khrushchev was a “silly fool”. He said the Soviets had developed their own bomb by trial and error. “You sat the Rosenbergs in the electric chair for nothing”, he said. “We got nothing from the Rosenbergs.