For university students studying physics, quantum mechanics is considered an extremely difficult subject, but one which must be taught increasingly early in their careers. We report a preliminary project which used a phenomenographic approach to explore the ways in which a small number of fundamental ideas are conceptualized by students who have been judged to have successfully mastered the material. The results suggest that the mental models used by these students are technically advanced but structurally unsophisticated. Whether a change in current teaching practices might lead to better conceptualization is a question that needs further exploration.