·The birth of Bell's inequality announced the local controversy of quantum mechanics theory, from pure speculation with philosophical color to scientific theory that can be falsifiable by experiment.

2022 Nobel Laureate in Physics

On the afternoon of October 4th, Beijing time, in Stockholm, the capital of Sweden, the Royal Swedish Academy of Sciences announced that the 2022 Nobel Prize in Physics will be awarded to Alain Aspect of France and John Clauser of the United States. ), and Anton Zeilinger of Austria for their contributions to quantum information science research.

The three scientists were awarded the Nobel Prize in Physics, well deserved. As the chairman of the Nobel Committee for Physics said, the laureate's research on entangled states has gone beyond explaining fundamental questions of quantum mechanics. That's because, based on the research of the three physicists, quantum entanglement has been experimentally confirmed and utilized in many physical systems. For example, in our laboratory, when the laser passes through a nonlinear crystal, a spontaneous parametric down-conversion process will occur, and a pump photon will be split into a pair of photons to satisfy the phase matching condition. This pair of photons is in a specific entangled state. No matter how far apart these two photons are, for example, one stays in my laboratory and the other is sent to space by the Micius satellite, as long as we measure and know the state of the photons in my laboratory, we don’t need to measure the state of the other photons in the distant Photons in space can instantly know their state when they do any operation.

Quantum entanglement is a very important physical resource that can be applied to quantum secure communication, enabling us to obtain a more secure and efficient communication method, and applied to quantum computing, so that we have a quantum computer with powerful computing power that is incomparable to classical. Precision measurement allows us to have more accurate measurement methods and so on.

In addition, as early as 12 years ago, in 2010, three of their physicists were awarded the Wolf Prize for their fundamental conceptual and experimental contributions to the foundations of quantum physics, in particular a series of increasingly complex tests of Bell's inequalities (Wolf Prize).

Moreover, Inside Science, a popular science website under the American Physical Union, has predicted that the three physicists will win the Nobel Prize in Physics for three consecutive years in 2019, 2020, and 2021.

However, before quantum entanglement is applied to real life, it has experienced twists and turns only in the stage of theoretical hypothesis and experimental verification.

First of all, Einstein believed that quantum entanglement, the super-distance interaction, was inconceivable and violated the special theory of relativity.

Einstein and his Princeton assistants Boris Podolsky and Nathan Rosen proposed a thought experiment known as the EPR paradox.

The EPR paradox describes a particle with spin 1/2 A and B, and the initial total spin is zero. Assuming that the particle has two possible spins, namely |up> and |down>, then, if the spin of particle A is |up>, the spin of particle B must be |down>, in order to keep the overall conservation. If the two particles are flying in opposite directions, they are getting farther and farther apart. Then, no matter how far apart they are, they should always be |up>|down>correlated to maintain overall conservation. If two particles are measured separately by observer Alice Bob. According to quantum mechanics, each particle should be in some superposition state, say, |up>, |down> with 50% probability each, as long as Alice and Bob haven't measured. Then, if Alice measures A, the superposition state of A collapses in an instant, for example, it collapses to |up>, because of conservation, B must be |down>. However, at this time, A and B are already very far apart, say tens of thousands of light-years. According to the theory of quantum mechanics, B should also have half the probability of |up> and |down>. Why can it be done? Always select | Down > What? Unless there is some way of "communicating messages" between A and B particles in a timely manner? Even assuming they can sense each other, that seems to be an instantaneous signal at a distance! And this action at a distance is contrary to the theory of relativity that the speed of light cannot be surpassed. So this constitutes a paradox.

It has nothing to do with the distance between particles; it can be measured at the same time or delayed, that is, the speed of light; it has nothing to do with the space environment, and electromagnetic shielding, gravitational shielding, etc. cannot block their associations, which are two particles in an entangled state correlation between.

Einstein, on his part, believed that such a phenomenon would never occur, and called it ghostly action at a distance, arguing that the problem stemmed from "the incompleteness of quantum mechanics." Einstein hoped to establish a more general theory of local realism to make up for the deficiencies of quantum theory and to eliminate the action at a distance.

And Bohr believed that this super-distance effect must exist, quantum mechanics is complete; the quantum world is non-local.

As one of Einstein's followers, in 1964, Bell (John Bell) defined an observable quantity, and based on the local hidden variable theory, the predicted measurement value is not greater than 2. Once the experimentally measured result is greater than 2, it means that the local latent variable theory is wrong. This is known as "Bell's Theorem", "Bell's Inequality".

The birth of Bell's inequality announced the local controversy of quantum mechanics theory, from pure speculation with philosophical color to scientific theory that can be falsifiable by experiment.

The original intention of Bell's research on hidden variable theory was to prove that the non-locality of quantum mechanics was wrong, but it backfired.

In 1969, John Clauser, then a graduate student at Columbia University, along with Michael Horne, Abner Shimony, and Richard Holt, transformed the aforementioned Bell's 1964 mathematical theorem into a Very specific experimental predictions.

In 1972, John Clauser, already a postdoctoral researcher, and graduate student Stuart Freedman were the first to experimentally demonstrate that two distant particles could be entangled.

John Clauser went on with three more experiments to test the foundations of quantum mechanics and entanglement. Each new experiment confirms and extends his results.

In 1982, Alan Aspect et al. improved the Bell's theorem experiment of Clauser and Freedman at the University of Paris 11, using the entanglement of the polarization states between the photon pairs radiated by the calcium ion cascade, and the experimental results violated the "Bell's inequality".

In 1998, Anton Zeilinger et al. completed the "Bell's Inequality" experiment at the University of Innsbruck, Austria, using entangled photon pairs generated by parametric down-conversion in nonlinear crystals to partially eliminate locality loopholes, and the experimental results are of decisive significance.

Subsequently, over the years, people have used various entangled example pairs to verify Bell's inequality because of imperfections and loopholes in previous experiments, such as locality loopholes and measurement loopholes.

The so-called locality loophole means that the corresponding time between the entangled particles exceeds the speed of light. For example, if the detection result of one particle is obtained, the result of the other particle will be obtained in an instant, but if the distance between the two particles is not long enough to It is proved that the time to travel through the speed of light is much longer than the time to obtain the result of another photon in the experiment, which has a local loophole.

The measurement loophole is because the detector efficiency is not 100%, so it can be understood that the detected particles violate Bell's inequality, while the undetected particles do not violate.

In 2015, Ronald Hanson's research group at the Delft University of Technology in the Netherlands reported their experiments to verify Bell's inequality in a diamond color center system: To avoid locality loopholes, just place two diamond color centers 1.3 kilometers apart. of two laboratories. The time required for the direct optical communication between the two color centers is about 4.27 microseconds, and the time to complete an experiment is 4.18 microseconds, which is 90 nanoseconds less than the optical communication time, so the locality loophole is solved. In addition, the measurement efficiency of color centers is as high as 96%, and the measurement loopholes are also blocked. They claim to have achieved a loophole-free experiment verifying Bell's inequality, supporting quantum theory with 96% confidence (2.1 standard deviations), thereby falsifying the local hidden variable theory. In their experiments, they achieved entanglement between electrons in diamond color centers using entangled photon pairs and entanglement-exchange techniques.

If these two loopholes are blocked, another loophole is free will, and the measurement base needs to be selected during the experiment. Some people think that the choice of the measurement base is affected by consciousness, resulting in loopholes. Thus was born the so-called Big Bell experiment.

In 2016, the Big Bell Experiment launched and convened more than 100,000 volunteers around the world. In the experiment, all volunteers need to continuously choose based on personal free will to form binary random numbers, and press 0 or 1 quickly and randomly in the clearance game, and continuously generate a data stream of more than 1,000 bits per second within 12 hours. , all recorded in the Internet cloud, and distributed in real time and randomly to relevant research groups distributed around the world to control the Bell inequality test experiments of these research groups. The Big Bell Experiment believes that human beings have true free will. Through the free will of a large number of participants, the Big Bell Experiment closes the loophole of free choice in a wider range and strongly denies Einstein's principle of locality.

So far, the "ghost-like action at a distance" of quantum entanglement has been confirmed to exist and has been pushed to reality step by step.