Quantum theory way to describe the operation of matter and energy at a quantum level, where energy and matter behaved as both particles and waves. Related topics, such as interspatial parasites and spatial scissions, were studied by quantum theorists.
When Icheb began to pursue his studies for Starfleet Academy, Seven of Nine produced a list for him of crewmen that could help him study for the Starfleet Academy entrance exam, including, Lieutenant B'Elanna Torres, who was "extremely well-versed in warp mechanics," and Ensign Harry Kim, who she said, "claims to have aced the quantum theory section." (VOY: "Imperfection")