A Nobel Prize for the second quantum revolution

Frenchman Alain Aspect, American John Clauser and Austrian Anton Zeilinger share the 2023 Nobel Prize in Physics for their groundbreaking experiments on the universe’s most puzzling property: quantum entanglement.

Here is a Nobel Prize that, at least in France, had been eagerly awaited. Four decades after highlighting one of the strangest properties of the world around us, physicist Alain Aspect finally won the Nobel Prize in Physics, along with American John Clauser and Austrian Anton Zeilinger. The general opinion was that it had been promised him for a long time. “If John Bell had not died prematurely in 1990, they would have had him together twenty years ago,” says physicist and philosopher Alexeï Grinbaum (CEA). Time has passed, and alongside the pioneers in the discovery of quantum entanglement (Clauser, Aspect), the Nobel Committee has chosen to honor a pioneer in its implementation in concrete technologies, as amazing and promising as computer science or quantum communication (Zeilinger).

Physicist Alain Aspect. Credit: Ecole polytechnique – J.Barande

“This Nobel Prize rewards both fundamental work in physics, carried out long ago to answer a profound question, and recent research that has made it possible to turn it into applications. It is a good illustration that research and its results can only be judged on long term”, assesses 2012 Nobel Prize in Physics Serge Haroche. The “second quantum revolution” that we are experiencing has its roots in the 1980s. And even further, in the 1960s. And even further, in the year 1935…

Albert Einstein, 1935

To really understand the importance of the Nobel Prize awarded on October 4, 2023, we must actually go back to a famous article by Albert Einstein. That year, the brilliant physicist, together with Boris Podolsky and Nathan Rosen, proposed a thought experiment that he saw as a definitive objection to quantum mechanics, which he had fought against for years: if we are to believe Niels Bohr and the rules of the theory. consorts, he explains in the substance, two photons can under certain conditions behave as a single physical system. In other words, any action on one must immediately have consequences on the other, and this no matter how far apart ! For Einstein, this entanglement is patently absurd. In order for one photon to respond in accordance with the other, information must necessarily pass between the two. And since the theory doesn’t predict that, that’s because it’s false. Or at least incomplete…

Niels Bohr and Albert Einstein debated for decades about the meaning of quantum mechanics. Credit: P. Ehrenfest

John Bell’s breakthrough

Years pass, and when Einstein dies in 1955, no one cares much about this fundamental objection. Quantum physics works after all. Since the 1920s, it has enabled spectacular advances in our mastery of matter and light. A revolution! So it doesn’t matter if it seems magical…

In 1964, however, a young Northern Irish physicist named John Bell brought the question to life. In a second-rate magazine – because being interested in this subject was not really seen at the time – he reveals the principle of a new experiment which should finally make it possible to choose between Einstein and Bohr. On paper, it’s simple: it’s a kind of logic game where the probability of winning can never exceed 75% unless quantum entanglements come into play. But in reality it is extremely difficult to implement. Out of reach in all cases of the technology of the time.

The time for experiments

It was in 1972 that John Clauser and Stuart Freedman (who died in 2012) for the first time managed to develop a Bell experiment, or rather a variant. “This experiment seemed to confirm the reality of quantum entanglement, but it did not make it possible to be sure that no information had been exchanged between the entangled photons” explains Nicolas Brunner (Associate Professor in the Department of Applied Physics at the University of Geneva ).

Alain Aspect in front of his experience in 1982. Credit: Lars Becker-Larsen

Alain Aspect conducted in 1982 in the Orsay Bell experiment which provided quasi-definitive evidence, confirmed and reconfirmed since, for the reality of entanglement. “Today it is still one of the strangest and most mysterious aspects of quantum physics,” emphasizes the Swiss physicist.

A revolution underway

But quantum entanglement has also become a tool. Under the remarkable impulse of Anton Zeilinger, to whom we owe new fundamental tests of this property, but also its first practical application (in cryptography or as a basis for new communication protocols), it is the engine of a new technological revolution. “The field of quantum information is booming,” confirms Alexeï Grinbaum. For him, it is also a paradigm shift, which this Nobel crowns: “if physicists who worked with quantum communication, cryptography or quantum computers were often looked down upon 20 years ago, this is no longer the case! he congratulates himself. Quantum communication networks are at being set up all over Europe, unbreakable cryptography protocols have been tested in circuits by China (thanks to Jian-Wei Pan, a former student of Zeilinger in Vienna), and more generally “a huge competition has been launched around the world on quantum technologies ,” emphasizes Serge Haroche.

Bohr was therefore right, but in revealing a fundamental aspect of quantum physics, Einstein was not wrong! Alain Aspect, John Clauser and Anton Zeilinger will receive their Nobel Prize in Stockholm on 10 December.

Leave a Comment