[en] A new measure of information in quantum mechanics is proposed which takes into account that for quantum systems, the only feature known before an experiment is performed are the probabilities for various events to occur. The sum of the individual measures of information for mutually complementary observations is invariant under the choice of the particular set of complementary observations and conserved in time if there is no information exchange with an environment unitary transformation. This operational quantum information invariant results in k bits of information for a system consisting of $k$ qubits. For a composite system, maximal entanglement results if the total information carried by the system is exhausted in specifying joint properties, with no individual qubit carrying any information on its own. We interpret our results as implying that information is the most fundamental notion in quantum mechanics. Based on this observation we suggest ideas for a foundational principle for quantum theory. It is proposed here that the foundational principle for quantum theory may be identified through the assumption that the most elementary system carries one bit of information only. Therefore an elementary system can only give a definite answer in one specific measurement. The irreducible randomness of individual outcomes in other measurements and quantum complementarity are then necessary consequences. The most natural function between probabilities for outcomes to occur and the experimental parameters, consistent with the foundational principle proposed, is the well-known sinusoidal dependence. (author)