Quantum to classical transitions in causal relations

Abstract

The landscape of causal relations that can hold among a set of systems in quantum theory is richer than in classical physics. In particular, a pair of time-ordered systems can be related as cause and effect or as the effects of a common cause, and each of these causal mechanisms can be coherent or not. Furthermore, one can combine these mechanisms in different ways: by probabilistically realizing either one or the other or by having both act simultaneously (termed a physical mixture). In the latter case, it is possible for the two mechanisms to be combined quantum-coherently. Previous work has shown how to experimentally realize one example of each class of possible causal relations. Here, we make a theoretical and experimental study of the transitions between these classes. In particular, for each of the two distinct types of coherence that can exist in mixtures of common-cause and cause-effect relations--coherence in the individual causal pathways and coherence in the way the causal relations are combined--we determine how it degrades under noise and we confirm these expectations in a quantum-optical experiment.Comment: 9 pages, 6 figure

Timelessness Strictly inside the Quantum Realm

“…With an identified order of classical records, their potential causal dependences are constrained; (the record of) an effect can never precede (the record of) its cause. This cannot simply/fully be transferred to inside QM, which allows more complex and entangled connections, alas, without producing durable records inside [ 122 , 156 ].…”

“…With an identified order of classical records, their potential causal dependences are constrained; (the record of) an effect can never precede (the record of) its cause. This cannot simply be transferred to inside QM, which allows more complex and entangled connections, alas, without producing durable records inside [105,130]. Taking into account that it is only classic records, which at the end can be unambiguously ordered in one or more light cones, findings where the future seems to influence or determine the past, thus do not disturb everyday reality.…”

Study of the optical response and coherence of a quadratically coupled optomechanical system

“…Fundamentally, quantum coherence can be described as a basic element of quantum entanglement [19-21, 21, 22] and discord [23,24]. The study of coherence appears to be useful in understanding the boundaries between classical and quantum physics [25,26]. Moreover, in the past few years, the fundamental role of quantum coherence has been investigated by considering it as physical resource [27][28][29][30].…”

Classical evolution in quantum systems

“…Similarly to the concept of nonlocallity, the violation of this and related inequalities enables us to uncover temporal quantum correlations [12,13]. The question of causality in quantum physics is another closely related concept, which also addresses the temporal quantumness of a process [14][15][16][17]. Furthermore, noncommutative features in the mathematical formulation of quantum physics lead to quantum effects based on timeordering [18,19].…”