Particle physics experiments

‘Scary’ Quantum Entanglement Experiments Win Nobel Prize in Physics

John Clauser (left), Anton Zeilinger and Alain Aspect won this year’s Nobel Prize in Physics for their research into quantum entanglement.Credit: J. Clauser (CC BY-SA 4.0), Matthias Röder/dpa/Alamy, The Royal Society (CC BY-SA 3.0)

Three quantum physicists have won the 2022 Nobel Prize in Physics for their experiments with entangled photons, in which particles of light become inextricably linked. Such experiments laid the foundation for an abundance of quantum technologies, including quantum computers and communications.

Alain Aspect, John Clauser and Anton Zeilinger will each share one-third of the 10 million crown ($915,000 US) prize.

“I was actually very surprised to get the call,” said Zeilinger, a physicist at the University of Vienna, at the press conference announcing the award. “This award would not be possible without the work of over 100 young people over the years.”

Aspect, a physicist at the University of Paris-Saclay, received the call during a committee meeting. “I was sitting near Aspect this morning when he got the call,” says Serge Haroche, an experimental physicist at the College de France in Paris, who shared the 2012 Nobel Prize in Physics for his work in quantum physics. When he left the room, Haroche added, Aspect’s colleagues correctly guessed it was Stockholm calling.

The trio’s experiments proved that the connections between quantum particles were not due to local “hidden variables,” unknown factors that invisibly link the two outcomes. Instead, the phenomenon stems from a true association in which the manipulation of one quantum object affects another far distant. German physicist Albert Einstein called the phenomenon “frightening action at a distance” – it is now known as quantum entanglement.

All three winners are pioneers in the fields of quantum information and quantum communications, says Pan Jianwei, a physicist at the University of Science and Technology of China in Hefei who participated in some of Zeilinger’s landmark experiments as a graduate student in the 1990s. Recognition was long overdue, Pan says. “We’ve been waiting for this for a very, very long time.”

The victory is “beautiful news”, recognizes Chiara Marletto, a theoretical physicist at the University of Oxford, in the United Kingdom. Modern versions of the experiments developed by the three laureates could be at the heart of one of the big open questions in physics today, she says – how to reconcile quantum mechanics with Einstein’s theory of general relativity. .

Particle pairs

Due to the effects of quantum entanglement, measuring the property of one particle in one entangled pair immediately affects the results of measurements of the other. This is what allows quantum computers to work: these machines, which seek to exploit the ability of quantum particles to exist in several states at the same time, perform calculations that would be impossible on a conventional computer. Today, physicists are using entanglement to develop quantum encryption and a quantum internet that would enable ultra-secure communications and new types of sensors and telescopes.

But whether the particles could be fundamentally linked in this way – so that the measurement of one determines the properties of the other, rather than just revealing a predetermined state – has been a matter of debate since physicists laid the foundations of quantum mechanics in the 1920s.

In the 1960s, physicist John Bell proposed a mathematical test, known as Bell’s inequality, to distinguish between the two ideas. This test stated that experimental results that appeared to be correlated beyond a particular value would only be possible through quantum entanglement, rather than being due to some type of hidden variable. Quantum mechanics predicts a higher degree of correlation than would be possible in classical or pre-quantum physics.

In 1972, John Clauser – now a physicist at JF Clauser & Associates in Walnut Creek, California – and his colleagues developed these ideas into a practical experiment that violated Bell’s inequality, supporting theories of quantum mechanics.

David Kaiser, a quantum physicist and historian of science at Cambridge’s Massachusetts Institute of Technology, said Clauser came across Bell’s work by chance while browsing the library at Columbia University in New York, where he was a doctoral student. . Clauser was captivated, and he wrote to Bell asking if anyone had tried to test his inequalities experimentally. Bell replied that no one had – and encouraged him to do so. The reaction from the rest of the community, however, has not been so warm. “People would say, in writing, that it’s not real physics — that the subject isn’t worth it,” Kaiser says.

Loopholes and teleportation

Despite Clauser’s success, experimental shortcomings remained that left room for hidden variables to create the illusion of quantum entanglement. It was these loopholes that Aspect set out to close in the 1980s. His experiments used a shifting setup, which meant that experimental decisions could not be seen as predetermining outcomes.

And in 1997, Zeilinger and his colleagues at the University of Vienna used the phenomenon of entanglement to demonstrate quantum teleportation, in which a quantum state is transmitted from one place to another. Quantum systems cannot be detected and reconstructed elsewhere, because measurement destroys their delicate quantum properties. But a state can be transferred between two distant particles, if each is entangled with half of a previously entangled pair.

Teleportation allows for super secure communications, as any eavesdropping would cause the particles to lose their delicate quantum states. It could also allow future quantum computers to transfer information. Since Zeilinger’s first experiments, physicists have succeeded in teleporting electrons, but also atoms and superconducting circuits.

In more recent experiments, Zeilinger, along with Kaiser and other collaborators, have sought to fill other gaps in tests of Bell’s inequality by using the properties of starlight emitted billions of years ago. years to define the experimental parameters.

Although physics is now the basis of a fledgling industry, such experiments could continue to provide insight into fundamental physics. One hope, Marletto says, is that they will show whether two particles can become entangled through gravitational interaction. General relativity is apparently incompatible with quantum mechanics, and such experiments could provide clues as to how to develop a quantum theory of gravity to replace it. “Gravity is the elephant in the room,” says Marletto.

Kaiser credits the three Nobel laureates with the persistence and ingenuity to probe what appear to be “fantastic” phenomena and ask, “Can the world really work like this?”

“At the time, it was just ephemeral research, with no application in sight,” says Haroche. “It’s a wonderful example of the link between basic science and application,” he adds. “A demonstration of the usefulness of useless knowledge.”