He worked for two years in the Mathematics Institute at the University of Kharkiv, where he became friends with Lev Landau with whom he collaborated for many years. After the Second World War, he went to Moscow and worked at the Institute for Physical Problems, where he was a head of the mathematics lab. Then he worked in the Institute for Theoretical and Experimental Physics. In 1953, he was awarded a Stalin prize for his work in theoretical physics. He made important contributions in the development of nuclear weapons in the USSR.