Non-locality is one of the most characteristic trait of Quantum Mechanics, and it is linked to the conceptually counter-intuitive ability of some objects (when adequately prepared) to instantaneously know about each other’s state, no matter how much space-separated they are.
At the development stage of Quantum Mechanics, non-locality had been an argument of intense debate among scientists due to its consequences that seemed to contradict physicists common sense.
The scientific debate on the argument faced a turning point with the publication in 1964 of the landmark paper by J. S. Bell On the Einstein-Podolsky-Rosen Paradox, which ended the discussion whether or not non-locality was a true aspect of physical reality, raising it to one of the fundamental aspect of our
comprehension of the physical world.
The scientific interest for non-locality has recently substantially increased due to its striking applications in Quantum Information Theory.
The major innovative traits of Quantum Information Theory, compared to Classical Information Theory, arise in fact from the usage of non-locality as a resource for information manipulation and transmission.
The aim of this thesis is to investigate experimentally the non-local properties of a quantum system.
This work is one of the first to prove that it is possible to share the non-local correlations of a two-photon entangled singlet state among three observers.
The sharing of non-local correlations was possible through the employment of a weak measurement scheme, used to entangle the polarization degree of freedom of photons with a path degree of freedom used as ancilla for the weak measurement.
In this thesis we briefly review all the theoretical tools necessary to understand the work (Quantum Mechanics, Optics, Quantum Optics) and we discuss in depth all the aspects of our work.
We demonstrate theoretically how non-locality sharing is achievable, showing that it is possible to simultaneously violate two CHSH inequalities among three observers sharing a two-photon singlet state.
We describe precisely our setup and its implementation.
We finally present the obtained results, which confirm our theoretical model and expectations.
The achievement of double violation and the realization of a simple weak measurement scheme may have important applications for Quantum Random Number Generation or for Quantum Key Distribution exploiting weak measurements.
Non-locality is one of the most characteristic trait of Quantum Mechanics, and it is linked to the conceptually counter-intuitive ability of some objects (when adequately prepared) to instantaneously know about each other’s state, no matter how much space-separated they are.
At the development stage of Quantum Mechanics, non-locality had been an argument of intense debate among scientists due to its consequences that seemed to contradict physicists common sense.
The scientific debate on the argument faced a turning point with the publication in 1964 of the landmark paper by J. S. Bell On the Einstein-Podolsky-Rosen Paradox, which ended the discussion whether or not non-locality was a true aspect of physical reality, raising it to one of the fundamental aspect of our
comprehension of the physical world.
The scientific interest for non-locality has recently substantially increased due to its striking applications in Quantum Information Theory.
The major innovative traits of Quantum Information Theory, compared to Classical Information Theory, arise in fact from the usage of non-locality as a resource for information manipulation and transmission.
The aim of this thesis is to investigate experimentally the non-local properties of a quantum system.
This work is one of the first to prove that it is possible to share the non-local correlations of a two-photon entangled singlet state among three observers.
The sharing of non-local correlations was possible through the employment of a weak measurement scheme, used to entangle the polarization degree of freedom of photons with a path degree of freedom used as ancilla for the weak measurement.
In this thesis we briefly review all the theoretical tools necessary to understand the work (Quantum Mechanics, Optics, Quantum Optics) and we discuss in depth all the aspects of our work.
We demonstrate theoretically how non-locality sharing is achievable, showing that it is possible to simultaneously violate two CHSH inequalities among three observers sharing a two-photon singlet state.
We describe precisely our setup and its implementation.
We finally present the obtained results, which confirm our theoretical model and expectations.
The achievement of double violation and the realization of a simple weak measurement scheme may have important applications for Quantum Random Number Generation or for Quantum Key Distribution exploiting weak measurements.
Abstract Bell non-locality plays a fundamental role in quantum theory. Numerous tests of the Bell inequality have been reported as the ground-breaking discovery of the Bell theorem. Up to now, however, most discussions of the Bell scenario have focused on a single pair of entangled particles distributed to only two separated observers. Recently, it has been shown surprisingly that multiple observers can share the non-locality from an entangled pair using the method of weak measurement without post-selection [Phys. Rev. Lett. 114 , 250401 (2015)]. Here we report an observation of double CHSH-Bell inequality violations for a single pair of entangled photons with strength continuous-tunable optimal weak measurement in a photonic system. Our results shed new light on the interplay between non-locality and quantum measurements and our design of weak measurement protocol may also be significant for important applications such as unbounded randomness certification and quantum steering.
Bell nonlocality plays a fundamental role in quantum theory. Numerous tests of the Bell inequality have been reported since the ground-breaking discovery of the Bell theorem.Up to now, however, most discussions of the Bell scenario have focused on a single pair of entangled particles distributed to only two separated observers. Recently, it has been shown surprisingly that multiple observers can share the nonlocality present in a single particle from an entangled pair using the method of weak measurements [Phys. Rev. Lett. {\bf 114}, 250401 (2015)]. Here we report an observation of double CHSH-Bell inequality violations for a single pair of entangled photons with strength continuous-tunable optimal weak measurements in photonic system for the first time. Our results not only shed new light on the interplay between nonlocality and quantum measurements but may also be significant for important applications such as unbounded randomness certification and quantum steering.
It is one of the most remarkable features of quantum physics that measurements on spatially separated systems cannot always be described by a locally causal theory. In such a theory, the outcomes of local measurements are determined in advance solely by some unknown (or hidden) variables and the choice of local measurements. Correlations that are allowed within the framework of a locally causal theory are termed classical. Typically, the fact that quantum mechanics does not always result in classical correlations is revealed by the violation of Bell inequalities, which are constraints that have to be satisfied by any classical correlations. It has been known for a long time that entanglement is necessary to demonstrate nonclassical correlations, and hence a Bell inequality violation. However, since some entangled quantum states are known to admit explicit locally causal models, the exact role of entanglement in Bell inequality violation has remained obscure. This thesis provides both a comprehensive review on these issues as well as a report on new discoveries made to clarify the relationship between entanglement and Bell inequality violation. In particular, within the framework of a standard Bell experiment, i.e., a Bell inequality test that is directly performed on a single copy of a quantum state r, we have derived two algorithms to determine, respectively, a lower bound and an upper bound on the strength of correlations that r can offer for any given Bell inequality. Both of these algorithms make use convex optimization techniques in the form of a semidefinite program. By examples, we show that these algorithms can often be used in tandem, in conjunction with convexity arguments, to determine if a quantum state can offer nonclassical correlations and hence violates a given Bell inequality. On the other hand, since a standard Bell experiment typically involves measurements over many copies of the quantum systems, we have also investigated the possibility of enhancing the strength of nonclassical correlation by, instead, performing collective measurements on multiple copies of the quantum systems. Our findings show that even without postselection, such joint measurements may also lead to stronger nonclassical correlations, and hence a better Bell inequality violation. Meanwhile, previous studies have indicated that entangled state admitting locally causal models may still lead to observable nonclassical correlations if, prior to a standard Bell experiment, the state is subjected to some appropriate local preprocessing. This phenomenon of hidden nonlocality was discovered more than a decade ago, but to date, it is still not known if all entangled states can demonstrate nonclassical correlations through these more sophisticated Bell experiments. A key result in this thesis then consists of showing that for all bipartite entangled states, observable nonclassical correlations, in the form of a Bell-CHSH inequality violation, can indeed be derived if we allow both local preprocessing and the usage of shared ancillary state which by itself does not violate the Bell-CHSH inequality. This establishes a kind of equivalence between bipartite entanglement and states that cannot be simulated by classical correlations. In summary, for a standard Bell experiment where no local preprocessing on a quantum state r is allowed, we have provided two algorithms that can be used in tandem to determine if r can be simulated by a locally causal theory, whereas in the scenario where local preprocessing is allowed, we have demonstrated that bipartite entangled states are precisely those which cannot always be simulated classically.
Most physicists agree that the Einstein–Podolsky–Rosen–Bell paradox exemplifies much of the strange behavior of quantum mechanics, but argument persists about what assumptions underlie the paradox. To clarify what the debate is about, we employ a simple and well-known thought experiment involving two correlated photons to help us focus on the logical assumptions needed to construct the EPR and Bell arguments. The view presented in this paper is that the minimal assumptions behind Bell’s inequality are locality and counterfactual definiteness but not scientific realism, determinism, or hidden variables as are often suggested. We further examine the resulting constraints on physical theory with an illustration from the many-worlds interpretation of quantum mechanics—an interpretation that we argue is deterministic, local, and realist but that nonetheless violates the Bell inequality.
Starting from the late 60’s many experiments have been performed to verify the violation Bell’s inequality by Einstein-Podolsky-Rosen (EPR) type correlations. The idea of these experiments being that: (i) Bell’s inequality is a consequence of locality, hence its experimental violation is an indication of non locality; (ii) this violation is a typical quantum phenomenon because any classical system making local choices (either deterministic or random) will produce correlations satisfying this inequality. Both statements (i) and (ii) have been criticized by quantum probability on theoretical grounds (not discussed in the present paper) and the experiment discussed below has been devised to support these theoretical arguments. We emphasize that the goal of our experiment is not to reproduce classically the EPR correlations but to prove that there exist perfectly local classical dynamical systems violating Bell’s inequality. The conclusions of the present experiment are: no contradiction between quantum theory and locality can be deduced from the violation of Bell’s inequality. The Copenhagen interpretation of quantum theory becomes quite reasonable and not metaphyisic if interpreted at the light of the chameleon effect. One can realize quantum entanglement by classical computers. In section (7) we prove that our experiment also provides a classical analogue of the type of logical (i.e. independent of statistics) incompatibilities pointed out by Greenberger, Home and Zeilinger. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
"In 1935, Einstein, Podolsky, and Rosen questioned whether quantum mechanics can be complete, as it seemingly does not adhere to a natural view of reality: local realism, which is the notion that an event can only be influenced by events in the past lightcone, and can only influence events in the future lightcone. This question sparked a philosophical debate that lasted for three decades, until John Bell demonstrated that not only are quantum mechanics and local realism philosophically incompatible, but they predict different statistical results for an appropriate set of measurements on entangled particles, which changed the debate to a scientific discussion. Since then, Bell inequality violations have occurred in a plethora of systems, hinting that local realism is indeed wrong. However, every experiment had imperfections that complicated the interpretation --- the experiments had so-called ""loopholes"" which allowed local realism to persist. In this manuscript, we present our work in using optimized sources of entangled photons to perform the long-sought loophole-free Bell test. This landmark experiment invalidates local realism to the best that science will allow. Beyond answering questions on reality, these Bell tests have a important application in generating provably-secure private random numbers, which then can be used as a seed for cryptographic applications. Not only do we demonstrate that nonlocality must exist, but we begin an experimental exploration in an attempt to understand and quantify this nonlocality. We do so by considering all theories that obey no-signaling (or relativistic causality). In our experiments, we observe the counter-intuitive feature of measuring more nonlocality with less entangled states. We also place a bound on the predictive power of any theory that obeys relativistic causality. And finally, we are able to measure quantum correlations only attainable through complex qubits. This work merely begins to probe the quantum boundary, beginning a journey that may someday find evidence of a beyond-quantum theory."