Delayed Choice and Quantum Causal Coherence

Quantum information technology is one of the fastest crowing sectors within information technology. This paper is a conjecture about expected boundary conditions of nonlocal effects and shows the consequences of definite or indefinite causal order of entangled quantum events leading to causality. It focuses on nonlocal causality in relativistic systems, and introduces the notion of quantum causal coherence as a nonlocal property of relativistic causality, reflecting the relation of entanglement and the principle of least action. It is shown, that the principle of least action is the self-consistency mechanism of nonlocal causality in relativistic scenarios, especially in timelike scenarios of entangled quanta. From the view of relativistic scenarios, this paper examines the possibility of causal loops in quantum causal relations and investigates quantum causal relations in well established experiments.