After returning from military service in World War I, he worked at the telegraph research division of the German Post in Berlin as a co-worker of Karl Willy Wagner, and, from 1921, he was lead engineer at the central laboratory of Siemens & Halske AG in the same city.

In 1928 he became full professor of general and theoretical electrical engineering at the Technische Hochschule in Danzig, and later held the same position in Berlin.

[1] Küpfmüller was appointed as director of communication technology Research & Development at the Siemens-Wernerwerk for telegraphy.

From 1952 until his retirement in 1963, he held the chair for general communications engineering at Technische Hochschule Darmstadt.

About 1928, he did the same analysis that Harry Nyquist did, to show that not more than 2B independent pulses per second could be put through a channel of bandwidth B.