Alan Turing

Alan Turing was an English mathematician. During World War II, he was a code-breaker with whom both Rachel Jensen and Toshiko Sato's grandparents worked. (PROSE: Who Killed Kennedy, TV: Greeks Bearing Gifts) During the war he designed a Rift Predictor for Torchwood Three in 1941 (PROSE: The Twilight Streets) and helped the Eighth Doctor crack an alien code in 1944. (PROSE: The Turing Test)

He committed suicide in the mid-1950s, after having been hounded out of academia by Professor Jeffrey Broderick. (AUDIO: Artificial Intelligence)

Duplicates and alternatives
The Eleventh Doctor and Amy Pond met a robot duplicate of Alan Turing in the Museum of Lost Opportunities. Although appearing to possess many of Turing's memories and attitudes, the robot did not remember Turing's previous encounter with the Doctor. (COMIC: The Child of Time)

In an alternative timeline, Turing was the inventor of the Turing Shroud, which served as a schematic of an early computer, only to be arrested on charges of sexual deviancy by the Star Chamber. He was kept locked up in the Tower of London for decades until he was executed by this timeline's version of Sabbath, who had been manipulated into believing that Turing's death would separate his world from the rest of the universe as alternate timelines began to collapse, but Turing's death actually accelerated the damage due to his status as the main focal point of this word's divergence from other histories. (PROSE: The Domino Effect)