In 1960, he moved to the United States to become an associate professor of physics at Yale University. He became a U.S. citizen in 1970. He was appointed a professor in 1961 and was associate director of the Heavy Ion Accelerator Lab from 1960 to 1963. He was the founder, and from 1963 to 1989, the director, of Yale's A. W. Wright Nuclear Structure Lab. From 1970 to 1977, he was chairman of the physics department. In 1972, he was appointed the Henry Ford II Professor of Physics and was in this position until 1993.