Karplus continued his work at the highest level in theoretical physics for more than 10 years, at Harvard from 1950 to 1954 and then at the University of California, Berkeley, publishing 50 research papers, mostly in QED but also in other areas of physics, including the Hall effect, Van Allen radiation, and cosmic rays. He also thoroughly enjoyed experimental work, investigating the chemistry of Land Camera instant pictures and setting up an experimental germanium purification assembly line for transistors. His paper, with J. M. Luttinger, Hall Effect in Ferromagnetics has over 1100 journal citations.