Violazione della disuguaglianza CHSH su una coppia di fotoni intrecciati usando le misure deboli

Quantum Mechanics is one of the cornerstones of modern Physics, with its predictions verified by countless experiments and its properties giving rise to huge technological advantages in fields. Nevertheless, the debate on its very foundations is still ongoing, since some peculiar traits of Quantum Mechanics seem to be in contradiction with physical reality at a macroscopic scale. This is the case, for example, of entanglement at the center of a famous paradox proposed in 1935 by A. Einstein, B. Podolski and N. Rosen. In their work, these three scientists suggested that Quantum Mechanics might be not a complete theory, since it cannot describe with arbitrary precision every “element of reality” of an entangled system. This reasoning brought the idea that Quantum Mechanics could be a statistical approximation of a class of deterministic classical theories dubbed Local Hidden Variable Theories, in which the observables of a physical system are fixed by some “hidden” variables inaccessible to the experimenter. In 1964, J. S. Bell found out that any Local Hidden Variable Theory must satisfy certain inequalities (called Bell Inequalities) that can be violated by entangled states in Quantum Mechanics. Because of the experimental difficulty of these tests, the first Bell inequality violation, with a space-like separation between the measurements performed on the entangled particles, was obtained only in 1982 by A. Aspect and colleagues. Unfortunately, that result was afflicted by some loopholes, compromising (to some extent) the validity of the obtained violation. The same holds for the Bell Inequality tests carried un- til 2015, when B. Hensen et al., M. Giustina et al. and by L. K. Shalm et al. independently achieved the first loophole-free Bell inequality violations. Anyway, these experiments are characterized by the fact that none of them can measure the full inequality on each single physical system, needing to repeat the experiment on multiple copies of the system to achieve the violation. In fact, the whole inequality violation would require measuring incompatible observables, a task impossible in the usual (projective) quantum measurement framework because of the wave function collapse. These constraints can be partially relaxed by means of a novel measurement protocol called “weak measurement”, a form of indirect measurement exploiting a weak coupling between measured system and measuring device, preventing the wave function collapse while inducing just a faint decoherence on the system state. This allows performing joint and sequential weak measurements of non-commuting observables on each entangled pair, being able to test the whole Bell inequality with each entangled pair produced. This is the goal of this work: the experimental violation of the so-called Clauser-Horne-Shimony-Holt inequality obtained by measuring the full inequality with each photon pair generated by our setup, a conceptual paradigm shift with respect to all other Bell inequality violation experiments run so far. To do that, we designed a high-visibility entangled photon pair source relied on the Sagnac interferometer, in which the photons are prepared in a polarization singlet state are sent to two different branches, where four birefringence-based weak measurements (two in sequence for each branch) occur. Finally, the photons are detected by two synchronized 32 × 32 arrays of Single-Photon Avalanche Diodes, allowing to extract for each pair the inequality value.