He later held an assistant position with Werner Heisenberg in Leipzig between 1929 and 1934 and lectured in Kraków between 1934 and 1938. Due to the rising Antisemitism in Poland he emigrated to Denmark and arrived in Copenhagen in March 1938 where Niels Bohr admitted him to his institute. From 1940, after Hendrik Anthony Kramers (from 1916) and Léon Rosenfeld (from 1934), he was Niels Bohr 's personal assistant for almost fifteen years, whom he assisted even into the early sixties after both returned to Copenhagen in 1945.