**Advances in quantum physics only challenge all of our beliefs about the world around us. This year, the Nobel Prize in Physics rewards a trio of researchers who have irrefutably demonstrated, after 50 years of work, a more than controversial reality: the phenomenon of quantum entanglement – where the quantum state of two particles is linked regardless of the distance between them. It is the basis of the development of current quantum computers and has made it possible to understand what Einstein called “frightening action at a distance”.**

Until the end of the 19th century, it was considered that reality was accessible to us, and that scientists were external observers of phenomena which they could then describe objectively. The quantum physicsthe one working within the infinitely small, triggers a lively debate on the relationship of science to reality.

Indeed, we can objectively know the world around us by measuring it. But the act of measuring in the quantum world modifies and therefore disturbs the object studied. De facto, it is impossible to know its condition before the measurement. Hence the question: are particles “things” in themselves, can we attribute to them an autonomous physical reality outside of observation? Einstein amused himself by saying: *Do you really believe the moon isn’t there when you’re not looking at it? *“. These are the bases of what is called quantum entanglement.

It should be known that quantum entanglement is the phenomenon in which two particles (or more) exist in a so-called entangled state, that is to say that despite the distance which separates them, they behave as a whole: a modification on one of them leads to a change on the other.

Working independently, each of the three researchers awarded the 2022 Nobel Prize in Physics has forged new experiments demonstrating and studying quantum entanglement. If an observer determines the state of such a particle, its entangled counterparts will instantly reflect that state, whether they are in the same room as the observer or in a galaxy on the other side of the universe! Their results established the violation of so-called Bell inequalities and paved the way for new technologies based on quantum information, currently used to develop the quantum computersthe quantum cryptography and the future quantum internet.

## Bell’s inequalities, a demonstration of quantum entanglement

First elucidated by Erwin Schrödinger in 1935, leading to his famous cat paradox, entanglement was dismissed by Albert Einstein as “frightening action at a distance” and sparked a long philosophical debate over the physical interpretation of Quantum mechanics. Was it a complete theory, or was quantum entanglement due to “hidden variables” because its laws made no sense in the macroscopic world.

In 1964, CERN (European Organization for Nuclear Research) theorist John Bell proposed a theorem known as Bell’s inequalities, which put this question to the test. Concretely, he explains that if hidden values are at stake, the correlation between the results of a large number of measurements will never exceed a certain value; conversely, if quantum mechanics is complete and therefore a valid theory, this value may be exceeded. This is indeed what is happening: all the experiments that have put these inequalities into practice, including those of the three Nobel winners, show that they are transgressed and that quantum physics is indeed a complete theory.

Specifically, John Clauser (JF Clauser & Associates, USA) was the first to experimentally study Bell’s theorem, obtaining measurements that clearly violated a Bell inequality and thus supported quantum mechanics. Then Alain Aspect (Paris-Saclay University and École Polytechnique, France) put the results on firmer ground by imagining ways to perform measurements of entangled photon pairs after they have left their source, thus eliminating the effects of the environment in which they were emitted. Eventually, using refined tools and a long series of experiments, Anton Zeilinger (University of Vienna, Austria) started using entangled quantum states to demonstrate, among other things, the quantum teleportationwhich allows a quantum state to be transferred from one particle to another.

As summarized by communicated of CERN, these delicate and pioneering experiments not only confirmed quantum theory, but also laid the foundation for a new field of science and technology, which has applications in computing, communication, sensing and the simulation.

## The Universe is not real locally, a basic principle of quantum computing

Currently, entanglement is therefore accepted as one of the main features of quantum mechanics and is being implemented in cryptography, quantum computing and a future “quantum internet” at over $1 billion a year. . One of his earliest successes in cryptography was sending messages using pairs of entangled photons, creating cryptographic keys in a secure manner — any eavesdropping will destroy the entanglement, alerting the recipient of the hack.

It would thus be a flagrant illustration that the Universe is not locally real, demonstrated by the Nobel-winning scientists this year. As explained in an article by *Scientific American*, “real” means that objects have defined properties independent of observation: an apple can be red even when no one is looking at it, which is not the case in the quantum world. The properties of objects are interdependent on observation.

“Local” means that objects can only be influenced by their environment and any influence cannot travel faster than light. This is also not the case in quantum physics “because” of quantum entanglement. The trio of scientists has thus demonstrated that objects are not influenced solely by their environment, a modification to a particle will have repercussions on its entangled particle, which is several light years for example.

In 2017, Dr. Zeilinger used this technique via a Chinese satellite called Micius to have a encrypted video chat of 15 minutes with Jian-Wei Pan from the Chinese Academy of Sciences, one of his former students. The satellite, built partly thanks to the discoveries of John Clauser, uses several properties of quantum mechanics applied to photons, the elementary particles of light. The satellite is capable of manufacturing and emitting pairs of entangled photons, in two telescopes separated by 1203 kilometers.

Although he acknowledged that the award honors future applications of his work, Dr. Zeilinger points out in an interview with *New York Times* : “ *My advice would be: do what you find interesting and don’t worry too much about possible applications *“. For his part, Dr Clauser says: *I still confess today that I still don’t understand quantum mechanics, and I’m not even sure I really know how to use it well. *“.

However, in an article by *ScienceNews*Nicolas Gisin, physicist at the University of Geneva in Switzerland underlines: *This award is well deserved, but comes a bit late. The majority of this work has been done in the *[années 1970 et 1980]*but the Nobel committee was very slow and is now rushing after the quantum technologies boom* “.

This boom is happening globally. Gisin concludes: *In the United States, Europe and China, billions – literally billions of dollars are being poured into this area. So that changes completely. Instead of having a few pioneering individuals in the field, we now have very large crowds of physicists and engineers working together* “.

Although some of the quantum applications are in their infancy, the experiments of Clauser, Aspect and Zeilinger introduce quantum mechanics and its implications to the macroscopic world. Their contributions validate some of the key, once-controversial ideas of quantum mechanics, and promise new applications that may one day find their way into everyday life.

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The Universe is not locally real: a discovery rewarded with the 2022 Nobel Prize in Physics

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