Abstract This chapter presents a detailed discussion of Bell inequalities and their experimental tests. Starting from the Aharonov-Bohm version of the Einstein-Podolsky-Rosen (EPR) paradox, we review EPR’s criteria for realism and completeness and Bell’s locality condition, before deriving the Clauser-Horne-Shimony-Holt (CHSH) inequality. This is followed by an exposition on Bell’s original inequality and other Bell inequalities by Wigner, as well as Clauser and Horne. We then discuss the milestones in the experimental violation of Bell inequalities, from the experiment of Freedman and Clauser in the 1970s, to the efforts of Aspect et al. in the 1980s, the experiments by the teams of Zeilinger and Gisin in the 1990s, and culminating the loophole-free tests in 2015. Finally we turn to a brief discussion of the interpretations of quantum mechanics, focusing on contrasting the realist views held by, e.g., Einstein and Bell, and the informational views with proponents such as Brukner and Zeilinger
Abstract This chapter presents a detailed discussion of Bell inequalities and their experimental tests. Starting from the Aharonov-Bohm version of the Einstein-Podolsky-Rosen (EPR) paradox, we review EPR’s criteria for realism and completeness and Bell’s locality condition, before deriving the Clauser-Horne-Shimony-Holt (CHSH) inequality. This is followed by an exposition on Bell’s original inequality and other Bell inequalities by Wigner, as well as Clauser and Horne. We then discuss the milestones in the experimental violation of Bell inequalities, from the experiment of Freedman and Clauser in the 1970s, to the efforts of Aspect et al. in the 1980s, the experiments by the teams of Zeilinger and Gisin in the 1990s, and culminating the loophole-free tests in 2015. Finally we turn to a brief discussion of the interpretations of quantum mechanics, focusing on contrasting the realist views held by, e.g., Einstein and Bell, and the informational views with proponents such as Brukner and Zeilinger
In 1935, Einstein, Podolsky, and Rosen (EPR) argued that (then new) quantum mechanics was incomplete. At the heart of EPR’s paradox was the strange non-local nature of an entangled state which allowed EPR to simultaneously assign local values for position and momentum of a particle. In 1964, John Bell proposed an inequality (Bell’s inequality) that must be satisfied by any local realistic theory. The experimental confirmation of quantum physics, i.e., the experimental test of Bell’s inequality, required a pair of particles in an entangled state. The experiments first performed in the early 1970s, followed by a series of experiments until now, have confirmed the violation of Bell’s inequality, strongly implying that local realistic descriptions of nature are not consistent with experimental observations. The early simplistic view of entanglement has now been significantly expanded to include mixed states and multiple particles. Moreover, theoretical and experimental studies on the nature of entanglement have led to quantum information science where entanglement is an essential resource. In this article, we briefly review the early experiments on Bell’s inequality and experimental attempts to close “loopholes” as well as some key experiments on quantum information.
We emphasize the role of the precise correlations loophole in attempting to connect the CHSH type inequalities with the EPR-argument. The possibility to test theories with hidden variables experimentally by using such inequalities is questioned. The role of the original Bell inequality is highlighted. The interpretation of the CHSH inequality in the spirit of Bohr, as a new test of incompatibility, is presented. The positions of Bohr, Einstein, Podolsky, Rosen, Bell, Clauser, Horne, Shimony, Holt, De Broglie, Hertz, and Boltzmann on interrelation of theory and experiment are enlightened.
The Nobel Prize in Physics 2022 was awarded to three experiment physicists: Alain Aspect, John F. Clauser and Anton Zeilinger, for their contributions to “experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”. In this paper, the history, basics, and applications related with the Bell inequalities are reviewed. The discussion of fundamental issues on quantum mechanics dated back to the year 1927 when the fifth Solvay conference was held, highlighting intense arguments between Albert Einstein and Niels Bohr. At the time, Einstein expressed, publicly for the first time, his critical attitude towards quantum mechanics. While the arguments did not reach a decisive conclusion, Bohr’s probabilistic interpretation of quantum mechanics, the Copenhagen interpretation, had since been well circulated among the community. Einstein’s arguments with Bohr lasted many years after the conference. The arguably most famous one was the Einstein-Podolsky-Rosen (EPR) paradox, proposed in a 1935 paper written by the three authors. They stated, by flawless reasoning, that quantum mechanics is ‘incomplete’. Bohr replied immediately. So did Erwin Schrödinger, who, in particular, coined the term ‘entangled’ for the first time to describe the bizarre quantum mechanical property. Nevertheless, the EPR paper received few citations at its publication, until 1990’s when quantum information science started blossoming. From today’s perspective, the paper resembles the ‘sleeping beauty’ who slumbered for decades before fully awakening. It has been widely accepted that it was John S. Bell who eventually resolved the EPR paradox. But many important figures before him should not go unnoticed, including John A. Wheeler, Chien-Shiung Wu, David Bohm, to name a few. In 1964, Bell wrote down a mathematical inequality which satisfies local hidden variable theory but which can be violated in quantum mechanics. The significance is that Bell’s inequality brings the rather philosophic debates on the EPR paradox down to be experimentally testable. Hence, an experiment is here appealing, as is the case in all fields of physics. To the end, John F. Clauser, alongside Stuart J. Freedman, conducted an experiment in 1972 to test a Bell inequality, the Bell-Clauser-Horne-Shimony-Holt (CHSH) inequality, proposed by him and his collaborators in 1969. The results, however, leaves loopholes open. One of the loopholes was closed in a later experiment led by Alain Aspect in 1982. The Bell-CHSH inequality was later tested under a much stricter condition by Anton Zeilinger’s team. Two most important loopholes were both closed by three teams in 2015 independently. Zeilinger’s team was among them. Almost all experiments to date have confirmed quantum mechanical predictions. Zeilinger’s team also performed the first teleportation of unknown quantum states, the first entanglement swapping, along with other contributions pioneering the quantum information science. In retrospect, the foundational study of quantum theory has been developed over a century. It once drew much of quantum physics founders’ attention, went virtually unnoticed while other physical fields advanced, had for quite a while been considered to have very weak relationship with major physics, and finally revived in recent decades leading to rapid progress in quantum information science and quantum technology. We believe that the Nobel Prize in Physics this year is not only an award and recognition to past achievements, but also heralds a new era of science in future.
This review is related to the Einstein-Bohr debate and to Einstein–Podolsky–Rosen’s (EPR) and Bohm’s (EPRB) Gedanken-experiments as well as their realization in actual experiments. I examine a significant number of papers, from my minority point of view and conclude that the well-known theorems of Bell and Clauser, Horne, Shimony and Holt (CHSH) deal with mathematical abstractions that have only a tenuous relation to quantum theory and the actual EPRB experiments. It is also shown that, therefore, Bell-CHSH cannot be used to assess the nature of quantum entanglement, nor can physical features of entanglement be used to prove Bell-CHSH. Their proofs are, among other factors, based on a statistical sampling argument that is invalid for general physical entities and processes and only applicable for finite “populations”; not for elements of physical reality that are linked, for example, to a time-like continuum. Bell-CHSH have, furthermore, neglected the subtleties of the theorem of Vorob’ev that includes their theorems as special cases. Vorob’ev found that certain combinatorial-topological cyclicities of classical random variables form a necessary and sufficient condition for the constraints that are now known as Bell-CHSH inequalities. These constraints, however, must not be linked to the observables of quantum theory nor to the actual EPRB experiments for a variety of reasons, including the existence of continuum-related variables and appropriate considerations of symmetry.
The assumption of local realism imposes constraints, such as Bell inequalities, on quantities obtained from measurements. In recent years, various tests of local realism have gained popularity in undergraduate laboratories, giving students the exciting opportunity to experimentally contradict this philosophical assumption. The standard test of the CHSH (Clauser–Horne–Shimony–Holt) Bell inequality requires 16 measurements, whereas a test of Freedman's inequality requires only three measurements. The calculations required to test Freedman's inequality are correspondingly simpler and the theory is less abstract. We suggest that students may benefit from testing Freedman's inequality before proceeding to the CHSH inequality and other more complicated experiments. Our measured data violated Freedman's inequality by more than six standard deviations.
Bell inequalities represent a milestone for contemporary physics, both for quantum foundation investigations and technological applications (e.g., quantum communication and entanglement certification). Although loophole-free tests have been recently performed, a strong debate is still ongoing on the actual meaning of Bell inequality tests, for example, on the possible additional hypotheses and eventual loopholes to be included in Bell's theorem, as well as on the implications for certain interpretations of quantum mechanics. A recent work [S. Virzì , ] challenges some of the statements appeared in this debate, achieving an experimental estimation of the entire Bell-Clauser-Horne-Shimony-Holt (Bell-CHSH) parameter from a single entangled pair owing to a weak-interaction-based measurement approach. Here, we analyze the implications of this result for quantum mechanics foundations investigation, illustrating how it can tackle some of the aforementioned interpretations of Bell inequality tests and, more in general, of quantum mechanics itself.