It was also in 1926 that Klein was appointed as docent at Lund University and became, for the next five years, Bohr 's closest collaborator both on correspondence and complimentarity, and apparently contributed to the development of the uncertainty principle, as Heisenberg recalled:
Ito ay din sa 1926 na Klein ay hinirang bilang guro at Lund University at naging, para sa susunod na limang taon, Bohr 's closest tagatulong ang pareho sa pagsusulatan at complimentarity, at apparently ambag sa pag-unlad ng kawalang-katiyakan batayan, bilang Heisenberg-alaala: