In this course we are interested in understanding the quantum physics of combined systems, i.e., of systems that consists of more than one party at different locations. It might seem strange, why we are especially interested in this situation, but as we will see later on, there are fundamental principles in quantum physics that only become apparent when considering at least two separated parties. We will call two parties separated, when any action on the first party will not have any effect on outcomes of measurements made by the second party. We will see that the study of such localized systems will allow us to answer one of the fundamental questions of quantum physics, namely whether the randomness encountered in a measurement is based on ignorance or constitutes a fundamental property of nature.
We had discussed the principle of complementarity in the first Quanth-course: There are certain quantities that are not jointly measurable, i.e., it is not possible to measure one without disturbing the results of successive measurement of the other (Footnote: for a more detailed description see…). One possible explanation for this behaviour could be that any measurement device needs a small interaction to perform the measurement. As quantum objects tend to be quite ‘delicate’ things, his interaction will always have a non negligible impact on successive measurement results. If this was true, one could imagine quantum physics to be similar to classical physics with some additional rules for interactions when performing measurements. We will see in this course, that no such model of physics can be found.
To show this, we will present the Bell theorem, which is one of the most remarkable findings of physics. The Bell theorem does actually not only apply to quantum physics, but rather to any theory which can be cast in the form “classical physics + some extra rules”. It will be a consequence of the theorem that no such theory can make the same predictions for certain experiments as quantum physics. With this, we can experimentally test predictions from quantum physics against any other theory. Until now, all performed experiments have shown perfect agreement with the predications made within quantum physics. To reach this goal we will start with a very general approach, first presenting a story to derive the Bell theorem before discussing its consequences.