In the usual derivation of Bell's inequalities, Locality is assumed to justify statistical independence of the outcomes of observations performed in remote stations; as the inequalities are observed to be violated, it is concluded that these outcomes are statistically dependent, and hence, that non-Local effects exist. Here it is shown that the simplest description of Bell's experiment according to the canon of von Neumann's theory of measurement assumes statistical independence of those outcomes. In other words, the straightforward description of Bell's experiment according to Quantum Mechanics (which, of course, violates Bell's inequalities) does not assume non-Locality. Summing up this and other related results it is concluded that, within the framework of Quantum Mechanics, there is no reason to believe in the existence of non-Local effects. In addition to its foundational significance, this conclusion has practical impact in the fields of quantum-certified and device-independent randomness generation and on the vulnerability of Quantum Key Distribution schemes, which are based on the existence of quantum non-Locality as an axiom.
In the usual derivation of Bell's inequalities, Locality is assumed to justify statistical independence of the outcomes of observations performed in remote stations; as the inequalities are observed to be violated, it is concluded that these outcomes are statistically dependent, and hence, that non-Local effects exist. Here it is shown that the simplest description of Bell's experiment according to the canon of von Neumann's theory of measurement assumes statistical independence of those outcomes. In other words, the straightforward description of Bell's experiment according to Quantum Mechanics (which, of course, violates Bell's inequalities) does not assume non-Locality. Summing up this and other related results it is concluded that, within the framework of Quantum Mechanics, there is no reason to believe in the existence of non-Local effects. In addition to its foundational significance, this conclusion has practical impact in the fields of quantum-certified and device-independent randomness generation and on the vulnerability of Quantum Key Distribution schemes, which are based on the existence of quantum non-Locality as an axiom.
The interpretation of the meaning of Quantum Mechanics has faced controversy since its inception. Bell's inequalities are a touchstone in this controversy. Their observed violation demonstrates that at least one of the hypotheses involved in their derivation and test is false in Nature. In principle, one has to choose between accepting that Locality is false, what implies a possible contradiction with the Theory of Relativity, or accepting that Realism is false, what means to give up the existence of a physical world independent of the observer. The right answer has consequences both foundational and practical, and theoretical discussions have searched it for decades. We report the results of a Bell's experiment designed and performed to add observational information to the discussion. Three proposals to reveal the false hypothesis are carried out, namely: search of attractors in time series of observations, variation of randomness of binary series of outcomes between space-like and not-space-like separated conditions of observation, and test of a bound of Kolmogorov's complexity. The results are consistent with the absence of the specific form of Realism usually involved in the derivation of Bell's inequalities, while remaining compatible with Locality within the sensitivity of the tests. Independently of the foundational problem and of any interpretation, some bare observations have immediate practical impact on the best use of device-independent quantum Random Number Generators and Quantum Key Distribution.
The problem of the existence of nonlocal effects in Quantum Mechanics is discussed. The problem is divided in two: the first ('soft') one is to explain the violation of Bell's inequalities as a statistical magnitude. This can be achieved by a simple model within non-Boolean Locality and Realism. This result shows that quantum non-Locality as a consequence of the statistical violation of Bell's inequalities is inexistent. The second ('hard') problem is to explain the violation as it is calculated from series of detection outcomes. L.Sica has demonstrated that, in order to violate Bell's inequalities, the series recorded at (say) Bob when the setting at station Alice is alfa, can be different from the series that would have been recorded at Bob if that setting had been alfa'instead. Therefore, non-Locality in the series of detection outcomes does exist. It cannot be experimentally verified because of its counterfactual nature, but is observed in computer simulations. An appropriate computer code is based on the simple model mentioned plus a contextual instruction. It explains 'how' (Sica's) non-Locality arises, and solves the hard problem. 'Why' the contextual instruction exists is explained by Hellwig and Kraus' postulate of covariant quantum state collapse. In consequence, (Sica's) non-Locality is not in contradiction with Relativity but, quite the opposite, it is implied by Relativistic covariance.
Bell nonlocality is the experimentally testable incompatibility between quantum correlations and explanations based on local hidden variables. Its significance now extends beyond the foundations of quantum mechanics: a Bell-inequality violation can certify entanglement, randomness, measurement incompatibility, and cryptographic security without requiring a detailed model of the devices producing the observed data. This article develops the mathematical structure of Bell experiments and explains its role in device-independent quantum information. Conditional probability distributions are classified into local, quantum, and no-signalling sets; the Clauser-Horne-Shimony-Holt inequality is derived, and its classical, quantum, and algebraic limits are distinguished. Tsirelson’s bound is obtained from the CHSH operator, while Popescu-Rohrlich correlations demonstrate that no-signalling alone does not identify the quantum set. The relation between entanglement and nonlocality is examined through local entangled states, hidden nonlocality, and activation. Nonlocal games, semidefinite relaxations, self-testing, certified randomness, and device-independent quantum key distribution are treated as operational consequences of Bell violation. Particular attention is given to entropy accumulation, finite-data security, loophole-free experiments, and the limitations of device independence. Bell nonlocality emerges as both a foundational separation between classical and quantum causal models and a quantitative resource for cryptographic and informational tasks.
Assuming that there is no way of sending signals propagating faster than light and that free will exists, the loophole-free observed violation of Bell's inequalities demonstrates that at least one of three fundamental hypotheses involved in the derivation and observation of the inequalities is false: Locality, Realism, or Ergodicity. An experiment is proposed to obtain some evidence about which one is the false one. It is based on recording the time evolution of the rate of non-random series of outcomes that are generated in a specially designed Bell's setup. The results of such experiment would be important not only to the foundations of Quantum Mechanics, but they would also have immediate practical impact on the efficient use of quantum-based random number generators and the security of Quantum Key Distribution using entangled states.
Abstract Solid experimental evidence has now been obtained that confirms the violation of Bell’s inequality in tests of maximally entangled qubit pairs. This violation is widely interpreted as definitive proof of the impossibility of describing quantum phenomena in terms of locally defined elements of reality. In a series of recent papers, we have noticed, however, that this conclusion inadvertently, yet crucially, relies on the assumed existence of an absolute frame of reference, with respect to which it would be possible to describe independently of each other the hypothetical elements of reality and the measurement devices that test them. Otherwise, a non-zero geometric phase may appear in the description of the former with respect to a closed sequence of settings of the latter, leading to the violation of the inequality. Following this observation, we discuss an explicit statistical model, which fully reproduces the predictions of Quantum Mechanics for the Bell experiment.
In a recent note, Hance and Hossenfelder (arXiv:2211.01331) recall that "locally causal completions of quantum mechanics are possible, if they violate the assumption [called statistical independence or measurement independence] that the hidden variables do not in any way depend on measurement settings" and that, consequently, the experimental violations of Bell inequalities "show that maintaining local causality requires violating statistical independence". However, Hance and Hossenfelder also argue that "we should (...) look for independent experimental evidence that can distinguish the two different options: non-locality and statistical independence, or locality and violations of statistical independence" and that "the unwillingness to consider theories without statistical independence may be the reason we do not yet have a locally causal theory for the foundations of physics that is consistent with general relativity". Here, we recall that there is a third option, namely, rejecting that measurement outcomes are governed in any way by hidden variables. Moreover, we argue that some recent results in the search for principles singling out the sets of quantum correlations for Bell and Kochen-Specker contextuality scenarios point out that this third option is scientifically more plausible and answers the question of why and how nature produces quantum nonlocality.