Louis de Broglie encouraged Watanabe to study thermodynamics and wave mechanics.

In 1939, at the beginning of World War II, he left Germany and stayed with Niels Bohr for a time.

His argument that quantum mechanics is time-asymmetric (irreversible; non-invariant under the time reversal transformation) is repeated in a number of his papers (1955; 1965; 1966; 1972).

Watanabe's work was later rediscovered by Yakir Aharonov, Peter Bergmann and Joel Lebowitz in 1964, who later renamed it the Two-state vector formalism (TSVF).

[6] In 1956, he became a researcher at the IBM Watson Laboratory and started to build his own information theory based on quantum mechanics.

His elder brother, Takeshi Watanabe, was Vice Minister of Finance for International Affairs and director general of Asia Development Bank.

His wife, Dorothea Dauer Watanabe, was a professor of German (language and literature) at the University of Hawaii.