Kai Manne Börje Siegbahn (20 April 1918 – 20 July 2007) was a Swedish physicist.Siegbahn was born in Lund, Sweden, son of Manne Siegbahn the 1924 physics Nobel Prize winner. Siegbahn earned his doctorate at the University of Stockholm in 1944. He was professor at the Royal Institute of Technology 1951–1954, and then professor of experimental physics at Uppsala University 1954–1984, which was the same chair his father had held.