2022 Nobel Prize in Physics was awarded to 3 scientists for their experiments with entangled photons

The Royal Swedish Academy of Sciences has  decided to award the 2022 Nobel Prize in Physics to Alain Aspect, John F. Clauser and Anton  Zeilinger for experiments with entangled photons, establishing the violation of Bell inequalities  and pioneering quantum information science. Using groundbreaking experiments, Alain  Aspect, John Clauser and Anton Zeilinger 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. 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 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. 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 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. Entangled pairs of photons, particles of  light, can be sent in opposite directions through optical fibres and function as signals in  a quantum network. Entanglement between two pairs makes it possible to extend the distances between  the nodes in such a network. There is a limit to the distance that photons can be sent through  an optical fibre before they are absorbed or lose their properties. Ordinary light signals  can be amplified along the way, but this does not work with entangled pairs. An amplifier has  to capture and measure the light, which breaks the entanglement. However, entanglement swapping  means it is possible to send the original state further, thereby transferring it over longer  distances than had otherwise been possible. Two pairs of entangled particles are  emitted from different sources. One particle from each pair is brought together  in a special way that entangles them. The two other particles (1 and 4 in the diagram)  are then also entangled. In this way, two particles that have never been  in contact can become entangled. This progress rests on many years  of development. It started with the mind-boggling insight that 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 question was whether there could be  a more complete description of the world, where quantum mechanics is just one part. This  could, for example, work through particles always carrying hidden information about what they  will show as the result of an experiment. All the measurements then show the properties  that exist exactly where the measurements are conducted. This type of information  is often called local hidden variables. The Northern Irish physicist John Stewart  Bell (1928–1990), 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. However, quantum mechanics can violate this  inequality. It predicts higher values for the correlation between the results than  is possible through hidden variables. 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. The experiment involves sending a pair of  entangled particles in opposite directions. In practice, photons that have a property  called polarisation are used. When the particles are emitted the direction  of the polarisation is undetermined, and all that is certain is that the  particles have parallel polarisation. This can be investigated using a filter  that allows through polarisation that is oriented in a particular direction. This  is the effect used in many sunglasses, which block light that has been polarised in a  certain plane, for example by reflecting of water. If both the particles in the experiment are sent  towards filters that are oriented in the same plane, such as vertically, and one slips through  – then the other one will also go through. If they are at right angles to each other, one will  be stopped while the other will go through. The trick is to measure with the filters set  in different directions at skewed angles, as then the results can vary: sometimes both  slip through, sometimes just one, and sometimes none. How often both particles get through the  filter depends on the angle between the filters. Quantum mechanics leads to a correlation  between measurements. The likelihood of one particle getting though depends on  the angle of the filter that tested its partner’s polarisation on the opposite  side of the experimental setup. This means that the results of both  measurements, at some angles, violate a Bell inequality and have a stronger  correlation than they would if the results were governed by hidden variables and were already  predetermined when the particles were emitted. John Clauser immediately started working  on conducting this experiment. He built an apparatus that emitted two  entangled photons at a time, each towards a filter that tested  their polarisation. In 1972, along with doctoral student Stuart Freedman  (1944–2012), he was able to show a result that was a clear violation of a Bell inequality and  agreed with the predictions of quantum mechanics. In the years that followed, John Clauser and  other physicists continued discussing the experiment and its limitations. One of these was  that the experiment was generally inefficient, both when it came to producing and capturing  particles. The measurement was also pre-set, with the filters at fixed angles.  There were therefore loopholes, where an observer could question the results:  what if the experimental setup in some way selected the particles that happened  to have a strong correlation, and did not detect the others? If so, the particles  could still be carrying hidden information. Eliminating this particular loophole was  difficult, because entwined quantum states are so fragile and difficult to manage; it  is necessary to deal with individual photons. French doctoral student Alain Aspect was  not intimidated, and built a new version of the setup that he refined over several  iterations. In his experiment, he could register the photons that passed through  the filter and those that did not. This meant more photons were detected  and the measurements were better. In the final variant of his tests, he was also  able to steer photons towards two different filters that were set at different angles.  The finesse was a mechanism that switched the direction of the entangled photons after they had  been created and emitted from their source. The filters were just six metres away, so the switch  needed to occur in a few billionths of a second. If information about which filter the photon  would arrive at influenced how it was emitted from the source, it would not be arriving at that  filter. Nor could information about the filters on one side of the experiment reach the other side  and affect the result of the measurement there. In this way, Alain Aspect closed an important  loophole and provided a very clear result: quantum mechanics is correct and  there are no hidden variables. These and similar experiments laid the foundation for the current intense research  in quantum information science. 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 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.