Evidence of entanglement, or: Is Schr\"{o}dinger's cat really entangled?

Abstract

Quantum effects and, in particular, entanglement are by now widely recognized in all areas of physics and related fields. However, we feel that the precise notion of entanglement---though mathematically well-defined---still generates confusion even in the academic environment. Our aim in this article is to point out and clarify some aspects of the notion of entanglement, in particular with respect to observable quantities. We do this by illustrating common misconceptions with Schr\"{o}dinger's cat paradox, explain them in mathematical terms and provide examples where misunderstandings arise and how they can be solved.