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.

Frenchman Alain Aspect, American John Clauser and Austrian Anton Zeilinger have each conducted groundbreaking experiments using entangled quantum states, where two particles behave like a single unit even when they are separated. Their results have cleared the way for new technology based upon quantum information.

Clauser, Aspect, and Zeilinger have figured in Nobel speculation for more than a decade. In 2010 they won the Wolf Prize in Israel, seen as a possible precursor to the Nobel.

Control particles that are in entangled states

• 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. What happens to one of the particles in an entangled pair determines what happens to the other particle, even if they are far apart.

• Albert Einstein talked about spooky action at a distance and Erwin Schrödinger 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.

• For a long time, the question was whether the correlation was because the particles in an entangled pair contained hidden variables, instructions that tell them which result they should give in an experiment. 

• In the 1960s, John Stewart Bell developed the mathematical inequality that is named after him. This states that if there are hidden variables, the correlation between the results of a large number of measurements will never exceed a certain value. However, quantum mechanics predicts that a certain type of experiment will violate Bell’s inequality, thus resulting in a stronger correlation than would otherwise be possible.

• John Clauser developed John Bell’s ideas, leading to a practical experiment. When he took the measurements, they supported quantum mechanics by clearly violating a Bell inequality. This means that quantum mechanics cannot be replaced by a theory that uses hidden variables.

• Some loopholes remained after John Clauser’s experiment. Alain Aspect developed the setup, using it in a way that closed an important loophole. He was able to switch the measurement settings after an entangled pair had left its source, so the setting that existed when they were emitted could not affect the result.

• Using refined tools and a long series of experiments, Anton Zeilinger started to use entangled quantum states. Among other things, his research group has demonstrated a phenomenon called quantum teleportation, which makes it possible to move a quantum state from one particle to one at a distance.

The era of quantum information

• The ineffable effects of quantum mechanics are starting to find applications. There is now a large field of research that includes quantum computers, quantum networks and secure quantum encrypted communication.

• 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.

• 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 kilometres of optical fibre, 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 particles.

Quantum teleportation

• 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.

EPR paradox and Bell’s inequality

• Quantum mechanics allows a single quantum system to be divided up into parts that are separated from each other but which still act as a single unit. This goes against all the usual ideas about cause and effect and the nature of reality. How can something be influenced by an event occurring somewhere else without being reached by some form of signal from it? A signal cannot travel faster than light – but in quantum mechanics, there does not seem to be any need for a signal to connect the different parts of an extended system.

• Albert Einstein regarded this as unfeasible and examined this phenomenon, along with his colleagues Boris Podolsky and Nathan Rosen. They presented their reasoning in 1935: quantum mechanics does not appear to provide a complete description of reality. This has come to be called the EPR paradox, after the researchers’ initials.

• The Northern Irish physicist John Stewart Bell, who worked at CERN, the European particle physics laboratory, took a closer look at the problem. He discovered that there is a type of experiment that can determine whether the world is purely quantum mechanical, or whether there could be another description with hidden variables. If his experiment is repeated many times, all theories with hidden variables show a correlation between the results that must be lower than, or at most equal to, a specific value. This is called Bell’s inequality.

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