Shulou(Shulou.com)11/24 Report--

On October 4, 2022, at about 17: 45 Beijing time, the 2022 Nobel Prize in Physics was awarded to French scholar Alain Aspect Aspe, American scholar John   Krause and Austrian scholar Anton   Zeilinger for "experimenting with entangled photons, falsifying Bell inequalities and pioneering quantum information science".

Alan Aspe (Alain Aspect) was born in 1947 in Arjen, France. He received his doctorate from the University of Paris XI in 1983 and is currently a professor at the University of Paris Thackeray and the Paris Institute of Technology.

John Krause (John Clauser) was born in Pasadena, California in 1942 and received his doctorate from Columbia University in 1969. Founder and research physicist of J.F Clauser & Assoc, USA.

Anton Tsai Ling (Anton Zeilinger) was born in 1945 in Reed, Inhe region, Austria. He received his doctorate from the University of Vienna in Austria in 1971 and is now a professor at the University of Vienna.

Quantum mechanics is not only a theoretical or philosophical problem, but also has a wide range of applications. A great deal of research has been focused on using the special properties of a single particle system to build quantum computers, improve measurement methods, and build quantum networks and secure quantum encrypted communications.

Quantum mechanics allows for a situation in which two or more particles can share a physical state no matter how far apart they are, which is called quantum entanglement. Since the theory was proposed, it has been one of the most controversial elements in quantum mechanics. Albert Einstein called it "ghostly teleaction", which Schrodinger said was the most important feature of quantum mechanics.

This year's winners explored these entangled quantum states, and their experiments laid the foundation for the ongoing revolution in quantum technology.

Beyond daily experience when two particles are in an entangled state, as long as the characteristics of one particle are measured, the equivalent measurement result of the other particle can be determined immediately.

At first glance, this may not be surprising. We can change the angle and compare the particles to black and white balls. Imagine an experiment in which a black ball is sent in one direction and another white ball in the opposite direction. If the observer catches a ball and sees that it is white, it is immediately known that the ball moving in the other direction is black.

The reason why quantum mechanics is so special is that there is no definite state of the "ball" in quantum mechanics before it is measured. It's as if both balls are gray until someone sees one of them. At this point, the ball may appear black or white. And the other ball immediately changes to the opposite color.

But the question is, how do we know that the initial color of these balls is not fixed? Even if they look gray, maybe they contain a hidden label indicating what color the balls should be when someone sees them.

Pairs of entangled particles in quantum mechanics can be compared to oppositely colored balls thrown in the opposite direction. When Bob catches a ball and sees that it is black, he can immediately know that Alice has caught a white ball. The theory of hidden variables holds that these balls always contain hidden information about what color to display. However, quantum mechanics holds that before someone sees them, the balls are gray, and then one of them randomly turns white and the other turns black. Bell inequality shows that there are some experiments that can distinguish between the two cases-experiments show that the description of quantum mechanics is correct.

An important part of the research that won this year's Nobel Prize in physics is the Bell inequalities inequality. Bell inequality enables scientists to distinguish between quantum mechanics and hidden variables through experiments. Experiments show that, as predicted by quantum mechanics, these spheres are gray and do not contain any hidden information. In the experiment, which ball becomes black and which becomes white is determined by probability.

The entangled quantum state of the most important resource in quantum mechanics provides a new possibility to store, transmit and process information.

If a pair of entangled particles travel in the opposite direction at the same time, and one of the particles is entangled with the third, an interesting phenomenon will occur. They will be transformed into a new shared state. The third particle loses its independence, but its quantum state properties are transferred to the particle that is entangled with it (one of the original entangled particle pairs). Entanglement has now been transferred from the original pair to individual particles. This way of transferring unknown quantum states from one particle to another is called quantum teleportation. In 1997, Anton Cailinger and his colleagues carried out the experiment of quantum teleportation for the first time.

It is worth noting that quantum teleportation is the only way to transfer quantum information from one system to another without losing any information. It is absolutely impossible to measure all the properties of a quantum system and then transmit that information to reconstruct the whole system. The quantum system can be completely described by the probability superposition of quantum states, which means that a quantum system contains multiple quantum states at the same time, and each quantum state has a certain probability to appear in the measurement.

Once measured, the quantum system collapses into a quantum state, that is, the state observed by the measurement system. On the other hand, all the states superimposed with the measured final state of the quantum system will disappear completely after observation, and they can no longer be measured by any method. However, through quantum teleportation, we can transfer the completely unknown quantum state information to the new particles intact, but at the cost of destroying the information carried by the original particles.

Scientists have proved this through experiments, and the next step is to try quantum teleportation between two pairs of entangled particles. If one of the two pairs of entangled particles gathers together in a particular way, the undisturbed particles in the two pairs may become entangled, even though they have never touched each other. In 1998, Anton Cailinger's team first proved the exchange of entanglement between particle pairs.

Entangled photon pairs can be transmitted in the opposite direction through optical fibers and act as signals in quantum networks. The entanglement between two groups of entangled particles makes it possible to expand the distance between quantum network nodes. In general, the distance over which photons can travel through optical fiber is limited before they are absorbed or lost their quantum properties. Although ordinary optical signals can be amplified all the way through optical fiber, this is not suitable for entangled photon pairs. Optical signal amplifiers need to capture and measure photons to achieve amplification, which are destroying the entanglement of photon pairs. The entanglement swapping between particle pairs means that the original quantum state can be transmitted farther, achieving a longer ultra-long distance transmission than other ways.

Two pairs of entangled particles are emitted from different sources. One particle in each pair (2 and 3 in the figure) is entangled in a special way. As a result, the other two particles (1 and 4 in the figure) will also be entangled. In this way, two particles that have never come into contact with each other can be entangled.

From paradox to inequality, in fact, this progress is based on years of research and development. It began with the incredible discovery that quantum mechanics allows a single quantum system to be divided into separate units while still acting as multiple units as a whole.

This runs counter to all common ideas about causality and the nature of reality. How can a system be affected by other local systems without being affected by the signals it transmits? The laws of physics dictate that signals cannot travel faster than the speed of light-but in quantum mechanics, there seems to be no need for signals to connect different parts of the extended system.

Albert Einstein thought it was not feasible. He studied the phenomenon with colleagues Boris Podolski (Boris Podolsky) and Nathan Rosen (Nathan Rosen). They came up with their inference in 1935: quantum mechanics does not seem to provide a complete description of reality. According to the initials of the researchers, this inference is called the EPR paradox.

The question is whether quantum mechanics is only part of a more complete description of the world. One way to explain, for example, is that particles always carry hidden information indicating what experimental results they will show. It is inferred that all measurement activities contain information about the location of the measurement. This type of information is often referred to as local hidden variables.

John Stewart Bell (John Stewart Bell,1928-1990), a Northern Ireland physicist who works at the European Centre for Nuclear Research (CERN), studied the problem carefully. He found that there was an experiment that could verify whether the world fully conformed to the laws of quantum mechanics, or whether there could be another description with hidden variables. If this experiment is repeated many times, all theories related to hidden variables show that the correlation between the results must be less than or at most equal to a certain value, that is, the Bell inequality.

However, quantum mechanics can violate this inequality, that is, the correlation between results can be greater than a specific value.

When John Krause was a student in the 1960s, he became interested in the basics of quantum mechanics. When he read John Bell's idea, he couldn't help thinking about the possibility of this method. In the end, he and three other researchers proposed an experiment that could be carried out in real life to test the Bell inequality.

The experiment involves sending a pair of entangled particles in the opposite direction. In practice, photons with polarization properties are used. When a particle is emitted, the direction of polarization is uncertain, and the only thing that can be determined is that the particle has parallel polarization.

The polarization properties of photons can be studied by using filters that allow polarized light to pass through a specific direction. This kind of filter is used in many sunglasses to block polarized light on a certain plane. for example, the light reflected by water contains polarized light.

If both particles in the experiment are sent to parallel filters, such as two vertically placed filters, if one particle can pass-- then the other will also pass. If two filters are at right angles to each other, one of the particles will be blocked and the other will pass through. The point is that when using filters placed at different dips, the results may be different: sometimes both particles can pass through, sometimes only one, sometimes none. The probability that two particles pass through the filter at the same time depends on the angle between the filters.

Quantum mechanics leads to the correlation between measurement results. The possibility that one particle passes through the filter depends on the angle at which the filter is set by another particle during the experiment. This means that at some angles, the correlation between the two measurements will violate the Bell inequality. If the results are controlled by hidden variables, they can be determined in advance at the time of particle emission, and there will be a stronger correlation between the results.

The violated inequality John Krause immediately began the experiment. He built a device that emits two entangled photons at a time, each aimed at a filter that detects polarization. In 1972, together with doctoral student Stuart Friedman (Stuart Freedman,1944-2012), he showed a clear violation of Bell's inequality, which was consistent with the prediction of quantum mechanics.

Over the next few years, John Krause and other physicists continued to discuss the experiment and its limitations. One of the limitations is that the experiment is inefficient in preparing and trapping particles. And because the measurement is pre-set and the angle of the filter is fixed, there are loopholes that observers can question: what if the experimental device happens to select particles with strong correlation in some way and no other particles are detected? If so, the particles may still carry hidden information.

This particular loophole is difficult to eliminate because the entangled quantum states are so fragile and difficult to manage. Therefore, it is necessary to deal with individual photons. Undeterred by the difficulties, Alan Aspe, who was still a doctoral student in France at the time, set up a new version of the experiment and improved it several times. In his experiments, he could record which photons passed through the filter and which did not. This means that more photons are detected and the measurement is better.

In his final version of the test, he was also able to direct photons to two filters with different angles. This strategy is a mechanism that can change the direction of entangled photon pairs after they have been prepared. The filter is only six meters away, so the change needs to be done in a few 1/1000000000 seconds. If the information about which filter the photon will reach affects the way it is emitted from the light source, then it will not reach the filter. The information about the filter on the other side of the experiment cannot reach the other side and affect the measurement results there.

In this way, Alan Aspe fills an important loophole and provides a very clear result: quantum mechanics is correct and there are no hidden variables.

In the era of quantum information, these and similar experiments have laid the foundation for the in-depth study of quantum information science.

The ability to manipulate and manage quantum states and all their properties enables us to implement a tool that has unexpected potential. This is the basis of quantum computing, the transmission and storage of quantum information, and quantum encryption algorithms. Now, a system with more than two particles (all entangled) is coming into practice, and Anton Zelinger and his colleagues are the first to explore.

John Krause used calcium atoms. After he irradiates calcium atoms with a special light, he can emit entangled photons. He measured the polarization of photons with filters on both sides. After a series of measurements, he proved that they violated Bell's inequality.

Alan Aspe developed the experiment, which uses a new way to excite atoms to emit entangled photons at a higher rate. He can also switch between different settings so that the system does not contain any advance information that may affect the results.

Anton Zelinger later conducted more tests on the Bell inequality. He irradiates the laser on a special crystal to prepare entangled photon pairs and uses random numbers to switch measurement settings. One experiment uses signals from distant galaxies to control filters and ensure that signals do not interfere with each other.

These increasingly sophisticated tools bring us closer to practical application. It has been proved that entangled states can be established between photons sent through tens of kilometers of optical fiber, as well as between photons from satellites and ground stations. In a very short time, researchers around the world have discovered many new ways to take advantage of the most powerful properties of quantum mechanics.

The first quantum revolution gave us transistors and lasers, but we are now entering a new era because of the modern tools that can manipulate entangled quantum systems.

Official website of the Nobel Prize: https://www.nobelprize.org/

This article comes from the official Wechat account: global Science (ID:huanqiukexue).

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