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Sin-Itiro Tomonaga or Shinichirō Tomonaga (朝永 振一郎 Tomonaga Shin'ichirō, March 31, 1906 – July 8, 1979) was a Japanese physicist, influential in the development of quantum electrodynamics, work for which he was jointly awarded the Nobel Prize in Physics in 1965 along with Richard Feynman and Julian Schwinger.

Tomonoga was born in Tokyo in 1906. He was the second child and eldest boy of a Japanese philosopher, Sanjūrō Tomonaga. He entered the Kyoto Imperial University in 1926. Hideki Yukawa, also a Nobel Prize winner, was one of his classmates during undergraduate school. During graduate school at the same university, he worked as an assistant in the university for three years. After graduate school, he joined Nishina's group in Riken. In 1937, while working in Leipzig, he collaborated with the research group of Werner Heisenberg. Two years later, he returned to Japan due to the outbreak of the Second World War, but finished his doctoral degree on the study of nuclear materials with his thesis on work he had done while in Leipzig.

In Japan, he was appointed to a professorship in the Tokyo University of Education (a forerunner of Tsukuba University). During the war he studied the magnetron, meson theory, and his "super-many-time" theory. In 1948, he and his students re-examined a 1939 paper by Sidney Dancoff that attempted, but failed, to show that the infinite quantities that arise in QED can be canceled with each other. Tomonaga applied his super-many-time theory and a relativistic method based on the non-relativistic method of Pauli and Fierz to greatly speed up and clarify the calculations. Then he and his students found that Dancoff had overlooked one term in the perturbation series. With this term, the theory gave finite results; thus Tomonaga discovered the renormalization method independently of Julian Schwinger and calculated physical quantities such as the Lamb shift at the same time.

In the next year, he was invited by Robert Oppenheimer to work at the Institute for Advanced Study in Princeton Township. He studied a many-body problem on the collective oscillations of a quantum-mechanical system. In the following year, he returned to Japan and proposed the Tomonaga-Luttinger liquid. In 1965, he was awarded the Nobel Prize in Physics (alongside Julian Schwinger and Richard P. Feynman) for the study of QED, specifically for the discovery of the renormalization method. He died in Tokyo in 1979.　

He performed a rakugo in German during a campus festival at the University of Tokyo, demonstrating his broad cultural interests.

He honored Yoshio Nishina as his teacher in physics throughout his life. Now his tomb is next to that of Yoshio Nishina and the epitaph on his tombstone reads "He rests within the hearing of his teacher".

• Schweber, Sylvan S., 1994. QED and the men who made it : Dyson, Feynman, Schwinger, and Tomonaga. Princeton Univ. Press.
• Tomonaga's Nobel Prize Lecture

• Nobel Prize biography
• Shinichiro Tomonaga
• fundamental work in quantum electrodynamics, with deep-ploughing consequences for the physics of elementary particles.