Causality and no-go theorems

Abstract

The aim of the paper is to investigate the role played by causality, and more specifically the no-signaling condition, in the assessment of the quantum theory. To this end, we discuss why it is important that even a non-relativistic theory such as Quantum Mechanics doesn’t imply a violation of this condition. Then, we use this argument to prove an original result stating that the destructive behaviour of the measurement process on the entanglement properties of quantum systems is a necessary and unavoidable feature of the quantum theory. Finally, we critically review the no-cloning theorem. The original formulation of the theorem states that a linear quantum cloning machine, designed in order to successfully clone states that coincide with appropriate basis vectors, fails to copy states that are a non-trivial superposition of those basis vectors; we will furthermore prove that such a linear cloning device, even with the hypothesis that it can only clone basis vectors successfully, may provide a violation of the no-signaling condition and therefore cannot exist.

Keywords

• quantum mechanics
• entanglement
• quantum nonlocality
• quantum cloning