The device-independent formalism is a set of tools to analyze experimental data and infer properties about systems, while avoiding almost any assumption about the functioning of devices. It has found applications both in fundamental and applied physics: some examples are the characterization of quantum nonlocality and information protocols for secure cryptography or randomness generation. This thesis contains novel results on these topics and also new applications such as deviceindependent test for dimensionality. After an introduction to the field, the thesis is divided in four parts. In the first we study device-independent tests for classical and quantum dimensionality. We investigate a scenario with a source and a measurement device. The goal is to infer, solely from the measurement statistics, the dimensionality required to describe the system. To this end, we exploit the concept of dimension witnesses. These are functions of the measurement statistics whose value allows one to bound the dimension. We study also the robustness of our tests in more realistic experimental situations, in which devices are affected by noise and losses. Lastly, we report on an experimental implementation of dimension witnesses. We conducted the experiment on photons manipulated in polarization and orbital angular momentum. This allowed us to generate ensembles of classical and quantum systems of dimension up to four. We then certified their dimension as well as its quantum nature by using dimension witnesses. The second part focuses on nonlocality. The local content is a nonlocality quantifier that represents the fraction of events that admit a local description. We focus on systems that exhibit, in that sense, maximal nonlocality. By exploiting the link between Kochen-Specker theorems and nonlocality, we derive a systematic recipe to construct maximally nonlocal correlations. We report on the experimental implementation of correlations with a high degree on nonlocality in comparison with all previous experiments on nonlocality. We also study maximally nonlocal correlations in the multipartite setting, and show that the so-called GHZ-state can be used to obtain correlations suitable for multipartite information protocols, such as secret-sharing. The third part studies nonlocality from an operational perspective. We study the set of operations that do not create nonlocality and characterize nonlocality as a resource theory. Our framework is consistent with the canonical definitions of nonlocality in the bipartite setting. However, we find that the well-established definition of multipartite nonlocality is inconsistent with the operational framework. We derive and analyze alternative definitions of multipartite nonlocality to recover consistency. Furthermore, the novel definitions of multipartite nonlocality allows us to analyze the validity of information principles to bound quantum correlations. We show that `information causality' and `non-trivial communication complexity' are insufficient to characterize the set of quantum correlations. In the fourth part we present the first quantum protocol attaining full randomness amplification. The protocol uses as input a source of imperfect random bits and produces full random bits by exploiting nonlocality. Randomness amplification is impossible in the classical regime and it was known to be possible with quantum system only if the initial source was almost fully random. Here, we prove that full randomness can indeed be certified using quantum non-locality under the minimal possible assumptions: the existence of a source of arbitrarily weak (but non-zero) randomness and the impossibility of instantaneous signaling. This implies that one is left with a strict dichotomic choice regarding randomness: either our world is fully deterministic or there exist events in nature that are fully random.
The device-independent formalism is a set of tools to analyze experimental data and infer properties about systems, while avoiding almost any assumption about the functioning of devices. It has found applications both in fundamental and applied physics: some examples are the characterization of quantum nonlocality and information protocols for secure cryptography or randomness generation. This thesis contains novel results on these topics and also new applications such as deviceindependent test for dimensionality. After an introduction to the field, the thesis is divided in four parts. In the first we study device-independent tests for classical and quantum dimensionality. We investigate a scenario with a source and a measurement device. The goal is to infer, solely from the measurement statistics, the dimensionality required to describe the system. To this end, we exploit the concept of dimension witnesses. These are functions of the measurement statistics whose value allows one to bound the dimension. We study also the robustness of our tests in more realistic experimental situations, in which devices are affected by noise and losses. Lastly, we report on an experimental implementation of dimension witnesses. We conducted the experiment on photons manipulated in polarization and orbital angular momentum. This allowed us to generate ensembles of classical and quantum systems of dimension up to four. We then certified their dimension as well as its quantum nature by using dimension witnesses. The second part focuses on nonlocality. The local content is a nonlocality quantifier that represents the fraction of events that admit a local description. We focus on systems that exhibit, in that sense, maximal nonlocality. By exploiting the link between Kochen-Specker theorems and nonlocality, we derive a systematic recipe to construct maximally nonlocal correlations. We report on the experimental implementation of correlations with a high degree on nonlocality in comparison with all previous experiments on nonlocality. We also study maximally nonlocal correlations in the multipartite setting, and show that the so-called GHZ-state can be used to obtain correlations suitable for multipartite information protocols, such as secret-sharing. The third part studies nonlocality from an operational perspective. We study the set of operations that do not create nonlocality and characterize nonlocality as a resource theory. Our framework is consistent with the canonical definitions of nonlocality in the bipartite setting. However, we find that the well-established definition of multipartite nonlocality is inconsistent with the operational framework. We derive and analyze alternative definitions of multipartite nonlocality to recover consistency. Furthermore, the novel definitions of multipartite nonlocality allows us to analyze the validity of information principles to bound quantum correlations. We show that information causality' and non-trivial communication complexity' are insufficient to characterize the set of quantum correlations. In the fourth part we present the first quantum protocol attaining full randomness amplification. The protocol uses as input a source of imperfect random bits and produces full random bits by exploiting nonlocality. Randomness amplification is impossible in the classical regime and it was known to be possible with quantum system only if the initial source was almost fully random. Here, we prove that full randomness can indeed be certified using quantum non-locality under the minimal possible assumptions: the existence of a source of arbitrarily weak (but non-zero) randomness and the impossibility of instantaneous signaling. This implies that one is left with a strict dichotomic choice regarding randomness: either our world is fully deterministic or there exist events in nature that are fully random.
Quantum theory admits correlations between distant parties that are stronger than those permitted by classical physics. Such correlations, which are termed nonlocal, have been studied extensively since their discovery by John Bell in 1964. This thesis makes progress on various aspects of quantum nonlocality both from a foundational and practical perspective. We begin by studying a foundational question on the relationship between nonlocality and quantum entanglement. We show that the strongest form of nonlocality, also called pseudo-telepathy or full nonlocality, can be demonstrated by non-maximally entangled states and prove that all pure entangled states can demonstrate full nonlocality in the many-copy scenario. We also provide non-trivial examples of fully nonlocal mixed states. We then turn to the study of another phenomenon in quantum theory which is essential in almost all demonstrations of non-classicality—measurement incompatibility. We generalise the notion of measurement incompatibility and show that our notion has strong operational relevance: it captures the randomness in the outcomes of an untrusted measurement device against a classical adversary. We use our formalism to derive new bounds on the experimental losses and noise that can be tolerated in quantum communication protocols before they become insecure. We then begin a systematic study of routed Bell experiments, a variant of standard Bell experiments that was introduced to demonstrate nonlocality over long distances in the presence of losses and noise. We formalize the notion of genuine long-range quantum correlations in routed Bell experiments and develop numerical and analytical tools to characterize them. We show that routed Bell experiments allow for the demonstration of long-range quantum correlations at arbitrarily large distances using only entangled qubit states. We then prove the security of fully device-independent (DI) quantum key distribution (QKD) protocols based on routed Bell tests and quantify the resulting improvement in key rate and distance compared to standard DI QKD protocols.
Physics has two main ambitions: to predict and to understand. Indeed, physics aims for the prediction of all natural phenomena. Prediction entails modeling the correlation between an action, the input, and what is subsequently observed, the output.Understanding, on the other hand, involves developing insightful principles and models that can explain the widest possible varietyof correlations present in nature. Remarkably, advances in both prediction and understanding foster our physical intuition and, as a consequence, novel and powerful applications are discovered. Quantum mechanics is a very successful physical theory both in terms of its predictive power as well as in its wide applicability. Nonetheless and despite many decades of development, we do not yet have a proper physical intuition of quantum phenomena. I believe that improvements in our understanding of quantum theory will yield better, and more innovative, protocols and vice versa.This dissertation aims at advancing our understanding and developing novel protocols. This is done through four approaches. The first one is to study quantum theory within a broad family of theories. In particular, we study quantum theory within the family of locally quantum theories. We found out that the principle that singles out quantum theory out of this family, thus connecting quantum local and nonlocal structure, is dynamical reversibility. This implies that the viability of large scale quantum computing can be based on concrete physical principles that can be experimentally tested at a local level without needing to test millions of qubits simultaneously. The second approach is to study quantum correlations from a black box perspective thus making as few assumptions as possible. The strategy is to study the completeness of quantum predictions by benchmarking them against alternative models. Three main results and applications come out of our study. Firstly, we prove that performing complete amplification of randomness starting from a source of arbitrarily weak randomness - a task that is impossible with classical resources - is indeed possible via nonlocality. This establishes in our opinion the strongest evidence for a truly random event in nature so far. Secondly, we prove that there exist finite events where quantum theory gives predictions as complete as any no-signaling theory can give, showing that the completeness of quantum theory is not an asymptotic property. Finally, we prove that maximally nonlocal theories can never be maximally random while quantum theory can, showing a trade-off between the nonlocality of a theory and its randomness capabilities. We also prove that quantum theory is not unique in this respect. The third approach we follow is to study quantum correlations in scenarios where some parties have a restriction on the available quantum degrees of freedom. The future progress of semi-device-independent quantum information depends crucially on our ability to bound the strength of these correlations. Here we provide a full characterization via a complete hierarchy of sets that approximate the target set from the outside. Each set can be in turn characterized using standard numerical techniques. One application of our work is certifying multidimensional entanglement device-independently.The fourth approach is to confront quantum theory with computer science principles. In particular, we establish two interesting implications for quantum theory results of raising the Church-Turing thesis to the level of postulate. Firstly, we show how different preparations of the same mixed state, indistinguishable according to the quantum postulates, become distinguishable when prepared computably. Secondly, we identify a new loophole for Bell-like experiments: if some parties in a Bell-like experiment use private pseudorandomness to choose their measurement inputs, the computational resources of an eavesdropper have to be limited to observe a proper violation of non locality.
Despite the success of quantum mechanics in predicting the outcomes of experiments in many branches of physics, the foundations of the theory have remained subject of research and dispute. At the basis of this struggle with the theory lie the phenomena of nonlocality and entanglement. Since it was first predicted by Bell in 1964, nonlocality was not only verified in numerous experiments, but also identified as a useful resource for quantum information processing. Thus, the study of nonlocality is important both from a fundamental point of view and with respect to new applications in quantum information theory, such as secure cryptography and randomness generation. The identification of entanglement as a resource for quantum information led to a strong theoretical effort devoted to its characterisation and detection. Many of the resulting mathematical tools find application in several domains of physics. Although the only known way to create nonlocal correlations is to measure entangled quantum systems, it has been shown that entanglement and nonlocality constitute two inequivalent properties. Therefore, in the light of the success of entanglement theory, it is of interest to also devise a resource theory of nonlocality. The task of this thesis is to develop such a theoretical framework for the characterisation of nonlocality as a resource. To gain a better understanding of nonlocal correlations it will be necessary to investigate correlation scenarios that go beyond the situation originally considered by Bell. In doing so, this thesis provides new description of nonlocality that also have implications for the characterisation of quantum correlations and the detection of new forms of nonlocality. The first question we address is how nonlocality can consistently be defined in a scenario of arbitrarily many parties that may collaborate among each other. To this end we recognise which are the allowed physical operations in this situation and then define nonlocality as the resource that cannot be created by these operations. Our approach shows that the conventional definition of multipartite nonlocality, adopted by the community so far, is inconsistent with this operational definition; we further propose and analyse new models that do not suffer from these inconsistencies. Furthermore, we show that our findings have implications for the characterisation of quantum correlations. A recent approach to describe the set of quantum correlation consists in using principles inspired from information theory. By using a special instance of the models we defined earlier we show a fundamental limitation of this approach: no bipartite information principle is sufficient to single out the set of quantum correlations from the set of nonsignalling correlations. We then developed a description of nonlocality in an even more generalised scenario of several parties. Motivated by a result of Popescu we study scenarios where the parties are allowed to perform not only a single but sequences of measurements on their systems. Characterising nonlocality also in this scenario in operational terms and defining local models compatible with this definition, we show that a new form of nonlocality can be detected Lastly, we examine the problem of detecting the presence of nonlocality in a multipartite scenario when one is given only partial access to the global system. We find that one can verify that the total system must display nonlocality, even though the accessible subsystems only exhibit local correlations.
The question of certifying quantum nonlocality under a relaxation of the assumptions in the Bell theorem has gained traction, with potential for device-independent applications under weak seeds and cross-talk. Recently, it was shown that quantum nonlocality can be certified even under a simultaneous arbitrary (but not full) relaxation of the assumptions of Measurement Independence (MI) and Parameter Independence (PI), using states of local dimension d=poly((1−ε)−1) for an ε∈[0,1)-relaxation. Here, we derive three results strengthening the state-of-art. Firstly, we show that states of constant local dimension d are already sufficient to certify quantum nonlocality under arbitrary MI and PI relaxation, albeit in a non-robust manner. Secondly, and as a theoretical paradigm to derive the above, we introduce the notion of \textit{measurement-dependent parameter-dependent locality} as the set of input-output behaviors under simultaneous relaxations of measurement and parameter independence. We provide a rigorous characterisation of the vertices of the polytope of joint input-output behaviors that obey a μ-relaxation of MI and ε-relaxation of PI. We highlight a relation between nonlocality certification under PI relaxation and that under detection inefficiencies by pointing out alternative extremal correlations to the Eberhard correlations that also allow to achieve detection efficiency of η=2/3 in the two-input scenario. Finally, we study the implication of the relaxed assumptions for device-independent randomness certification. We analytically derive the quantum guessing probability for one player's outcomes in the CHSH Bell test, as a function of the noise in the test as well as of a leakage of an average amount of I(X:B)<1 bits of input information per measurement round.
In the past century, experimental discoveries have witnessed phenomena in Nature which challenge our everyday classical intuition. In order to explain these facts, quantum theory was developed, which so far has been able to reproduce the observed results. However, I believe that our understanding of quantum mechanics can be significantly improved by the search for an operational meaning behind its mathematical formulation, which would help to identify the limitations and possibilities of the theory for information processing. An intriguing property of quantum theory is its intrinsic randomness. Indeed, Einstein, Podolsky and Rosen in 1935 questioned the completeness of quantum theory. They argued the possibility of the existence of a complete theory where variables to which we have not access determine the behaviour of physical systems, and the randomness observed in quantum mechanics is then due to our ignorance of these variables. These hidden variables theories, however, were proved not to be enough for explaining the predictions of quantum theory, as shown in the no-go theorems by Bell on quantum-nonlocality and by Kochen and Specker on quantum-contextuality. In the past decades, many experiments have corroborated the nonlocal and contextual character of Nature. However, no intuition behind these phenomena has been found, in particular about what limits their strength. In fact, special relativity alone would allow for phenomena which are more nonlocal than what quantum theory allows. Hence, much effort has been devoted to find the physical properties of quantum theory that restricts these phenomena. In this thesis, we study the constraints that arise on nonlocal and contextual phenomena when a certain exclusiveness structure compatible with quantum theory is imposed in the space of events. Here, an event denotes the situation where an outcome is obtained given that a measurement is performed on the physical system. Regarding nonlocality, we introduce a notion of orthogonality that states that events involving different outcomes of the same local measurement are exclusive, and construct constraints that the correlations among observers should satisfy. We denote this by Local Orthogonality principle (LO), which is the first intrinsically multipartite principle for bounding quantum correlations. We prove that LO identifies the supra-quantum character of some bipartite and multipartite correlations, and gets close to the quantum boundary. When studying contextuality, the same abstract event may correspond to outcomes of different measurements, which introduces a non-trivial structure in the space of events. For its study, we develop a general formalism for contextuality scenarios in the spirit of the recent approach by Cabello, Severini and Winter. In our framework, nonlocality arises as a particular case of contextuality, which allows us to study a generalization of LO. Both in nonlocality and contextuality, we find close connections to problems in combinatorics and hence use graph-theoretical tools for studying correlations. Finally, this thesis also studies the detection of nonlocal correlations. Most results on quantum nonlocality focus on few particles' experiments, while less is known about the detection of quantum nonlocality in many-body systems. Standard many-body observables involve correlations among few particles, while there is still no multipartite Bell inequality to test nonlocality merely from these data. In this thesis, we provide the first proposal for nonlocality detection in many-body systems using two-body correlations. We construct families of Bell inequalities from two-body correlators, which can detect nonlocality for systems with large number of constituents. In addition, we prove violations by systems which are relevant in nuclear and atomic physics, and show how to test these inequalities by measuring global spin components, hence opening the problem to experimental realizations.
Nonlocal correlations are useful for device-independent (DI) randomness certification (P ironio S. et al. , Nature (London) , 464 (2010) 1021). The advantage of this DI protocol over the conventional quantum protocol is that randomness can be certified even when experimental apparatuses are not trusted. Quantum entanglement is the necessary physical source for the nonlocal correlation required for such DI task. However, nonlocality and entanglement are distinct concepts. There exist entangled states which produce no nonlocal correlation and hence are not useful for the DI randomness certification task. Here we introduce the measurement-device–independent randomness certification task where one has trusted quantum state preparation devices but the mesurement devices are completely unspecified. Interestingly we show that there exist entangled states, with local description, that are a useful resource in such task but are useless in the corresponding DI scenario.