In the last decades, an increasing interest in quantum technologies has brought to the flourishing of many theoretical and experimental protocols which exploit quantum resources. Indeed, some fundamental properties of quantum mechanics, e.g. entanglement and superposition principle, are widely believed to allow improvements in several fields, as for example in communication, cryptographic protocols, randomness extraction and quantum computation. However, all these tasks require the use of genuinely quantum states and measurements. Hence, the necessity to study protocols to discern devices that are truly exploiting quantum resources from those that do not. Furthermore, it would be desirable to perform such a distinction, without knowing in detail the internal functioning of the adopted device, which can be hard or impossible to verify. Such an approach is known as "device-independent".
In this context, causal inference, or the quantification of causal relationships among phenomena, turns out to be extremely useful. Indeed, causal inference allows to verify whether a device is correctly implementing the desired causal structure, i.e. a scheme of cause-effect relations among variables, just by observing its input/output statistics (i.e. device-independently). Then, the discrepancies between classical and quantum causal predictions can be used to detect the presence of quantum effects, within a given causal structure.
The aim of this project is precisely to find such discrepancies and exploit them to detect the presence of a quantum signature within a given causal structure, thus allowing a deeper knowledge of quantum processes and, consequently, a stronger control on future applications in the field of device-independent certification protocols for quantum communication and computation.
The new tools offered by this research are multifold. From a fundamental point of view, the proposed work would offer new insights in the field of quantum causality, since it would experimentally demonstrate that the presence of unobservable quantum variables in a given causal structure modifies the nature of cause-effect relationships within the structure, on a deeper level than previously thought. Moreover, it provides a new technique to detect quantum behaviours. Indeed, the quantum effects that can be observed in the proposed work could not be revealed before, since no instrumental inequality is known which can be violated by quantum correlations, in the settings that we consider here, namely all binary variables.
These effects, thus, represent a very useful tool to reveal the presence of non-classical behaviors and deepen the current knowledge of the impact of quantum mechanics on the classical notions of cause and effect. This is the reason why this kind of new tools are of great interest for the theoretical study and the implementation of device-independent protocols to certify the functioning of quantum devices. Moreover, many quantum information scenarios are based on the instrumental causal structure: one significant example coming directly from the Quantum Information Lab, where it was developed a protocol which exploits the instrumental scenario to generate certified random number exploiting quantum resources [1].
From an experimental point of view this apparatus is a novelty itself, since in the same experimental setup several different elements work together, with the aim to switch between two different causal scenarios on-demand, and without changing the experimental conditions or any part of the apparatus. In particular, the multipurpose logical gate device which can change its logical function according to the random number is a fundamental ingredient that will make this apparatus capable to realize two different causal scenarios, just by combining two inputs TTL signals in different ways, depending on the causal structure which must be implemented.
[1] Agresti, Iris, et al. "Experimental device-independent certified randomness generation with an instrumental causal structure." Communications Physics 3.1 (2020): 1-7