Quantum mechanics is a well-established theory which allows to predict the behavior of physical systems. However, there is an ongoing debate on the interpretations of some of its most relevant feature, including superposition, wave-particle duality, or entanglement. A large research effort has been devoted in the last decades to develop quantitative tests able to discriminate purely quantum with classical behaviours. Notable examples are provided by Bell¿s inequalities, which aim at studying the presence of correlations incompatible with locality and realistic assumptions. Other examples are provided by the Wheeler delayed choice experiment, which aim at studying the counterintuitive feature of wave-particle duality. In this framework, recent results have been reported showing how to combine entanglement with this property. Furthermore, very recently, causal structures have been identified as significant tool to identify new tests for the discrimination of quantum and classical behaviour in a mutlipartite system.
This project aims at performing an experimental study to combine entanglement in a Bell scenario with a delayed choice structure. Modeling of causal structures will be employed to define suitable inequalities to be tested. The funding will be employed to buy a certified quantum random number generator from ID Quantique, which is currently not available in the Quantum Information Lab. The quantum random number generator will be employed throughout the test to avoid predetermined choice of the causal structures.
The innovative aspect of this proposal comes from its insertion at the multidisciplinary interface between causal inference, device-independence, quantum information and experimental quantum optics. It was not until recently that the interconnection between the theory of causality/device-independence and quantum information has been properly established. From the theoretical side, due to the lack of a solid mathematical framework related to the causal modeling in the delayed choice scenario (that only in the last three years has started to be formulated), most research in the field has only marginally considered what are the advantages and limitations imposed by causality on our ability to process information using quantum resources. Experimentally, due to the high technical difficulty of implementing complex quantum networks beyond the paradigmatic Bell structure, very few (but always high-profile) research has been pursued. Thus, now it is the best time to consolidate this research, to continue to work on the foundations but also pursue for practically and experimentally feasible relevant directions.
As stated before, the Quantum Information Lab has already performed different experiments in this topic and hence we are very confident about our possibilities to increase and improve the level of research within this line.
On the research side, we can begin to systematically manipulate more of the enhancement provided by quantum systems to guide future quantum technologies making use of it. This new way of thinking will allow us to explore completely new regimes of quantum information processing. In particular, those making use of complex patterns of causal interactions that not only will unveil new fundamental features of quantum theory but also lead to novel protocols and new experimental implementations. If successful, these results will certainly open a new research venue and likely provide fundamental building blocks for our understanding of quantum causality and its use as a potential resource in quantum information processing.