Using groundbreaking experiments, Alain Aspect (French), John Clauser (American) and Anton Zeilinger (Austrian) have demonstrated the potential to investigate and control particles that are in entangled states. What happens to one particle in an entangled pair determines what happens to the other, even if they are really too far apart to affect each other.
The laureates’ development of experimental tools has laid the foundation for a new era of quantum technology. Scientists Alain Aspect, John Clauser and Anton Zeilinger won the 2022 Nobel Prize in Physics for "experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science", the award-giving body said on Tuesday the October 4, 2022. The more than century-old prize, worth 10 million Swedish crowns ($902,315), is awarded by the Royal Swedish Academy of Sciences.
The physics prize has often taken centre stage among the awards, featuring household names of science such as Albert Einstein, Max Planck, Pierre Curie and Marie Curie, and rewarding breakthroughs that have reshaped how we see the world. Last year, scientists Syukuro Manabe, Klaus Hasselmann and Giorgio Parisi shared the physics prize for their work on complex physical systems such as Earth's changing climate, key in understanding global warming.
The fundamentals of quantum mechanics are not just a theoretical or philosophical issue. Intense research and development are underway to utilise the special properties of individual particle systems to construct quantum computers, improve measurements, build quantum networks and establish secure quantum encrypted communication.
Many applications rest upon how quantum mechanics allow two or more particles to exist in a shared state, regardless of how far apart they are. This is called entanglement, and has been one of the most debated elements of quantum mechanics ever since the theory was formulated.
Albert Einstein talked about spooky action at a distance and Erwin Schrodinger said it was quantum mechanics’ most important trait. This year’s laureates have explored these entangled quantum states, and their experiments laid the foundation of the revolution currently underway in quantum technology. When two particles are in entangled quantum states, someone who measures a property of one particle can immediately determine the result of an equivalent measurement on the other particle, without needing to check. What makes quantum mechanics so special is that its equivalents to the balls have no determined states until they are measured. It is as (considering two balls) if both the balls are grey, right up until someone looks at one of them.
Then, it can randomly take either all the black the pair of balls has access to, or can show itself to be white. The other ball immediately turns the opposite colour. But how is it possible to know that the balls did not each have a set colour at the beginning? Even if they appeared grey, perhaps they had a hidden label inside, saying which colour they should turn when someone looks at them. Quantum mechanics’ entangled pairs can be compared to a machine that throws out balls of opposite colours in opposite directions.
When Bob catches a ball and sees that it is black, he immediately knows that Alice has caught a white one. In a theory that uses hidden variables, the balls had always contained hidden information about what colour to show. However, quantum mechanics says that the balls were grey until someone looked at them, when one randomly turned white and the other black. Bell inequalities show that there are experiments that can differentiate between these cases. Such experiments have proven that quantum mechanics’ description is correct.
An important part of the research being rewarded with this year’s Nobel Prize in Physics is a theoretical insight called Bell inequalities. Bell inequalities make it possible to differentiate between quantum mechanics’ indeterminacy and an alternative description using secret instructions, or hidden variables. Experiments have shown that nature behaves as predicted by quantum mechanics. The balls are grey, with no secret information, and chance determines which becomes black and which becomes white in an experiment.
Entangled quantum states hold the potential for new ways of storing, transferring and processing information. Interesting things happen if the particles in an entangled pair travel in opposite directions and one of them then meets a third particle in such a manner that they become entangled. They then enter a new shared state. The third particle loses its identity, but its original properties have now been transferred to the solo particle from the original pair. This way of transferring an unknown quantum state from one particle to another is called quantum teleportation. This type of experiment was first conducted in 1997 by Anton Zeilinger and his colleagues. Remarkably, quantum teleportation is the only way to transfer quantum information from one system to another without losing any part of it.
It is absolutely impossible to measure all the properties of a quantum system and then send the information to a recipient who wants to reconstruct the system. This is because a quantum system can contain several versions of every property simultaneously, where each version has a certain probability of appearing during a measurement.
As soon as the measurement is conducted, only one version remains, namely the one that was read by the measuring instrument. The others have disappeared and it is impossible to ever know anything about them. However, entirely unknown quantum properties can be transferred using quantum teleportation and appear intact in another particle, but at the price of them being destroyed in the original particle.
Once this had been shown experimentally, the next step was to use two pairs of entangled particles. If one particle from each pair are brought together in a particular way, the undisturbed particles in each pair can become entangled despite never having been in contact with each other. This entanglement swapping was first demonstrated in 1998 by Anton Zeilinger’s research group.
John Clauser became interested in the fundamentals of quantum mechanics as a student in the 1960s. He could not shake of John Bell’s idea once he had read about it and, eventually, he and three other researchers were able to present a proposal for a realistic type of experiment that can be used to test a Bell inequality. Being able to manipulate and manage quantum states and all their layers of properties gives us access to tools with unexpected potential. This is the basis for quantum computation, the transfer and storage of quantum information, and algorithms for quantum encryption. Systems with more than two particles, all of which are entangled, are now in use, which Anton Zeilinger and his colleagues were the first to explore.
John Clauser used calcium atoms that could emit entangled photons after he had illuminated them with a special light. He set up a filter on either side to measure the photons’ polarisation. After a series of measurements, he was able to show they violated a Bell inequality.
Alain Aspect developed this experiment, using a new way of exciting the atoms so they emitted entangled photons at a higher rate. He could also switch between different settings, so the system would not contain any advance information that could affect the results.
Anton Zeilinger later conducted more tests of Bell inequalities. He created entangled pairs of photons by shining a laser on a special crystal, and used random numbers to shift between measurement settings. One experiment used signals from distant galaxies to control the filters and ensure the signals could not affect each other.
These increasingly refined tools bring realistic applications ever closer. Entangled quantum states have now been demonstrated between photons that have been sent through tens of kilometers’ of optical fiber, and between a satellite and a station on the ground.
In a short time, researchers around the world have found many new ways to utilise the most powerful property of quantum mechanics. The first quantum revolution gave us transistors and lasers, but we are now entering a new era thanks to contemporary tools for manipulating systems of entangled.
The trio’s discoveries are “potentially going to change our world in terms of really practical things, like being able to do quantum computing; solutions that will help us with everything from vaccines, to tech, to weather prediction. “There are just so many different types of computations that we can do through quantum information science that we can’t do with classical computers’’.