via wikipedia
5 years. him and his family.
"During the Second Red Scare in the 1950s, the United States federal government accused him of communist sympathies. In 1950, despite protests by his colleagues and without any evidence of the allegations, he was stripped of his security clearance. He was given a deferred deportation order by the Immigration and Naturalization Service, and for the following five years, he and his family were subjected to partial house arrest and government surveillance in an effort to gradually make his technical knowledge obsolete.
After spending five years under house arrest, he was released in 1955 in exchange for the repatriation of American pilots who had been captured during the Korean War. He left the United States in September 1955 on the American President Lines passenger liner SS President Cleveland, arriving in mainland China via Hong Kong."