Passage: He moved to Institute for Theoretical Physics (later the Niels Bohr Institute) in Copenhagen on the Sheldon Traveling Fellowship from Harvard, and remained in Denmark. In 1953 he was appointed staff member in CERN's Theoretical Study Group, which was based in Copenhagen, a position he held until he became professor at the newly formed Nordic Institute for Theoretical Physics (Nordita) in 1957. He was a visiting professor at the University of California, Berkeley in Spring 1959. In 1971 he became a naturalized Danish citizen.

Question: When did he become a citizen of Denmark?
1971