Forty years after the advent of quantum mechanics the problem of hidden variables, that is, the possibility of imbedding quantum theory into a classical theory, remains a controversial and obscure subject. Whereas to most physicists the possibility of a classical reinterpretation of quantum mechanics remains remote and perhaps irrelevant to current problems, a minority have kept the issue alive throughout this period. (See Freistadt [5] for a review of the problem and a comprehensive bibliography up to 1957.) As far as results are concerned there are on the one hand purported proofs of the non-existence of hidden variables, most notably von Neumann’s proof, and on the other, various attempts to introduce hidden variables such as de Broglie [4] and Bohm [1] and [2]. One of the difficulties in evaluating these contradictory results is that no exact mathematical criterion is given to enable one to judge the degree of success of these proposals.

The Problem of Hidden Variables in Quantum Mechanics

These notes are based on a course of lectures given by Professor Nelson at Princeton during the spring term of 1966. The subject of Brownian motion has long been of interest in mathematical probability. In these lectures, Professor Nelson traces the history of earlier work in Brownian motion, both the mathematical theory, and the natural phenomenon with its physical interpretations. He continues through recent dynamical theories of Brownian motion, and concludes with a discussion of the relevance of these theories to quantum field theory and quantum statistical mechanics.

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Dynamical Theories of Brownian Motion

True randomness does not exist in classical physics, where randomness is necessarily a result of forces that may be unknown but exist. The quantum world, however, is intrinsically truly random. This is difficult to prove, as it is not readily distinguishable from noise and other uncontrollable factors. Now Pironio et al. present proof of a quantitative relationship between two fundamental concepts of quantum mechanics — randomness and the non-locality of entangled particles. They first show theoretically that the violation of a Bell inequality certifies the generation of new randomness, independently of any implementation details. To illustrate the approach, they then perform an experiment in which — as confirmed using the theoretical tools that they developed — 42 new random bits have been generated. As well as having conceptual implications, this work has practical implications for cryptography and for numerical simulation of physical and biological systems. Here it is shown, both theoretically and experimentally, that non-local correlations between entangled quantum particles can be used for a new cryptographic application — the generation of certified private random numbers — that is impossible to achieve classically. The results have implications for future device-independent quantum information experiments and for addressing fundamental issues regarding the randomness of quantum theory. Randomness is a fundamental feature of nature and a valuable resource for applications ranging from cryptography and gambling to numerical simulation of physical and biological systems. Random numbers, however, are difficult to characterize mathematically1, and their generation must rely on an unpredictable physical process2,3,4,5,6. Inaccuracies in the theoretical modelling of such processes or failures of the devices, possibly due to adversarial attacks, limit the reliability of random number generators in ways that are difficult to control and detect. Here, inspired by earlier work on non-locality-based7,8,9 and device-independent10,11,12,13,14 quantum information processing, we show that the non-local correlations of entangled quantum particles can be used to certify the presence of genuine randomness. It is thereby possible to design a cryptographically secure random number generator that does not require any assumption about the internal working of the device. Such a strong form of randomness generation is impossible classically and possible in quantum systems only if certified by a Bell inequality violation15. We carry out a proof-of-concept demonstration of this proposal in a system of two entangled atoms separated by approximately one metre. The observed Bell inequality violation, featuring near perfect detection efficiency, guarantees that 42 new random numbers are generated with 99 per cent confidence. Our results lay the groundwork for future device-independent quantum information experiments and for addressing fundamental issues raised by the intrinsic randomness of quantum theory.

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Random numbers certified by Bell's theorem.

Quantum theory: Concepts and methods

Classical mechanics was first envisaged by Newton, formed into a powerful tool by Euler, and brought to perfection by Lagrange and Laplace. It has served as the paradigm of science ever since. Even the great revolutions of 19th century phys icsnamely, the FaradayMaxwell electro-magnetic theory and the kinetic t h e o r y w e r e viewed as further support for the complete adequacy of the mechanistic world view. The physicist at the end of the 19th century had a coherent conceptual scheme which, in principle at least, answered all his questions about the world. The only work left to be done was the computing of the next decimal. This consensus began to unravel at the beginning of the 20th century. The work of Planck, Einstein, and Bohr simply could not be made to fit. The series of ad hoc moves by Bohr, Eherenfest, et al., now called the old quantum theory, was viewed by all as, at best, a stopgap. In the period 1925-27 a new synthesis was formed by Heisenberg, Schr6dinger, Dirac and others. This new synthesis was so successful that even today, fifty years later, physicists still teach quantum mechanics as it was formulated by these men. Nevertheless, two foundational tasks remained: that of providing a rigorous mathematical formulation of the theory, and that of providing a systematic comparison with classical mechanics so that the full ramifications of the quantum revolution could be clearly revealed. These tasks are, of course, related, and a possible fringe benefit of the second task might be the pointing of the way 'beyond quantum theory'. These tasks were taken up by von Neumann as a consequence of a seminar on the foundations of quantum mechanics conducted by Hilbert in the fall of 1926. In papers published in 1927 and in his book, The Mathemat ical Foundations of Quantum Mechanics, von Neumann provided the first completely rigorous

The mathematical foundations of quantum mechanics

On the basis of three physical axioms, we prove that if the choice of a particular type of spin 1 experiment is not a function of the information accessible to the experimenters, then its outcome is equally not a function of the information accessible to the particles. We show that this result is robust, and deduce that neither hidden variable theories nor mechanisms of the GRW type for wave function collapse can be made relativistic and causal. We also establish the consistency of our axioms and discuss the philosophical implications.

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The Free Will Theorem

Diophantine Problems Over Local Fields I

We strengthen "The Free Will Theorem" [1] in several ways, by replacing the axiom FIN by a weaker axiom MIN, and also by allowing the particles' responses to depend on past half-spaces rather than on on past light cones. This change allows us to directly apply our theorem to relativistic theories of the GRW type.

The Strong Free Will Theorem

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Simon Kochen

Papers

The Problem of Hidden Variables in Quantum Mechanics

The Free Will Theorem

Diophantine Problems Over Local Fields I

The Strong Free Will Theorem

A note on the Borel-Cantelli lemma