Nobel Prize in Physics for 2022 to three scientists in the field of quantum entanglement

The researchers were pioneers in the study of quantum entanglement. All three won the 2010 Wolff Prize for Physics, thus continuing the tradition of the Wolff Prize as a Nobel predictor.

The 2022 Nobel Prize in Physics was awarded to three scientists working in the field of quantum mechanics: Alain Aspect, John P. Clauser and Anton Zeilinger, thanks to "experiments with entangled photons that provide a violation of Bell's law ( Bell's theorem) while establishing the science of quantum information".

All three won the 2010 Wolf Prize for Physics. This proved once again that the Wolf Prize became a Nobel predictor.

Quantum entanglement (Wikipedia)

Concentration of information on quantum entanglement on the knowledge site

Bell's theorem

In physics, Bell's theorem is a general name for a family of results that prove that quantum mechanics contradicts the simple sense of the principle of locality. The principle of locality in this sense is the assumption that a measurement carried out in one place in space cannot immediately affect the result of a measurement carried out in another place in space.

Alan Espa is a physicist who deals with quantum theory, winner of the Wolf Prize in Physics for 2010. He is best known for experiments he conducted in 1981. The results of the experiment were a confirmation of Bell's theorem and quantum theory and refuted the claim of Albert Einstein, Boris Podolsky and Nathan Rosen that quantum theory is incomplete and must be replaced by theories Local hidden variables. Date of birth: June 15, 1947 (age 75 years), Agen, France. Awards: Holwalk Prize (1991), Wolf Prize in Physics (2010), Albert Einstein Medal (2012), Fellow of the Royal Society (2015);

John P. Clauser (John Clauser). From the reasons of the Wolff Prize committee: "For the conceptual achievements and experimental work that contributed to the foundations of quantum physics, and especially for experiments that have become more and more sophisticated in examining the correctness of Bell inequalities or their extensions while exploiting entangled quantum states." The strange properties of entangled quantum states, first noted by Einstein, Podolsky and Rosen who suspected that quantum mechanics was not a perfect theory, were first dramatically outlined in the seminal work of John Bell. Bell showed that certain statistical correlations between properties of two distant particles produced in an entangled state cannot be explained by a theory based on local deterministic states even if there are additional unmeasurable properties ('hidden variables'). The series of experiments with entangled photons, which this award distinguishes, began with the work of John Clauser (b. 1942, USA) and his collaborators, who showed how to apply Bell's inequality to a particular practical experiment, in which they produced two entangled photons and measured correlations between their properties on by two detectors far from each other. This groundbreaking attempt demonstrated consistency with the expectations of quantum mechanics and eliminated the possibility of local deterministic states. Since it was still possible to interpret the results of the experiment as arising from macroscopic classical states that existed before the entangled photons were created, Alain Aspe (b. 1947, France) and his collaborators conducted additional experiments. At the beginning, a larger number of correlations were measured in a single experiment, thereby increasing the accuracy of the measurement. Later, in another experiment, the properties of the detectors (the polarization direction of the polarizers placed in front of the detectors) were determined only after the entangled photon pairs were created, while they were on their way to the detectors. In this experiment too, Bell's inequality was violated, as expected, by quantum mechanics."

Anton Zeilinger (born 1945, Austria) (Anton Zeilinger) and his partners added another factor. In one experiment, a random factor was added to determine the properties of the detectors. In addition, following a theoretical proposal by Greenberger, Horn and Zeilinger, another experiment was performed with three entangled particles (instead of two entangled particles) which showed, even more unequivocally, the contradiction between the prediction of quantum mechanics and that of local deterministic states.

Allen collected (Alain Aspect)

University of Paris-Saclay and Ecole Polytechnique, Palaiseau, France

John P. Clouser (John Clauser)

JF Clauser & Assoc., Walnut Creek, California, USA

Anton Zeilinger (anton zeilinger),

University of Vienna, Austria

Intertwined situations - from theory to technology

Each of the three winners of the 2022 Nobel Prize in Physics performed groundbreaking experiments using entangled quantum states, in which two particles behave as if they were a single unit even when separated and far apart. Their findings paved the way for the development of innovative technologies based on quantum information.

The indescribable implications of quantum mechanics are beginning to reach technological applications. Today there is an extensive field of research that includes quantum computers, quantum networks and encrypted and secure quantum communication. A key element in all this development is the answer to the question of how quantum mechanics allows two or more particles to exist in what is known as an 'entangled state'. What happens to one of the particles in the entangled pair determines what happens to the other particle, even if they are far apart. 

For a long time, the question was whether the correlation between them is due to the fact that the particles in the intertwined pair include hidden variables, a kind of 'instructions' that direct them to the required result in the experiment. In the sixties of the last century, John Stewart Bell developed the mathematical inequality named after him. This law states that: "If latent variables exist, the correlation between the results of a large number of experiments will never exceed a specified value". However, quantum mechanics predicts that a certain experiment will be able to disprove Bell's theorem, that is, there may be a stronger correlation than predicted. 

John P. Clauser (John Clauser) developed Bell's ideas that led to a practical experiment. When he analyzed the results of the experiments, they supported quantum mechanics in clear violation of Bell's theorem. This finding states that quantum mechanics cannot be replaced by a theory that uses hidden variables.

A number of loopholes still remain after Klauser's experiment. Alain Aspect developed an innovative testing system that prevented these loopholes. He managed to change the measurement data after the twisted pair had already left its source, so that the original data in the system could no longer affect the outcome of the experiment.

Using more advanced means and a long series of experiments, Anton Zeilinger began to investigate entangled quantum states. Among other things, his research group demonstrated a quantum phenomenon called 'quantum teleportation', which allows the quantum state of one particle to be transferred to another particle located elsewhere.

"It became increasingly clear that a completely new type of quantum technology was emerging. We can see that the research of this year's Nobel laureates in physics in the field of entangled states is of great importance, even beyond the fundamental questions regarding the interpretation of quantum mechanics", says Anders Irbäck, chairman of the Nobel Committee for Physics.

How quantum entanglement became a powerful tool

Using groundbreaking experiments, Alain Aspect, John P. Clauser, and Anton Zeilinger demonstrated the potential to study and control particles in entangled states. What happens to one particle that is part of an entangled pair determines what happens to the other particle in the pair, even if they are far apart. The development of the experimental tools of the three laureates of the 2022 Nobel Prize in Physics laid the foundation for a new era of quantum technology.

The foundations of quantum mechanics are not just theory or thought. Vigorous research and development are currently being used in a direct way to achieve use of the special properties of a system that includes distinct particles in order to develop quantum computers, improve measurements, build quantum networks and develop secure and encrypted quantum communication.

Many applications are based on the question of how quantum mechanics allows two or more particles to exist in a common state, regardless of the distance between them. This phenomenon is called 'entanglement', and it has been one of the most controversial issues in the field of quantum mechanics since the formulation of this theory. Albert Einstein spoke of "strange activity" at distances and Erwin Schrödinger stated that this is the most important virtue in the field of quantum mechanics.

This year's Nobel Prize winners investigated these entangled quantum states, and their experiments laid the foundations for the scientific revolution taking place today in the field of quantum technology.

Not really an everyday experiment

When two particles are in entangled states, whoever measures the property of one of the particles can immediately determine the result of a parallel measurement of the other particle, without actually conducting the experiment. At first glance, this does not seem so strange. If we think of balls instead of particles, we can imagine an experiment where a black ball is pointed in one direction and a white ball in the opposite direction. An observer who catches a ball and sees that it is white, can immediately say that the ball that went in the other direction is definitely black.  

What makes quantum mechanics special is the fact that the equivalents of spheres are without measurable states, until the moment of measurement itself. It's as if the two balls are gray in color, right up until the moment someone watches them. At this moment, each ball can randomly be black or white in color. The other ball will get the opposite color at that moment. However, how do you know if the balls were indeed a fixed color at the beginning of the experiment? Even if they looked gray, maybe there was some kind of hidden instruction inside them that told them what color they should turn into once someone was watching them.

Does color exist when no one is watching?

Entwined pairs in quantum mechanics can be compared to a machine that throws balls of opposite colors in opposite directions. When Bob catches a ball and recognizes that it is black, he immediately knows that Alice caught a white ball. In a theory where latent variables were used, the balls always contained latent information as to the color in which they would appear. However, quantum mechanics claims that both balls were gray until someone observed them, when one randomly turned white and the other black. Bell's inequality claims that there are experiments that can distinguish between these cases. Such experiments proved that quantum mechanics is correct.

An important part of the research for which this year's Nobel Prize in Physics was awarded is a theoretical insight called 'Bell inequalities'. This theorem makes it possible to distinguish between the uncertainty principle of quantum mechanics and an alternative description using secret instructions, or hidden variables. Experiments have proven that nature behaves as predicted by quantum mechanics. The balls are gray in color, without any hidden information, and only absolute randomness determines which of them will become black and which will become white.    

The most important resource of quantum mechanics

In entangled quantum states the ability to develop new ways of storing, transferring and processing information is stored. Interesting things happen when the particles of a entangled pair move in different directions and then one of them meets a third particle with which it forms an entangled pair. At this point they enter a new joint state. The third particle loses its identity, but its original properties have now been transferred to the first particle of the original pair. This way of transferring an unknown quantum state from one particle to another is called teleportation. This type of experiment was first performed in 1997 by Anton Zeilinger and his colleagues.

Remarkably, quantum copying 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 transfer the information to a receiving party who wishes to reproduce the original system. The reason for this lies in the fact that a quantum system is able to contain several versions of each of the properties at the same time, where each version has a certain probability of appearing during the measurement.

As soon as the measurement is made, only one of the possible versions remains, namely - the one captured by the measuring device. The other versions have disappeared, and nothing can ever be known about them. However, completely unknown quantum properties can be transferred using quantum copying without changing the other particle, but at the cost of the disappearance of these properties in the original particle.

Once this phenomenon was proven in experiments, the next step was to use two pairs of intertwined particles. If one particle from each pair is brought closer in a certain way, the unmoved particles in each pair can become entangled, even though they have never been in any contact with each other. This interlacing switch was first demonstrated in 1998 by Anton Zeilinger's research group.

Entwined pairs of photons, particles of light, can be sent in opposite directions through optical fibers and used as signals in quantum networks. Interweaving two pairs can allow the distance between the nodes of such a network to be increased. Admittedly, there is a limit to the distance through which photons can travel through an optical fiber before they fade or lose their properties. Although normal light signals can be amplified along the track, this amplification is not possible when twisted pairs are used. For this purpose, an amplifier must be used that captures and measures the light, but using an amplifier breaks up the quantum entanglement. At the same time, interleaving means that it will be possible to send the original state on, while moving it over greater distances than ever before.

Entwined particles that will never meet

Two pairs of entangled particles are emitted from different sources. One particle from each of the pairs approaches the other particle in a special way that allows them to intertwine. The remaining two particles are also interwoven at this stage. In this way, two particles that have never met or been in contact with each other become an entangled pair.

From paradox to inequality

This scientific progress is based on many years of developments. It began with the mind-boggling insight that quantum mechanics allows a single quantum system to split into separate parts that will still function as a single unit. This insight contradicts all the scientific ideas we have received regarding cause and effect and the nature of reality. How can something be affected by an event happening elsewhere without some kind of signal coming from it? A signal cannot move faster than light - however in quantum mechanics it does not seem that any signal is needed to connect the various parts of the extended system. Albert Einstein considered such a phenomenon impossible and tested its feasibility, together with his colleagues Boris Podolsky and Nathan Rosen. They presented their hypothesis in 1935: "Quantum mechanics does not provide a perfect description of reality". This assertion became the EPR paradox, named after the family names of the three researchers. The question was whether there could be a more perfect description of the world, with quantum mechanics being only one part of it. This situation could, for example, function when particles always treasure hidden information about their behavior at the end of an experiment. All subsequent experiments showed that the properties exist exactly where the measurements are made. This type of information is sometimes referred to as 'local hidden variables'. Northern Irish physicist John Stewart Bell [1928–1990], who worked at the CERN particle accelerator, focused on this problem even more. He discovered that there is a type of experiment that can determine whether the world is based on quantum mechanics in its entirety, or whether an additional description containing hidden variables is possible. Repeating his experiment many times, all the theories that were supposed to contain latent variables showed a correlation between the results that had to be less than, or at most equal to, a certain value. This theory is formulated in the form of a mathematical formula called Bell's inequality. However, quantum mechanics can violate this formula. It predicts higher values ​​for the correlation between outcomes than is possible in systems containing latent variables.

John P. Clouser (John Clauser) Became intrigued by the basics of quantum mechanics as a student in the sixties. He could not shake Bell's ideas from the moment he read about them, and eventually, he and three other researchers were able to present a proposal for a practical type of experiment that could be used to test Bell's theory.

The experiment involved sending a pair of entangled particles in opposite directions. In practice, photons with a property called polarization are used. When the particles are emitted the polarization direction is undetermined, and all that is certain is that the particles have a parallel polarization. This can be tested using a filter that allows the passage of particles with a certain polarization. This is the result in which sunglasses are used, which block light rays that have been polarized in a certain plane.

If the two particles in the experiment are sent towards filters located in the same plane, such as the horizontal plane, and one of them manages to pass through, then the other will pass as well. If they are placed at different angles to each other, then one will brake while the other can pass. The idea is to measure with the filter system in different directions, and get an unequivocal determination thanks to the findings: in some cases both particles will pass, sometimes only one of them and sometimes none of them. The number of times the two particles pass the filter depends on the angles between the filters.

Quantum mechanics states that there will be a correlation between the measurements. The probability that one particle will succeed in passing depends on the angle of the filter that checks the polarization of the other particle on the opposite side of the measurement system. That is, the results of the two measurements, at certain angles, violate Bell's inequality and have a stronger correlation than would be obtained if the results were based on latent variables that were predetermined even before the particles were emitted.

Violation of the inequality

John P. Clouser immediately set to work on conducting this experiment. He built a device that emits two entangled photons at the same time, with each of them directed towards a filter that checks their polarization. In 1972, together with his doctoral student Stuart Friedman, he was able to show a result that was a clear violation of Bell's inequality while agreeing with the predictions of quantum mechanics. In the years that followed, John P. Clauser and other physicists continued to discuss the experiment's findings and its limitations. One of the limitations was that the experiment was generally inefficient, both in terms of production and in terms of capturing the particles. The measurement was also predetermined in light of the fact that the filters were placed at fixed angles. That is, in this experiment there were several technical loopholes, when the observer can doubt the results: is it possible that the measurement system itself, in some way, chose particles that previously had a strong correlation, and did not measure others? If so, the particles could still store hidden information within them. Removing these particular loopholes was challenging, given the fact that entangled quantum states are unstable and difficult to control; It is necessary to handle individual photons. The French student in the research group Alain Aspe was not afraid of this, and built a new version of the measurement system while introducing changes and improvements. As part of his experiment, he could measure the photons that passed through the filter and those that did not. This means that more photons were measured and that the measurement itself was more accurate.

In the latest version of the measurement system, it could also direct photons at two filters placed at different angles. The innovative idea was the ability to change the direction of movement of the entangled photons after their production and emission from the source. The filters were placed at a distance of six meters from each other, so that the step of changing the directions had to happen within a few billionths of a second. If information regarding the identity of the filter to which the photon will reach has already been determined at the moment of the photon's departure from the source, then in this system this photon will not reach this filter. In addition, information regarding the filter on one side cannot reach the other side and affect the results of the measurement there. In this way, Allen closed a mandatory loophole and provided a completely clear result: quantum mechanics is correct and there are no hidden variables.  

The age of quantum information

These and similar experiments laid the foundations for today's vigorous research in the scientific field of quantum information. The ability to control and change quantum states, and consequently change their properties, gives us access to tools with extraordinary capabilities. This is the basis for quantum computing, the transfer and storage of quantum information as well as the development of algorithms for quantum encryption. Systems with more than two particles, all of which are entangled, are used today, the same systems that Anton Zeilinger and his colleagues were the first to study.