Quantum mechanics is considered conceptually to be a demanding topic due to the classically counterintuitive nature of its basic concepts and entities. A study is carried out where a picture of students' conceptual understanding of quantum entities (quantum `particles') is constructed. This is done by analysing students' qualitative problem solving. The problem used as a research tool in this study is based on an example of the gradual formation of interference pattern for either low intensity light or electron beam passing through a two-slit system.