Quantum information theory
QIT is about information and probabilities and observables, and how they relate to each other within the understanding and explanations provided by quantum mechanics.
In addition to the the empirical and scientific theoretical foundation, quantum mechanics is what you would arise if the analysis started from probability theory, and then generalised it so that the numbers called “probabilities” can be negative numbers as well as numbers from zero to one. As such, the theory could possibly have been invented by mathematicians in the nineteenth century without any input from experiment.
The Qbit is the fundamental component of quantum information. It can be understood through an analysis of a 2-level system.