There were quite a few defectors from the West that went over to the Soviet Union. The USSR treated western defectors with a very high degree of importance and lavishness, giving them access to the same privileges as the important Party members.

A few of the more important or influential defectors:

George Koval: Koval was a spy that stole nuclear secrets from the US and brought them to the Soviet Union in 1948. He was an American who became a Soviet spy, and he is one of the prime infiltrators of the Manhattan Project. Koval's stolen secrets were extremely important for the development of the USSR's own nuclear weaponry projects. Koval was drafted into the US Army in 1943 and worked at atomic research labs, and the secrets he stole included intimately detailed research on how to produce and process polonium, plutonium, and uranium. Koval left the US in 1948 and never returned, and in 1949 the USSR detonated its first nuclear weapon. In prime Putin fashion, never missing an opportunity to flip the bird to the US, he ordered Koval to be named a Hero of Russia in 2007.

Alfred Sarant and Joel Barr were two more atomic spies that defected to the USSR (Sarant defected to the People's Republic of Poland and then to the USSR while Barr defected to Czechoslovakia and then to the USSR) and they were both influential in the production of the Soviets' first hydrogen bomb. Sarant and Barr both defected in 1951, and worked inside the Rosenberg Spy Ring.

Julius and Ethel Rosenberg: These two were an interesting case. They're two of the few cases of US citizens being outright convicted and executed for the formal charge of treason. The US Constitution has very strict definitions of what constitutes treason, and the Rosenbergs were convicted along those lines. They were charged with high treason and conspiracy to commit espionage relating to passing on information and secrets about the US hydrogen bomb projects. While there is little doubt that Julius was definitely working for the Soviets (he led the spy ring and coordinated the activities and information handling), his wife was executed for the same charges. Ethel's charge, conviction, and execution were most likely the by-product of rampant McCarthyism and the Second Red Scare, as there was little evidence that Ethel was involved at all. She was convicted on the evidence given by her brother, who later stated that he perjured himself to protect his own wife, who was the actual typist of the classified documents that Ethel's brother stole. The prosecution encouraged her brother to give that testimony, and another convicted spy working with the Rosenbergs (Morton Sobell, served 17 years of a 30 year espionage sentence) outright admitted that he (Sobell) was a spy, that Julius was the ringleader, and that Ethel had nothing at all to do with it.

Both Rosenbergs were executed on the same day by electrocution in an electric chair in capital punishment ward of Sing Sing Correctional Facility in New York State.