Physics gets weird in the microscopic world of atoms and particles. This year’s Nobel laureates in physics have explored “quantum entanglement” in photons to open up a whole range of possible applications of entanglement.
For example, advances in quantum computing are set to revolutionize the way we can process information.
One of the Nobel laureates (who will share a third of the $1 million prize), Anton Zeilinger, 77, a professor of physics at Austria’s University of Vienna, told BBC News: “I’ve always been interested in quantum mechanics since first moment. moments when I read about it. And I was really struck by some of the theoretical predictions because they didn’t fit the usual intuitions that one might have.”
Quantum entanglement, referred to by Einstein as “spooky action at a distance,” is a strange phenomenon where the states of two “entangled” particles are linked no matter how far apart they are.
But none of the particles exist in just one state. The position, momentum, and other physical observables of each particle are undetermined until they are measured. That is, they live in what is called a “superposition” of states simultaneously. Only when measured does the particle “collapse” into a single state – the state to which the particle collapses is determined by the probability of the particle being in that state.
read more: Quantum entanglement of many individuals observed for the first time
Quantum mechanics vs. hidden variables
Imagine two regular balls as a basis for particles. One is white, the other is black. A machine spits balls 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 a “hidden variable” description – the balls contained “hidden” information about the color to be displayed.
However, in quantum mechanics, both balls are gray when they are spat out of the machine. Only when one ball is caught does it 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 group created an entangled pair of particles, A and B. Shooting the pair in opposite directions, particle A encounters a third particle, C. By entangling with the new particle, A and C are in a new common situation. While particle C has lost its uniqueness, its identifying properties are transferred to particle B through its entanglement with particle A.
This impressive effect is called quantum transport and is not compatible with a “hidden variable” approach.
Zeilinger and his colleagues were the first to perform experiments of this type.
Zeilinger’s quantum transport experiments were based on 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 “hidden variables” as an alternative.
Einstein and his colleagues argued that, if changes in one entangled particle can affect the other instantaneously regardless of distance, then either (a) there is some interaction between particles traveling faster than the speed of light (which is impossible ) or (b ) there is an unmeasured property of the particles that predetermines 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 made on each of the entangled particles independently. Latent variables imply that the correlation between measures is mathematically constrained. This constraint would later be called “Bell’s inequality”.
Such a hypothetical constraint would be found in the immediate environment of the particle. Bell himself stated 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 confirmation of Bell’s theorem challenging the Einstein-Podolsky-Rosen paradox was performed in 1972 by Clauser and his colleagues. This was followed by further experiments in the 1980s led by Aspect.
It turns out that even Einstein was wrong sometimes (or so it seems).
So why do we care?
All this may seem a bit like a theoretical physics controversy.
In fact, that the experiments done by Clauser, Aspect, and Zeilinger show that nature seems to agree with quantum mechanics has real implications. These and similar experiments in recent years laid the foundation for the great interest and research in quantum information.
The prize was awarded “for experiments with entangled photons, which establish the violation of Bell inequalities and the pioneering science of quantum information”, underscoring the importance of experimental verification of this theoretical work in the development of new technologies.
Because of quantum entanglement it is possible to store and transfer quantum information and use algorithms for quantum encryption. Advances in quantum computing are set to revolutionize the way we can process information.
read more: Australian researchers develop a coherent quantum simulator
Quantum computers are predicted to be thousands and even millions of times faster and more powerful than modern classical computers. Quantum encryption may not be broken. Systems with more than two entangled particles, pioneered by Anton Zeilinger and his team, are now in use.
“This is useful for the military and banking, in secure communications,” Clauser told the BBC. “The biggest application that I know of is the Chinese launched a satellite several years ago that they use for secure communications thousands of kilometers away.”
Decades of work, and much more to be done
Quantum information science is a “lively and rapidly developing field,” says Eva Olsson, a member of the Nobel Committee for Physics. “His predictions have opened doors to another world and also shaken the very foundations of how we interpret measurements.”
At the press conference announcing the award, Zeilinger emphasized that the award should inspire the next generation of physicists. “This award is an encouragement to young people – the award would not be possible without more than 100 young people who have worked with me over the years.”
For all those budding physicists out there struggling with quantum physics, Clauser’s own words in Washington Post after receiving the award it may give you some hope.
“This is all for work I did over 50 years ago,” Clauser says. “I struggled trying to understand quantum mechanics, unsuccessfully. I didn’t understand what I didn’t understand.”
The third physicist who participated in the award is Alain Aspect75, is an emeritus professor of physics at the Université Paris-Saclay and École Polytechnique in France.
You can read more about the announcement of the Nobel Prize in Physics here.