This self-contained essay collection is published to commemorate half a century of Bell’s theorem. Like its much acclaimed predecessor “Quantum [Un]Speakables: From Bell to Quantum Information” (published 2002), it comprises essays by many of the worlds leading quantum physicists and philosophers. These revisit the foundations of quantum theory as well as elucidating the remarkable progress in quantum technologies achieved in the last couple of decades. Fundamental concepts such as entanglement, nonlocality and contextuality are described in an accessible manner and, alongside lively descriptions of the various theoretical and experimental approaches, the book also delivers interesting philosophical insights. The collection as a whole will serve as a broad introduction for students and newcomers as well as delighting the scientifically literate general reader.

Table of Contents

Part I John Stewart Bell—The Man

1 Address to Participants at Quantum [Un]Speakables II

Mary Bell

2 John Stewart Bell, Quantum Information and Quantum Information Theory

Andrew Whitaker

3 Bell’s Universe: A Personal Recollection

Reinhold Bertlmann

Part II Bell’s Theorem—Fundamental Issues

4 Why QBism Is Not the Copenhagen Interpretation

and What John Bell Might Have Thought of It

N. David Mermin

5 On the Quantum Measurement Problem

Časlav Brukner

6 Causarum Investigatio and the Two Bell’s Theorems of John Bell

Howard M. Wiseman and Eric G. Cavalcanti

7 Whose Information? Information About What?

Jeffrey Bub

8 Quantum Theory: It’s Unreal

Terence Rudolph

9 The Universe Would Not Be Perfect Without Randomness:

A Quantum Physicist’s Reading of Aquinas

Valerio Scarani

10 Bell’s Theorem Tells Us Not What Quantum Mechanics Is, but What Quantum Mechanics Is Not

Marek Żukowski

Part III Contextuality

11 The Unspeakable Why

Adán Cabello

12 A Reconstruction of Quantum Mechanics

Simon B. Kochen

Part IV Bell Inequalities—Theory

13 A Quantum Mechanical Bound for CHSH-Type Bell Inequalities

Michael Epping, Hermann Kampermann and Dagmar Bruß

14 Bell Inequalities with Retarded Settings

Lucien Hardy

15 How to Avoid the Coincidence Loophole

Jan-Åke Larsson

16 Bringing Bell’s Theorem Back to the Domain of Particle Physics and Cosmology

Beatrix Hiesmayr

Part V Quantum Topics

17 Black Box Quantum Mechanics

Antonio Acín and Miguel Navascués

18 Quantum Measurement of Spins and Magnets, and the Classical Limit of PR-Boxes

Nicolas Gisin

19 The Dynamical Roles Played by Mass and Proper Time in Physics

Daniel M. Greenberger

20 On Spatial Entanglement Wave Functions

Michael Horne

Part VI Entanglement Features

21 Analysing Multiparticle Quantum States

Otfried Gühne, Matthias Kleinmann and Tobias Moroder

22 Few-Body Entanglement Manipulation

C. Spee, J.I. de Vicente and B. Kraus

Part VII Neutron Interferometry

23 Search for Hidden Observables in Neutron Experiments

Helmut Rauch

24 What Does Quantum Theory Tell Us? A Matter-Wave Approach

Yuji Hasegawa

Part VIII Bell Inequalities—Experiment

25 Nonlocality and Quantum Cakes, Revisited

Bradley G. Christensen and Paul G. Kwiat

26 An Early Long-Distance Quantum Experiment

Gregor Weihs

27 Quantum Information Experiments with Free-Space Channels

Yuan Cao, Qiang Zhang, Cheng-Zhi Peng and Jian-Wei Pan

28 Bell’s Theorem, Bell Inequalities, and the “Probability Normalization Loophole”

John F. Clauser

29 On Loopholes and Experiments

Marissa Giustina

30 New Dimensions for Entangled Photons: The Role of Information

Anton Zeilinger

About the Authors

Reinhold Bertlmann (Professor of Physics in Vienna) is known for his work on anomalies in quantum field theory and on gluon condensates and potential theory and for always wearing two socks of different color as an illustration of the properties of quantum entanglement, used by John Bell, with whom he then collaborated at CERN, as the red thread in his seminal paper “Bertlmann’s Socks and the Nature of Reality”.

Anton Zeilinger (Professor of Physics in Vienna) explores the foundations of quantum physics and their applications to quantum information science and technology. His experiments on the entanglement of photons, which Einstein called “spooky action at a distance”, have become central to the emerging field of quantum information science where Zeilinger performed pioneering realizations of quantum teleportation, quantum cryptography, and quantum computation.