Nobel Physics Prize explainer: Shedding light on entangled photons and their applications in quantum technologies

Physics gets weird in the tiny world of atoms and particles. This year’s Physics’ Nobel laureates have explored ‘quantum entanglement’ in photons to open up a whole array of potential applications of entanglement.

For example, developments in quantum computing are set to revolutionise the way in which we are able to process information.

One of the Nobel winners (who will share one-third of the US$1m prize), Anton Zeilinger, 77, a physics professor at Austria’s University of Vienna, told BBC News: “I was always interested in quantum mechanics from the very first moments when I read about it. And I actually was struck by some of the theoretical predictions because they did not fit the usual intuitions that one might have.”

Quantum entanglement, referred to by Einstein as “spooky action at a distance,” is a bizarre effect where the states of two “entangled” particles are linked no matter how far apart they get.

But neither particle exists solely in a single state. Each particle’s position, momentum, and other physical observables are undetermined until measured. That is, they live in what’s called a “superposition” of states all at once. Only once measured does the particle “collapse” into a single state – which state the particle collapses into is determined by the probability of the particle being in that state.

Read more: Quantum entanglement of many atoms observed for the first time

Quantum mechanics vs. hidden variables

Imagine two normal balls as stand-ins for particles. One is white, the other is black. A machine spits the balls out in opposite directions to two people (let’s call them Alice and Bob). If Bob catches the black ball, he immediately knows that Alice has the white ball. This is known as the “hidden variables” description – the balls contained “hidden” information about which colour to show.

However, in quantum mechanics, both balls are grey when they are spit out of the machine. Only when caught does one ball turn white and the other black.

These two descriptions – quantum mechanics vs. hidden variables – can be tested to see which is correct.

In 1997, Zeilinger’s team created an entangled pair of particles, A and B. Shooting the pair in opposite directions, particle A meets a third particle, C. Entangling with the new particle, A and C are in a new shared state. While particle C has lost its uniqueness, its identifying properties are transferred to particle B through its entanglement with particle A.

This mind-boggling effect is called quantum transportation and is incompatible with a “hidden variable” approach.

Zeilinger and his colleagues were the first to conduct experiments of this type.

Bell inequalities

Zeilinger’s quantum transportation experiments built upon theoretical work done decades earlier.

Physicist John Stewart Bell in 1964 wrote a paper in response to a 1935 paper co-authored by Albert Einstein, Boris Podolsky and Nathan Rosen. The 1935 paper presented a paradox to argue that quantum physics is an “incomplete” theory and posited the “hidden variables” alternative.

Einstein and co argued that, if changes to one entangled particle can influence the other instantaneously regardless of distance, then either (a) there is some interaction between the particles which travels faster than the speed of light (which is impossible), or (b) there is an unmeasured property of the particles predetermining their measured states (hidden variables).

Read more: Quantum entanglement can be used to encrypt messages – making data more secure

Bell wondered what would happen if measurements were taken on each of the entangled particles independently. Hidden variables suggests that the correlation between the measurements is mathematically constrained. This constraint would later be called the “Bell inequality.”

Such a hypothetical constraint would be localised to the immediate surroundings of the particle. Bell himself declared in his book Speakable and Unspeakable in Quantum Mechanics that if a hidden-variable theory “is local it will not agree with quantum mechanics, and if it agrees with quantum mechanics it will not be local.”

The first rudimentary experimental corroborating Bell’s theorem challenging the Einstein-Podolsky-Rosen paradox were performed in 1972 by Clauser and colleagues. This was followed up with further experiments in the 1980s led by Aspect.

It turns out even Einstein was wrong sometimes (or so it seems).

So, why do we care?

This might all seem a bit like theoretical physics in-fighting.

In fact, that experiments run by Clauser, Aspect and Zeilinger show that nature seems to agree with quantum mechanics does have real implications. These and similar experiments in recent years laid the foundation for the flurry of interest and research into quantum information.

The prize was awarded “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science,” highlighting the importance of verifying this theoretical work experimentally in the development of new technologies.

It’s because of quantum entanglement that the storage and transfer of quantum information and use of algorithms for quantum encryption are possible. Developments in quantum computing are set to revolutionise the way in which we are able to process information.

Read more: Australian researchers develop a coherent quantum simulator

Quantum computers are predicted to be thousands, even millions of times faster and more powerful than modern classical computers. Quantum encryption may be unhackable. Systems with more than two entangled particles, pioneered by Anton Zeilinger and his team, are now in use.

“This is useful for military and banking, in secure communications,” Clauser told the BBC. “The biggest application to my knowledge is the Chinese who launched a satellite several years ago that they use for secure communications over thousands of kilometres.”

Decades of work, and much more to be done

Quantum information science is a “vibrant and rapidly developing field,” says Eva Olsson, a member of the Nobel Committee for Physics. “Its predictions have opened doors to another world, and it has also shaken the very foundation of how we interpret measurements.”

At the press conference where the announcement of his receipt of the award was announced, Zeilinger stressed that the award should animate the next generation of physicists. “This prize is an encouragement to young people – the prize would not be possible without more than 100 young people who worked with me over the years.”

For all those budding physicists out there struggling with quantum physics, Clauser’s own words to the Washington Post upon receiving the award might give you some hope.

“This is all for work I did more than 50 years ago,” Clauser says. “I was struggling to try to understand quantum mechanics, unsuccessfully. Didn’t understand what I didn’t understand.”

The third physicist to share in the award is Alain Aspect, 75, is a professor emeritus in physics at the Université Paris-Saclay and École Polytechnique in France.

You can read more about the physics Nobel prize announcement here.

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