On October 4, Royal Swedish Academy of Sciences announced that French scientist Alain Aspect, American scientist John Clauser, and Austrian Scientist Anton Zeilinger won the 2022 Nobel Prize in Physics for their contributions to entangled photon experiments, proving violations of Bell inequality and the groundbreaking Quantum Information Science .
Chinese scientists have also made great contributions
Anton Salinger is a leader in quantum communication in my country and a supervisor of Academician of Chinese Academy of Sciences Pan Jianwei . In the official introduction of Nobel Prize , the achievements and contributions of Pan Jianwei's team were quoted in large quantities. In an interview with reporters, Academician Pan Jianwei said that it is well deserved to win the Nobel Prize by three scientists, including his supervisor, and "it's even a bit late."
Looking back at quantum physics 70, the contributions of the three pioneers are particularly important. As early as 2010, three scientists won the Wolf Prize in the field of physics for quantum mechanics non-locality test and light quantum information processing foundation experiments, but they were not awarded the Nobel Prize at that time.
In recent years, the field of quantum information science has made great progress. "There are two important events, one is the launch of my country's quantum scientific experimental satellite "Mozi", and the other is the implementation of the superiority of Google and Chinese scientists quantum computing , which makes the pioneering contributions of the three award-winning scientists today particularly important." Pan Jianwei said that they mainly found ways to realize quantum entanglement, and then used the generated entangled quantum pairs to conduct relevant quantum information processing experiments.
Pan Jianwei is very proud that the quantum communication experimental papers listed by Salinger this time have Pan Jianwei's name except for one theoretical article in quantum communication experiments. "The awards committee mentioned four quantum communication experimental articles by my mentor Anton Salinger. I am the first author of two of them and the second author of two of them." Pan Jianwei said.
At the same time, "The Awards Committee also mentioned three other articles, and these three articles are independent research work carried out by Chinese scientists. So, from this point of view, I not only joined Salinger's research team, but also participated in the creation of the field of quantum information physics. I feel very lucky." Pan Jianwei said.
More importantly, "Chinese scientists have also made great contributions in the process of turning the dream of award-winning scientists into reality." Pan Jianwei felt very proud of his achievements in this regard.
surpasses daily experience
When two particles are in the entangled state , people can immediately determine the equivalent measurement result of the other particle as long as they measure the characteristics of one particle.
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 where a black ball is sent in one direction and another white ball is sent in the opposite direction. If the observer catches a ball and sees it is white, then it can be immediately known that the ball traveling in the other direction is black.
The reason why quantum mechanics is so special is that before being measured, the "sphere" in quantum mechanics does not have a certain state. It was as if both balls were gray until someone saw one of them. At this time, the ball may appear black or white. And the other ball will immediately turn into the opposite color.
But the question is, how do we know that the initial color of these balls is not fixed? Even though they appear to be gray, maybe they contain a hidden label that marks what color these balls should turn into when someone sees them.
In quantum mechanics, pairs of entangled particles can be compared to balls of opposite colors thrown in opposite directions. When Bob caught a ball and saw it was black, he could immediately know that Alice caught a white ball. The theory about hidden variables suggests that these balls always contain hidden information about what color to display.However, quantum mechanics believes that before someone sees them, the balls are gray, and then one of them randomly turns white and the other turns black. Bell's inequality shows that there are experiments that can distinguish between these two situations-experiments prove that the description of quantum mechanics is correct. An important part of the study of
winning this year's Nobel Prize in Physics is the Bell inequalities. The Bell inequality allows scientists to distinguish between the two theories of quantum mechanics and hidden variables through experiments. Experiments show that, as quantum mechanics predicts, the balls are gray and contain no hidden information. In the experiment, which ball becomes black and which one becomes white is determined by probability.
The most important resource of quantum mechanics
entangled quantum state provides new possibilities for storing, transmitting and processing information.
If a pair of entangled particles travel in opposite directions at the same time, one of the particles entangles the third particle, 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 particles entangled with it (one of the original entangled particle pairs). Entanglement has now been transferred from the original pair to the individual particles. This way of transferring unknown quantum states from one particle to another is called quantum teleportation. In 1997, Anton Salinger and his colleagues implemented the first experiment of quantum teleportation.
It is worth noting that quantum teleportation is the only way to not lose any information when transmitting quantum information from one system to another. It is absolutely impossible to measure all the properties of a quantum system, then transmit this information and rebuild the entire system in this way. Quantum systems can be fully described as quantum states with probability superposition, which means that a quantum system contains multiple quantum states at the same time, and each quantum state has a certain probability of appearing during measurement.
Once the measurement is performed, the quantum system collapses into a quantum state, that is, the state observed by the measurement system. All states superimposed by the measured final state will completely disappear after observation, and no method can measure it. However, through quantum teleportation, we can transfer completely unknown information into 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 the quantum teleportation between two pairs of entangled particles. If one of the two entangled pairs of particles gathers together in a specific way, the undisturbed particles in the pairs may become entangled, although they never come into contact with each other. In 1998, Anton Cellinger's research team first demonstrated the exchange of entanglement between particles.
entangled photon pairs can be transmitted in the opposite direction through fiber and act as signals in the quantum network. The entanglement between the pairs of two groups of entangled particles makes it possible to expand the distance between the nodes of the quantum network. Generally, the distance that photons can transmit through optical fibers before being absorbed or lose their quantum characteristics is limited. Although ordinary optical signals can be amplified all the way through optical fibers, this does not apply to entangled photon pairs. Optical signal amplifiers need to capture and measure photons to achieve amplification, operations that are destroying the entanglement of photon pairs. The entangled exchange between particle pairs means that the original quantum state can be transmitted further, achieving longer ultra-long-distance transmission than other methods.
Two pairs of entangled particle pairs are emitted from different sources. One particle in each pair (2 and 3 in the figure) entangles 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 been in contact can be entangled together.
From paradox to inequality
In fact, this progress is based on years of research development. It began with an incredible discovery: quantum mechanics allows the division of a single quantum system into multiple units that are separated from each other while still manifesting as a whole.
This goes against all common views about the nature of cause and effect and reality.How can a system be affected by other local systems while not affected by the signals it transmits? Physical laws determine that the signal can't propagate faster than the speed of light - but in quantum mechanics, it seems that the signal is not needed to connect different parts of the extended system at all.
Albert Einstein believes that this is not feasible. He studied this phenomenon with colleagues Boris Podolsky and Nathan Rosen. They proposed their inference in 1935: quantum mechanics does not seem to provide a complete description of reality. Based on the first letter of the researchers, this inference is called the EPR paradox.
The question is whether the world can be described more fully, and quantum mechanics is only part of it. For example, one explanation is that particles always carry some hidden information that shows what kind of experimental results they will show. Based on this, all measurement behaviors contain information about the location of the measurement. This type of information is often called a local hidden variable.
Northern Ireland physicist John Stewart Bell (1928-1990) who worked at the European Centre for Nuclear Research (CERN) has studied this problem carefully. He found that there was an experiment that could verify whether the world fully complies with the laws of quantum mechanics, or whether there could be another description with hidden variables. If this experiment is repeated multiple times, all the theories related to the hidden variables show that the correlation between the results must be less than or at most equal to a certain value, i.e., the Bell inequality.
However, quantum mechanics can violate this inequality, that is, the correlation between the results can be greater than a specific value.
In the 1960s, when John Crowze was a student, he became interested in the basics of quantum mechanics. When he read John Bell's thoughts, he couldn't help but think about the possibility of this method. Finally, he and three other researchers proposed an experiment that could be implemented in reality to test the Bell inequality.
This experiment involves sending a pair of entangled particles in the opposite direction. In practice, photons with polarization characteristics are used. When the particle is emitted, the polarization direction is uncertain, the only thing that can be determined is that the particle has parallel polarization.
can study the polarization characteristics of photons by using filter that allows polarization through a specific direction. This filter is used in many sunglasses, which can block the polarized light on a certain plane, such as light reflected by water, which contains polarized light.
If both particles in the experiment are sent to filters placed in parallel, such as two filters placed in vertical directions, if one particle can pass—then the other will pass too. And if the two filters are at right angles to each other, one of the particles will be blocked and the other will pass. The key is that when measuring with filters placed at different inclinations, the results may vary: sometimes both particles can pass through, sometimes only one, and sometimes not. The probability of two particles passing through the filter simultaneously depends on the angle between the filters.
Quantum mechanics leads to correlations between measurement results. The possibility of one particle passing through the filter depends on the angle the filter is set when the other particle is performing the experiment. This means that at some angles, the correlation between the two measurements will violate the Bell inequality. If the result is controlled by a hidden variable, it can be determined in advance when the particle is emitted, and the results will also have stronger correlations.
Inequality violated
John Crowze started the experiment immediately. He built a device that emits two entangled photons at a time, each hitting the filter that detects polarization. In 1972, he, along with doctoral student Stuart Freedman (1944-2012), showed a result that clearly violated the Bell inequality and was consistent with the predictions of quantum mechanics.
In the next few years, John Crowze and other physicists continued to discuss this experiment and its limitations. One limitation is that the experiment is inefficient in preparing and capturing particles.And since the measurement is preset and the filter angle is fixed, there are loopholes, and observers can question: What if the experimental device happens to select particles with strong correlation in some way without detecting other particles? If so, particles may still carry hidden information. This special vulnerability of
is difficult to eliminate because the entangled quantum states are so fragile and difficult to manage. Therefore, it is necessary to deal with a single photon. Alan Aspe, who was still studying for a PhD in France at the time, was not intimidated by the difficulties. He established a new version of the experiment and iterated and improved it several times. In his experiments, he could record which photons passed through the filter and which did not. This means more photons are detected and the measurements are 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 changes its direction after the entangled photon pair is prepared. The filter is only six meters away, so the change needs to be completed in a few billionths of a second. If information about which filter the photon will reach affects how it emits from the light source, then it will not reach that filter. Information about the filter on the other side of the experiment also cannot reach the other side and affect the measurement results there.
Alan Aspe made up for an important loophole in this way and provided a very clear result: quantum mechanics is correct and there are no hidden variables.
Quantum Information Age
These experiments and similar experiments have laid the foundation for the current in-depth research on quantum information science.
's ability to manipulate and manage quantum states and all of its properties allows us to implement a tool that has potential beyond our expectations. This is the basis of quantum computing, transmission and storage of quantum information, and quantum encryption algorithms. Now, a system with more than two particles (all entangled) is entering practical applications, and Anton Salinger and his colleagues are the first to explore.
John Crowze used calcium atoms. After illuminating calcium atoms with a special light, he can emit entangled photons. He used filters on both sides to measure the polarization of the photons. After a series of measurements, he proved that they violated the Bell inequality.
Alan Aspe developed this experiment by emitting entangled photons at a higher rate through a new method of excitating atoms . It can also switch between different settings so that the system does not contain any pre-information that may affect the results.
Anton Salinger later tested the Bell inequality more. He prepared entangled photon pairs by irradiating laser light onto special crystals and switched measurement settings using random numbers. One experiment used signals from distant galaxies to control the filters and ensured that the signals did not affect each other.
, these increasingly perfect tools, bring us closer to practical applications. It has now been proved that entanglement states can be established between photons sent through tens of kilometers of optical fibers, as well as between satellite and the photons of ground stations. In a short time, researchers around the world have discovered many new methods that utilize the most powerful properties of quantum mechanics.
The first quantum revolution gave us transistor and lasers, but thanks to modern tools that can manipulate entangled quantum systems, we are now entering a new era.
my country has a number of achievements with important international influence
In recent years, my country has also attached great importance to the development of quantum information technology, and has broken through a series of important scientific issues and key core technologies in the field of quantum information technology, and has produced a number of achievements with important international influence.
"Overall, my country is in an international leading position in the research and application of quantum communications, and is on the same level as developed countries in quantum computing, and is developing rapidly in quantum precision measurement." Pan Jianwei said.
He said that the development goal of quantum communication is to build a global wide-area quantum communication network system.The development route of the wide-area quantum communication network is the development route of the wide-area quantum communication network .
my country's metropolitan quantum communication technology has initially met the practical requirements. my country has built the world's first long-distance optical fiber quantum confidential communication backbone network " Beijing-Shanghai Trunk Line ", and carried out technical verification and application demonstration of long-distance quantum confidential communication in the fields of finance, government affairs, electricity, etc. In terms of satellite quantum communication, my country has developed and launched the world's first quantum science experimental satellite "Mozi", which was the first in the world to realize satellite-ground quantum communication , and realized intercontinental quantum communication for the first time, fully verifying the feasibility of achieving global quantum communication based on satellite platforms. The core task of
quantum computing research is the coherent manipulation of multiple qubits . At present, quantum computing research has achieved "quantum superiority", that is, quantum computer 's computing power for specific problems exceeds that of traditional supercomputers, and achieving this goal requires coherent manipulation of about 50 qubits.
In 2020, scholars such as Pan Jianwei and Lu Chaoyang successfully developed the "Nine Chapters" of the quantum computing prototype with 76 photons, which promoted the cutting-edge research of global quantum computing to a new height. After Google's "Sycamore" quantum computer, my country has successfully achieved a milestone breakthrough in "quantum computing superiority" for the first time.
However, "my country started late in the field of quantum precision measurement, and overall there is a certain gap compared with developed countries. However, in recent years, the gap has been rapidly narrowed, and is comparable to the highest international level reported in several research directions." Pan Jianwei said.
