Quantum communication literally means the application of quantum science in communication.
This topic is very complicated. Quantum and communication have to be discussed separately in the previous semester. I can only explain it simply with less accurate metaphors, because an accurate explanation requires deducing mathematical disclosures. All popular science that can be seen now is comparative.
It is just accurate popular science and cannot be regarded as a guideline.
Quantum communication is divided into two types according to whether the transmitted information is classical or quantum.
The former is used for the transmission of quantum keys, which has not yet left the scope of optical fiber.
The latter is used for quantum teleportation and quantum entanglement distribution.
Teleportation is the highest pursuit of quantum science. Simply put, it is to copy a thing (information) 100% and then transmit it to the receiving location. This means that the same basic units that constitute the original must be selected to make a perfect copy of the original.
An exact copy, but this is absolutely impossible because quantum mechanics has uncertainties and does not allow accurate replication of information.
This characteristic that cannot be completely copied is called "God really throws dice" as the saying goes. Bohr beat Einstein on this point, which is absolutely impressive.
If it is expressed in writing, it can generally be described by Bell's inequality, uncertainty principle, etc.
Then based on this feature, quantum communication came into being. The most important thing at present is to greatly improve security. As for teleportation based on quantum entanglement, it is also planned!
So why is quantum communication so secure?
We have to start from the beginning. Matter is composed of atoms. The energy of atoms does not change continuously. It is done piece by piece. The smallest bit is called a quantum.
When we use quantum communication satellites to send codes, we must first send polarized photons, which represent a bit of 1 or 0.
Since light is an electromagnetic wave and has wave-particle duality, light can be subdivided into photons. At the same time, it is very unstable and can be polarized in unspecified directions. In this case, we have to specify the polarization angle in advance, such as towards zero angle.
The polarization represents 0, and the polarization toward 90 degrees represents bit 1.
Then the receiving device can only allow zero-angle and 90-degree polarized photons to pass through. Even the receiving device must set a fixed angle in advance, such as 45 degrees, so that the correct bits can be received smoothly. Polarized photons with other wrong angles will be blocked.
was filtered.
But what if a zero-angle photon wants to pass through the 0-bit opening and the 1-bit opening?
Then you have to roll the dice. This is the so-called "God rolls the dice".
In this way, when the photons emitted by the communication satellite are placed in the same direction as the receiver, the received bits are correct. Interestingly, the placement angle of the receiver is also random. The placement angle must be discussed every time
The problem
If both are random, a lot of unsuccessful bits will be generated, the unsuccessful bits will be deleted, and the rest will be the random password sent this time.
Since only one photon is sent at a time, if it is eavesdropped, it means that the message has been intercepted and read, and the receiver will definitely know it, because as mentioned before.
There is no concept of complete replication in quantum. It is impossible to intercept and then give the same photon information. This is where the "quantum non-cloning theorem" and the uncertainty principle come into play, truly achieving one encryption at a time.
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Randomly generated, unable to be eavesdropped or cracked, the security of quantum communication is naturally high. The current method for spreading passwords in quantum communication is called the BB84 protocol.
At present, the quantum distribution that humans have on hand does not communicate, it only distributes keys! It only distributes keys!
Quantum key distribution cannot actively protect against eavesdropping, but can only passively detect whether others are eavesdropping on you.
The detection of eavesdropping is still based on the principles of uncertainty and non-cloning. This is the current situation of mankind.
The current quantum keys still rely on conventional communication and cannot exceed the speed of light. The super-light quantum entanglement is just a fantasy. It is impossible to know whether the cat is dead or alive without opening the box! Quantum teleportation is too magical and perfect.
Quantum entanglement applications should be unattainable.
Currently, quantum keys are commercialized. The longest distance for quantum key distribution in the laboratory is 260 to 300 kilometers. The realization of long-distance quantum communication relies on relay stations. Relays are divided into quantum relays and trusted relays. Later,
It has been industrialized. The principle is that A transmits information to B, and B then transmits it to C. There is a certain risk when the key is released in the middle.
Quantum relay is not grounded. It uses quantum entanglement distribution technology to establish shared entangled pairs between adjacent sites, uses quantum storage technology to store the entangled pairs, and then uses long-distance free space transmission technology to achieve quantum entanglement conversion with extremely high fidelity.
high.
At present, the longest distance for the best principle experiment of quantum state teleportation is 143 kilometers, provided that the environment in the Canary Islands of Spain is good. It seems to be much better than the 16 kilometers in the capital of Xia and the 97 kilometers in the QH province.
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However, there are too many doubts about this Spanish experiment. No matter how you look at it, it looks like a scam to break Xia Guo's record.
If you want to reach a longer distance, you have to avoid the atmosphere's interference with the fragile quantum state of photons, and you have to use satellites to distribute photons. Thousands of kilometers in space are only about 8 kilometers on the ground. Going into space is imperative.
Xia Guo's achievements are the strongest, and they only achieve six-photon entanglement, eight-photon entanglement and ten-bit super-entanglement. Even if they achieve hundreds of kilometers of transmission on the ground, they can only theoretically transmit 1,000 kilometers in space. The distance is commercial.
It's still far away.
The system has given guidance, but the current manufacturing level is still lacking, and improvement cannot be achieved in one step.
Chen Xiao was not greedy at all. He decided to start with the relay station and turn the trusted relay into a quantum relay, and then slowly move from quantum keys to quantum state teleportation. Although it was definitely not 100%, it was
The reliability of this thing is really strong. As long as it is close to perfect quantum entanglement, many things can be omitted.
The idea is not that there is no idea, just divide the original information into two parts: classical information and quantum information, and transmit them to the recipient through their respective channels.
It can be copied after it is received. It is worth noting that classical information is obtained by the sender making certain measurements of the original substance, and quantum information is the remaining information that the sender did not extract during the measurement.
What is transferred in this process is only the quantum state of the original substance, not the original substance itself.
The sender may not even know anything about this quantum state, while the receiver places other particles in the quantum state of the original matter.
In this scheme, the non-locality of the entangled state plays an extremely important role.
Chen Xiao does not want to directly transfer items in one step, but just communicates. It does not need to be so perfect, and the difficulty is naturally reduced a lot.
At that time, communication based on quantum entanglement will be quantum communication in the true sense. Now it can only be called quantum encryption means communication, which is not a concept at all.
If the construction of quantum communication is really completed, there will be no need to worry about signal problems. The equipment is equipped with a quantum communication chip. You only need to use the principle of quantum entanglement and look at the quantum state here to understand the information transmitted by the other party.
The experiment was handed over to the Changtian Academy of Science and Technology to design and host the experiment, which has been involved before.
To be on the safe side, Chen Xiao invited relevant experts from the Xia Guo Academy of Sciences to discuss the matter, including two top experts, Pan Wei and Lu Yang.
To be honest, their team is very strong, and it is their credit that Xia Guo is ahead of the world. When they were first invited, they didn't take it seriously. How could Changtian Technology be involved in this aspect?
But then I thought about it. Changtian Technology has even developed organisms that can control nuclear fusion and automatically evolve. It may not be impossible, so I still have some small expectations.
When the academic seminar began, Pan Wei and Lu Yang knew that they were wrong. The progress of Changtian Technology was similar to that of Xia Guo Academy of Sciences.
"The distance between you has exceeded two hundred kilometers?"
"Has the quantum random generator reached 20Gbps? This one is already a little ahead of us! Unfortunately, it still cannot be truly random."
"You also have a 6-photon spin-orbital angular momentum experimental platform? Spin and orbital angular momentum can be transmitted simultaneously? You have also conducted experiments on 8-photon and 10-bit super-entanglement,"
"You also use cold atom quantum storage technology to achieve quantum switching with higher storage and reading functions! You can even play with the length of 350 meters of optical fiber! This is the quantum repeater that is urgently needed for long-range quantum communication, but
This distance is still too short.”
Pan Wei and Lu Yang no longer have the slightest contempt. Changtian Technology has achieved this step without saying a word. Doesn't Changtian Technology have no blind spots in scientific research? It is involved in any frontier field and has made great achievements.
A few years ago, when Changtian Technology was completing its 4G communication layout, it was experimenting with quantum technology, but because it had no applications, there was no publicity.
"We are deeply touched by your candid announcement. To be honest, we have nothing to show for it. We are so ashamed!" Pan Wei told the truth, "Being inferior to others means being inferior to others."
The person in charge of Changtian Technology’s quantum project is Guo Can.
This person was about the same age as Pan Wei, both young and promising people. He said humbly: "Academician Pan deserves the award. The fact that you have results that can be published means that there are more things that have not been revealed."
"Those are all guesses, they are all experiments, and they haven't been successful yet..."
"Don't be too self-effacing. At this point, conjectures are often more valuable than actual achievements. Maybe if a conjecture is confirmed, all the previous achievements will be worth mentioning. I invite you here just to show us what we have achieved." Let’s discuss ideas with everyone, inspire each other, and achieve theoretical breakthroughs earlier.” Guo Can waved his hand.
"Okay, what do you guys have in mind? Let's see if it works." Lu Yang was much more direct. He came here just for research, and the world's human connections don't matter at all.
"..."
Guo Can was stunned for a moment, and then distributed the research ideas. If Pan Wei and Lu Yang couldn't understand it, no one on the planet could understand it either.
When Guo Can got the information given by Chen Xiao, he was completely confused. He didn't understand these things, but where did the boss get them?
If you don’t fully understand it yourself, how can you translate it into newer results?
So he was full of expectations for the arrival of Pan Wei and Lu Yang. It was like a person who was not sure about a question, but two people with similar levels happened to come, and he had to show them his level.
After a while, Pan Wei and Lu Yang frowned and didn't say a word for a long time.
"How about it? Please tell me something. Are there any problems with the conceived principle and experimental plan?" Guo Can was very worried.
"I don't fully understand." Academician Lu Yang replied in a bachelor's manner.
Academician Pan Wei also shook his head: "I'm confused, and I always feel that the designed experiment is fine. If it were me, I would design it like this. The most critical question is, where do the principles and ideas come from? Span It’s a bit big, but it seems to make sense.”
Hearing this, Guo Can's heart dropped. It turns out that it's not that he is not sure, but that everyone is not sure! Since you are not sure, what are you afraid of when doing experiments? No one has a standard answer, so find it yourself. Answer.
"The principles and ideas came to me by accident. This is based on intuition, so I asked two academicians to come for reference." Guo Can did not say that the information was given by Chen Xiao. The sources of these things are basically kept confidential. Chen Xiao Don't want to attract too much attention.
"We have a head start, but we don't have any ideas anyway." Lu Yang was very honest. Researching to the most advanced level means building a new path on his own, without any predecessors as a reference.
Pan Wei also means the same thing. Although the subdivisions of their research are different, the basics are quantum applications. If Lu Yang has problems, he will naturally have problems too.
The experiment was carried out very quickly. It was really a race against time and every day.
The specific experimental idea of Changtian Technology is to use ultra-strong laser as the light source. The advantage of this is that it is more stable than the weak correlation light source and the quantum state is not easy to change.
The disadvantage is that once the laser is emitted, the photons do not burst out one by one. In this case, the problem of the photons carrying information must be solved.
The security of quantum communication lies in the fact that one photon is equal to a password. If such a bunch of photons all contain information, it can theoretically be easily intercepted and cracked.
In this way, it is necessary to ensure that a single photon is powerful enough. What Changtian Technology has to do is to capture specific photons. As for other photons in the beam, it is equivalent to the fuel when the rocket is launched, playing a propulsive role.
Specific photons are captured in a beam of light for encryption. After the receiving device is exposed to the laser, it will perform multi-slit interference on the beam to separate the photons, and then pass the polarization equipment to get the specific photons. The process is complicated, but it is all in
You can imagine how difficult it is to do it at the speed of light.
"The quantum state generator is stable and continues to increase, and the ultra-high voltage power supply device is turned on to ensure that the laser is powerful enough!"
"The quantum channel is being built and is already relatively stable, but the distance is too short."
"Nonsense, if the distance was not short, we would have achieved quantum communication long ago!"
"Where is the quantum test device? Is it open?"
"opened!"
"What about the device that records specific photon information? This is the password!"
"opened!"
"Then let's get started!"
Everyone was very nervous. Although it would take many experiments and conversion of frequency and polarization angle to see whether the idea would work, everyone hoped that it would only take one time.