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 have laid the foundations 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-Swedish-kronor (US$915,000) 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 prize would not be possible without the work of more than 100 young people over the years.”
Aspect, a physicist at the University of Paris-Saclay, received the call during a committee meeting. “I happened to be sitting near Aspect this morning when he got the call,” says Serge Haroche, an experimental physicist at the Collège de France in Paris who shared the 2012 Nobel Prize in Physics for work in quantum physics. When he left the room, Haroche added, Aspect’s colleagues guessed correctly that it was Stockholm calling.
The trio’s experiments proved that connections between quantum particles were not down to local ‘hidden variables’, unknown factors that invisibly tie the two outcomes together. Instead, the phenomenon comes from a genuine association in which manipulating one quantum object affects another far away. German physicist Albert Einstein famously called the phenomenon ‘spooky action at a distance’ — it is now known as quantum entanglement.
All three winners are pioneers of 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. The recognition was long overdue, Pan says. “We have been waiting for this for a very, very long time.”
The win is “beautiful news” agrees Chiara Marletto, a theoretical physicist at the University of Oxford, UK. The modern versions of the experiments pioneered by the three winners could be central to one of the great open questions of physics today, she says — how to reconcile quantum mechanics with Albert Einstein’s general theory of relativity.
Because of the effects of quantum entanglement, measuring the property of one particle in an entangled pair immediately affects the results of measurements on the other. It is what enables quantum computers to function: these machines, which seek to harness quantum particles’ ability to exist in more than one state at once, carry out calculations that would be impossible on a conventional computer. Today, physicists are using entanglement to develop quantum encryption and a quantum internet that would allow for ultrasecure communications and new kinds of sensors and telescopes.
But whether particles could be fundamentally linked in this way — such that measuring one determines the properties of another, rather than just revealing a predetermined state — had been a topic 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. This said that experimental results that seemed to be correlated beyond a particular value would be possible only through quantum entanglement, rather than being due to certain kinds of hidden variable. Quantum mechanics predicts a higher degree of correlation than would be possible in classical, or pre-quantum, physics.
In 1972, Clauser — now a physicist at J.F. Clauser & Associates in Walnut Creek, California — and his colleagues developed these ideas into a practical experiment that violated the Bell inequality, supporting the theories of quantum mechanics.
David Kaiser, a quantum physicist and historian of science at the Massachusetts Institute of Technology in Cambridge, says that Clauser had come across Bell’s work by chance while browsing in the library at Columbia University in New York City, where he was a PhD student. Clauser was captivated, and he wrote to Bell to ask him whether anyone had tried testing his inequalities experimentally. Bell replied that no one had — and encouraged him to do so. The reaction from the rest of the community wasn’t so warm, however. “People would say, in writing, that this isn’t real physics — that the topic isn’t worthy,” says Kaiser.
Loopholes and teleportation
Despite Clauser’s success, experimental loopholes 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 changing set-up that meant that experimental decisions could not be said to be predetermining the results.
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 gets transmitted from one location to another. Quantum systems cannot be detected and reconstituted somewhere else, because measurement destroys their delicate quantum properties. But a state can be transferred between two particles at a distance, if each is entangled with half of a previously entangled pair.
Teleportation allows for supersecure communications, because any eavesdropping would cause particles to lose their delicate quantum states. It might also enable future quantum computers to transfer information. Since Zeilinger’s initial experiments, physicists have succeeded in teleporting electrons, as well as atoms and superconducting circuits.
In more recent experiments, Zeilinger, together with Kaiser and other collaborators, has sought to seal further loopholes in tests of Bell’s inequality by using properties of starlight emitted billions of years ago to define experimental settings.
Although the physics is now the basis of a budding industry, these kinds of experiment could continue to provide insights into fundamental physics. One hope, says Marletto, is that they will show whether two particles can become entangled through a gravitational interaction. General relativity is apparently incompatible with quantum mechanics, and such experiments could provide hints on how to develop a quantum theory of gravity to replace it. “Gravity is the elephant in the room,” says Marletto.
Zeilinger “often anticipated the strangest and most counter-intuitive phenomena” in quantum physics, says Gabriela Barreto Lemos, a physicist at the Federal University of Rio de Janeiro in Brazil, who recalls warmly her time as a postdoctoral researcher in Zeilinger’s lab. “Whenever we presented him with new ideas, he would challenge us to go further, think more outside the box, be more imaginative,” she says.
Kaiser credits the three Nobel recipients with having had the persistence and ingenuity to probe what seem like “fantastical” phenomena, and to ask: “Can the world really work like this?”
“At the time, it was just blue-sky research, with no applications in view,” says Haroche. “It’s a wonderful example of the connection between basic science and application,” he adds. “A demonstration of the usefulness of useless knowledge.”