Artur Ekert: Bell inequalities: From Curiosity to Security


Artur Ekert
Merton College, University of Oxford
DIPC Josebe-Olarra Lecture Hall
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Artur Ekert: Bell inequalities: From Curiosity to Security
(Image by Duncan.Hull - Own work, CC BY-SA 4.0,

Bell inequalities's journey from rags to riches of quantum theory was a long one. Proposed by John Bell in 1964, the inequalities were designed to check whether quantum theory, with its inherent statistical predictions, is a complete description of physical reality or whether it is just a provisional construct, with an underlying hidden structure which, once discovered, would offer precise predictions. The subsequent pioneering experiments of outliers, such as John Clauser (1972) and Alain Aspect (1982), showed that Bell's inequalities can be violated. However, these experiments were barely noticed at that time. Quantum theory is admittedly strange, but it worked and the research community just carried on using it in an instrumental way, making successful statistical predictions while avoiding anything related to their interpretations. Bell inequalities were viewed as a philosophical topic with no practical value and hence not worthy of the attention of serious scientists. When Bell inequalities snagged my imagination, I was just a PhD student with nothing to lose. In 1991, I reformulated Bell inequalities as the test for eavesdropping in cryptography, paving the way for the most secure communication systems to date, known as the device independent quantum key distribution. The new narrative around Bell inequalities created an additional motivation to close all possible loopholes in the previous experiments. In the new context it seemed reasonable. Nature would have to be very malicious if it were to cheat selectively; on locality in some experiments and exploring detection loopholes in some others. In contrast, an eavesdropper has all the rights to be malicious. Closing the loopholes posed an experimental challenge but gradually, due to the efforts of several experimental groups, to mention only those of Anton Zeilinger (2015), the loopholes were closed and device independent cryptography became a realistic experimental proposition. Cryptography offered a lifeline to quantum foundations and in return the experimental tools developed to pursue esoteric philosophical questions gave cryptography unprecedented security. The curiosity and perseverance of the few brave souls who made this happen (and who are still alive) were finally rewarded with the 2022 Nobel Prize in Physics.