Is it difficult to understand the quantum communication satellite?In fact, it is very like a different place ...

More than two thousand years ago, Mozi wrote "The Light People, Ruoshuo." This sentence realized the phenomenon of "light along the straight line".

More than two thousand years later, on August 16, 2016, the quantum scientific experimental satellite "Mozi" named after "Mozi" was upgraded, and the future of "light" will continue to write the future of human information transmission.

On August 16, 2016, China launched the world's first quantum communication satellite "Mozi", picture source: Xinhua News Agency

Today, the "Mozi", which has been set for only 2 years, has been running on the track for 6 years, and has exceeded many expanded scientific research tasks.

Many people may see the word "quantum" and are puzzled. What is quantum? What are the work of the "Mozi" quantum science experimental satellite? What is it connected with "quantum"? Don't worry, let us know their secrets one by one.

One point

What is a quantum?

In our lives, we may often hear the phrase "in trouble, quantum mechanics", and we often see pseudo sciences such as "quantum speed reading" under the banner of "quantum", so "quantum", then "quantum" What is it?

This problem is not difficult to understand. All the objects in our lives have the smallest unit that constitutes its own, such as the most basic unit that constitutes "water" is "water molecules". The same is true of light. We shot a beam of light from a laser pen to subdivide this beam of light. The most basic unit to be divided into is "photon". We call the most basic unit that constitutes the material world. The "quantum" used in the "Mozi" during experiments is "optical quantum", that is, "photon".

Key second

What experiments did "Mozi" conduct?

Before answering this question, let's talk about the "online shopping" that everyone is more keen on.

Before the mobile phone has been developed, the fingerprint is developed and faces the face verification, we always need to enter the payment password on the screen when buying an order. Most people may have considered one thing: In case my password is intercepted, how can people intercepted? manage? Indeed, it is not only our payment password, but all communication information transmitted from A place to B place may be intercepted and cracked in the middle, causing our information to leak.

Gallery copyright picture, not authorized reprint

At this moment, some people may think that we make a password book, encrypted the information and other information of our payment password, and then use the same password to decrypt with the same password. Can we complete the information encryption? The reason is right, but unfortunately, the password may be decolved or even directly stolen, so that the encryption will have no effect.

Is there no complete secure encryption method? Don't worry, don't forget our protagonist today- "Mozi".

The birth of "Mozi" is an important mission to ensure communication security. Is traditional passwords safe enough? It doesn't matter, we use quantum technology to generate a "password book". This "password book" is random enough, and once it is intercepted by a third party, we will be known to both parties.

As a satellite of the quantum science experiment, the "Mozi" has three important experiments to complete: a thousand -kilometer -level quantum key (Yuè) distribution, a thousand -kilometer -grade star -level two -way quantum entanglement distribution, and a thousand -kilometer -grade star quantum quantum stealth state. After completing these three established scientific goals, the "Mozi" also carried out a number of important expansion experiments to continue to achieve new breakthroughs.

Seeing these "quantum" is going to be dark again? Don't be nervous, they are not difficult to understand, let me listen to you through the three established scientific experiments of Mozi.

Focus

What is the quantum key distribution?

what's it for?

Just now we said that in order to ensure real communication security, quantum technology is needed to make "password books". The quantum key distribution task of "Mozi" is to use the "Mozi" satellite to generate "password books" by sending photons to the ground from space to the ground.

So how does this photon form a "password book"? If you want to know this issue, we must first learn some knowledge about photons:

Polarization: Photon can vibrate in a certain direction, called "polarization". The polarizer of the photon can be divided into multiple types according to different angles, but here we only need to understand four of them: ↑ and → (that is, the polarization angle is 0 ° and the polarization angle and the 90 °), ↘ and） (that is, the polarization angle 45 ° and 135 °);

Several forms of photon polarization, the polarization angle of each photon is 180 °, so although 135 ° photon polarization in the comics is "↖", it is actually equivalent to "↘". The picture comes from Mozi Salon.

Observation: When receiving photons, you will use a pair of "special sunglasses" to observe these photons. When this pair of sunglasses are being placed, the shape is like "+", which can allow ↑ and → polarized photons to pass; when the tilt is placed, the shape is like "×", which can be allowed to pass the light and polarized photon passing;

Bit: Scientists have stipulated that after using "+" observations, the photon who obtain ↑ polarization represents Bit "0", and the photon that obtains → polarizes represents Bit "1"; similarly, after using "×" observation Then it represents Bit "0", and the photon that obtains the polarizer represents Bit "1";

Uncertainty: However, photons are special. Even the ↑ and → polarized photons can be placed through "+") through the "×" sunglasses (↗ ↗ ↘ ↘ ↘). For example, squeezed into the ↑ polarized photon of "×", it will become ↗ or ↘ after coming out, but which one is it? Don't ask, the probability is 50%, which is unclear. If you are still dizzy, then the following form will make you clearer:

After understanding these rules, we can start sending the photon and then making the "password book". The steps are as follows:

First of all, the sender will randomly generate a bunch of bit passwords, such as: 01001011;

Randomly select an observation form ("+" or "×");

According to the randomly generated bit and observation forms, a polarized photon is prepared, for example: when the bit generated by randomly generated is 0, the observation method is "+", the photon of ↑ polarization is prepared; When the method is "×", you can prepare a polarized photon;

Then send the photon to the receiver;

After receiving these photons, the receiver will randomly select an observation form "+" or "×" docking the observation of the observation and obtain the observation results.

Finally, the sender and the recipient called to communicate with their respective observation methods, retain the results of the same observation method, and other abandonment can get a completely randomly generated, safe initial "password book". Essence

This is a "random" job.

We also see that if the receiver uses "+" to observe "↗", two results will be obtained, namely: after observation of photons, the polarizer of the photon will change. In this way, if there is a third party intercepted the photon in the middle, once the "wrong" observation method is used, the polarization of the photon will change. Therefore, the receiver and observer can find the problem through "the answer" (this time the initial "password book") can be discovered, and the "password book" generated this time is abandoned.

In this way, the "0100" password obtained is a disposable quantum "password book" generated by both parties.

The above is one of the methods used by the quantum key to distribute this experiment by the "Mozi". This method is called the BB84 protocol, which was proposed by Charles Bennett and Gilles Brassard in 1984. This method can effectively discover the eavesdropping situation in the communication, so as to close the communication immediately and re -distribute the new quantum key.

The BB84 protocol proposed by Bramsad and Bennert. The picture comes from Mozi Salon.

In August 2017, the "Mozi" after one year of upgrading was the first time in the world to successfully realize the distribution experiment of a thousand -kilometer -level star -land quantum quantum key. Later, the "Mozi" also realized the quantum confidential communication of thousand -kilometers based on another quantum key distribution protocol E91. This lays a reliable technical foundation for the subsequent constructing the global quantum confidential communication network.

Key four

What is the "entangled" quantum?

What is the quantum entanglement distribution?

What do you think of when mentioning "entanglement"? The heart between the two people? Is there a spirit between couples? That's right, there are also "heart rhinos" between the two quantums. Scientists have the "heart -shaped rhinoceros" -style connection between scientists, called "quantum entanglement".

Two photons with quantum entanglement, the picture comes from Mozi Salon

There is such a mysterious term in quantum mechanics: superposition state, that is, for a quantum, when it is not observed, it may be a superposition of two states; once the quantum quantum is observed, the quantum superposition state will be Collapsed into one of the states. Quoting a classic example is Xue Dingzhang's "dead" and "alive" cat. The cats in the closed container are superimposed with "death" and "living". This is the cat's "superimposed state". Once we open the box, the stack of cats will collapse, and we can know that the cat is "dead" and "live".

Gallery copyright picture, not authorized reprint

The quantum entanglement is a strange stress of the combination of two quantums, and the "own fixed number in the underworld" -Once one of the quantum is observed, the state of the other quantum will be immediately known. Using a cat again as an example: I have two cats in the closed box. They are in the stack of "life" or "dead", and they are "entangled", that is, if I open a one, I open a one The box, seeing a cat with a superimposed state collapse "dead", then the stack of cats in another box will collapse, and it will definitely collapse into "life".

This quantum entanglement is not limited by distance, and is "non -domaining". Even if one cat is on the earth and the other is the other corner of the universe, this peculiar connection will still occur instantly. Einstein called the quantum entanglement phenomenon that span the space as "the super -distance effect of ghost charm". He was worried about it, and instead put forward the "fixed domain hidden variable theory" to explain this magical connection.

In order to verify the right or wrong, physicist Bell proposed an inequality and an experimental method. If the experimental results are in line with the same form, then the "domaining" theory of Einstein is supported; Einstein's "domaining" theory. It is a pity that the experimental results did not stand on Einstein. This "ghost -charm" in quantum mechanics is real, and quantum entanglement is indeed "non -domaining". Experiments are the only criteria for testing truth. Truth comes from strict experimental results, even if it is proposed by Einstein.

The experimental results showed that Einstein was wrong this time. Picture from Mozi Salon

To this day, many scientists are still undergoing this quantum entanglement experiment proposed by Bell: put two entangled quantum at a long distance, and then observe to see if the observation results are in line with Bell. Similarly, my country has also conducted quantum entanglement experiments between the two ground stations in Lijiang and Qinghai Dezureha, Yunnan, and is the farthest in the world -quantum entanglement experiments with a distance of 1,200 kilometers apart.

The role of "Mozi" is to launch a pair of entangled quantum to these two ground stations in space. The success of this experiment has laid the foundation for my country's entangled quantum confidential communication in the future. In 2020, Mozi realized a tangled thousand -kilometer -level quantum confidential communication.

The "Mozi" satellite is distributed in quantum entanglement to verify the Bells. The picture comes from Mozi Salon.

Focus

What is the quantum invisible state?

Can remote material transmission be achieved?

Before answering this question, let's first look at a long -distance love story:

There is a pair of couple A and B, of which B is more vicious, but the two sides still have a long -distance relationship and "entangle" each other. At the same time, they have two common friends C and D to observe A and B in both places.

One day, A met X. X is not as vicious as B and very friendly, so A and B cut off "entanglement" and be with X, and started a new "entanglement".

At this time, my friend C observed the incident and told my friend D after taking a picture. After D, I looked at the photo and conducted some friendship education on B. B suddenly realized after being educated. Like X into a kind person.

In this example, we can use A and B as two entangled quantum, C and D are scientists in the location where A and B are located, while X is an unknown quantum state. Scientists hope to pass the "quantum invisible transmission state" method. Transmit this unknown quantum state to B.

During the entire transmission process, the quantum x itself was not transmitted, but the state of X was transmitted to B -as the state of X in the alien love example was passed on to B, so that B's state changed from "vicious" to change from "vicious". Become "friendly".

This is the "quantum stealth". However, it should be noted that when the "quantum invisible states", the entanglement of A and B disappears, and new entanglement with A and X occur almost at the same time.

You will find that in the "quantum invisible transmission state", the state of quantum x is transmitted, and the particles itself has not been transmitted. This is very different from the remote transmission of material in science fiction works. It is unrealistic to send objects through quantum technology from A place to B place. At present, it is unrealistic.

Point 6

What can the "Mozi" do for us?

If you see here, then congratulations, you have basically learned what is the three main scientific experiments carried by the "Mozi" quantum science experiment satellite. But at this time you may ask: What impact will these three experiments have on me?

The discovery of each new theory and the realization of new technologies fundamentally represent a new step in humans on the unknown world. "Mozi" is the crystallization of the most advanced quantum communication technology in my country, and it is also the leading level internationally. Quantum communication technology has begun to appear in the field of civil and commercial. In the future, more mature quantum communication technology will be more widely used in many aspects such as communication, finance, and national defense.

Author: Popular Science China × Mozi Salon

This article is produced by popular science China and Mozi Salon.

The questionmation of this article comes from Science.

Some pictures in this article are from the copyright gallery and are not authorized to reprint.

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