■Editor's note
Recently, an article claiming to report a famous domestic quantum scientific research group with real name has spread in the WeChat circle. This article is extremely fierce in wording and involves famous figures in the domestic technology industry, which is easy to become popular. This was discussed in the group of friends of the author of this article, and the screenshot was as follows:
General reports require relevant departments or professional reporters to investigate and verify. But most of the content of this report is on the scientific level, and a qualified science author can explain this matter clearly.
This report article begins with the quantum uncloning theorem. The entire quantum communication (actually called quantum key allocation is more accurate) industry developed based on the quantum uncloning theorem. The first whistleblower published an article in September this year saying that this theorem is wrong.
This is an article just published not long ago, and the people reported have been working in this industry for 20 years. If you take a step back, even if this article is correct, it is a new understanding of the scientific community. You can't use this to report people you didn't know about before!
What is quantum unclone theorem ? Let us first explain what cloning is. Cloning is copying, copying. You copy a file on your computer and it is cloned. If the letter A represents all the information in your file, cloning means turning one piece of information into two pieces:
This expression is clear enough for you as a computer user, but it is not accurate at the physical level. Because your file is located in disk or flash chip , that part of the storage space originally had content, which may be formatted or contained discarded data. X represents the original content in the new storage space. The exact physical expression cloned is:
What is the specific value of A or X, your computer can complete the above operations. But if A and X are quantum information rather than classic information on ordinary computers, this operation is impossible. quantum state unclone theorem tells us that it is impossible to design a machine or program that replicates quantum data.
Why is the unclone theorem closely related to quantum communication? Quantum communication uses a single photon to transmit information (our current fiber communication , sending classic information, using an laser pulse , a very large number of photons to transmit). Photons have two basic polarization directions, horizontal or vertical. If used to transmit classic information, a photon can be used as an bit to represent 0 and 1 respectively. However, the quantum state of the photon can be any combination of these two basic polarizations, any linear combination of complex numbers as coefficients. It corresponds to not only the linear polarization of photons in any direction, but also the circular polarization of the rotation circle. The photon in quantum communication is an qubit , which is not either 0 or 1, but is composed of 0 and 1. Quantum information is much richer than classic information.
Assuming that the enemy found our communication fiber on the battlefield, they could cut it off, but eavesdropping is often more beneficial to the enemy than cutting off communication, which requires keeping communication uninterrupted at the same time. Quantum communication uses single photon communication, and the enemy must copy the state of this photon if eavesdropped, transfer one of the photons to our recipients, and the other for themselves to analyze. The principle of quantum state inaccuracy makes it theoretically impossible to eavesdrop on quantum communication.
Generally, the photon state is a combination of 0 and 1. If the quantum state of the received photon is not known in advance, any receiver can only randomly receive 0 or 1. No matter whether the received 0 or 1 cannot represent the original quantum state. Measurements in quantum mechanics always destroy the original state, so quantum states cannot be replicated. (After reading this, you may ask how do we receive quantum information? You need to read the reference book I gave. It will explain the clever protocol for quantum key allocation.)
The proof of the quantum non-cloning theorem is actually relatively simple, and it can't take a few lines of formulas. I will explain it to readers with a basic mathematics here, and other readers can skip the following paragraph.
We know that a quantum state is a linear combination of some basic states, a qubit is a combination of |0 and |1, and two qubits are a combination of 4 states: |0, 0, |0, 1, |1, 0, |1, 1. In quantum mechanics, we are used to writing quantum states in a semi-sharp bracket. A quantum state composed of two qubits (or two parts of a quantum system) can be expressed in the form of "direct multiplication" such as |A and B.
Any machine that operates on quantum states must of course comply with the Schrödinger equation of quantum mechanics. In different scenarios, the expression of the Schrödinger equation will be very different. But the Schrödinger equation has a basic property: it simply turns the quantum state from a linear combination of a basic state into a different linear combination of the same basic state; and this change itself is linear. That is to say, if state A becomes D and state B becomes E under the same operation, then A+B will become D+E.
For readers who have studied linear algebra , we can say that the evolution of quantum mechanics is always a linear transformation of quantum states, and to be more precise, a unitary transformation. Understanding the linear properties of quantum mechanics is enough to prove the non-cloning theorem:
assumes that it can create a quantum cloning (copiercing) machine, represented by C (it is a matrix in linear algebra, and the quantum state is a vector). That is to say, for any A and X, it can copy A:
Then it can naturally copy state B and state A+B:
However, using the linearity of quantum mechanics:
Did you see it? These two expressions are contradictory. So C is impossible.
The reporter said that this theorem, which is not difficult for professionals, was proved wrong after 40 years of being proposed. Since he has published his paper, he should be inspected by peer according to the rules of the scientific community. However, I asked my friends who were doing cutting-edge research in physics that they had never heard of the "Morning Journal of Basic Physics", but I had heard of the author, who had always wanted to overturn the theory of relativity.
If his article is correct, it will be very important; in the words of an industry insider: it is equivalent to discovering super-light speed . In my opinion, if the author has such important results, he doesn’t have to rely on the reporter to ask for attention. If this article is promoted to gain recognition from peers, even if he does not win the Nobel Prize for in the future, he will be more famous than the person he reports.
Regarding the non-clone theorem, the whistleblower said 4 points. In addition to listing his own paper, the second point quoted an article by an American scholar, but that article was considered wrong by the industry. The fourth point says this: The paragraph
wrongly describes the physical concept. What he meant is that your quantum communication is nothing more than sending horizontal or vertically polarized photons and preparing horizontal or vertically polarized photons. Can't I? Who said it cannot be cloned? If you know the quantum state of the incident photon in advance, you can of course prepare photons in the same state. But during the eavesdropping communication, you receive photons in unknown states. We have explained before that the photon is in an unknown mixed polarization state at this time, and your receiver can only randomly receive it into horizontal or vertical polarization. It is impossible to know its original state, and it is even more impossible to copy this unknown state.
The third point he talked about is very interesting and worth spending more time discussing.
The so-called laser is a cloning of quantums. It should be said that it is stimulated radiation, which is a physical phenomenon that the laser depends on.If most of the atoms in matter are brought to a high energy level, the atom will have two ways of radiation: first, the electrons will jump down to the low energy level by themselves and emit photons of a specific frequency, which is spontaneous radiation; secondly, if a photon of the same frequency passes by, another photon will be brought out, which is stimulated radiation. The photons brought out by the stimulated radiation are exactly the same as the direction of motion and polarization of the incident photons, and are in the same quantum state. In such matter, a photon passes through many atoms and can bring out a large number of photons in the same state like an avalanche. This does look like a cloned photon.
stimulated radiation
stimulated radiation The top experts in the industry know about the meme of the non-clone theorem. What is the answer? When a photon passes through an atom, the atom may generate stimulated radiation, but spontaneous radiation may still occur, and the direction of spontaneous radiation is random. The answer given to us by Schrödinger's equation is a superposition of all possibilities. Therefore, although the stimulated radiation itself seems to have achieved photon cloning, the photons that are actually brought out are mixed with spontaneous radiation photons, and are not cloning of incident photons. Some people have already rigorously proved in theory that due to this inevitable mixing, stimulated radiation cannot be used to clone photons. Experimental physicists tell us that using stimulated radiation can make optical signal amplifiers, but if the incident is a single photon, the output is completely noise. The
laser has a resonance cavity, selects a specific photonic state for amplification, and suppresses spontaneous radiation. This mechanism cannot be used to amplify photons in an unknown state.
To sum up, these whistleblowers' opinions on the theorem of quantum state non-cloning can basically be determined to be completely scientifically wrong. They have some other opinions, but it is not easy to explain a scientific concept and achievements to ordinary readers, so they will not waste space to further analyze it. Will the reported research group respond? I remember that there was a famous saying in Guo Degang : If you care about it, you will lose.
Finally, there is no problem with the research on quantum communication in science. Look at the papers published by this research team in top scientific journals like Nature. If there is any problem, you must first hold the review of these top international journals accountable. However, the application value and commercial value of quantum communication are subject to debate.
Reference book: "Understanding Quantum Mechanics from Scratch" "Quantum Information Brief Introduction"
This article was published on the WeChat public account on December 5, 2022. Physics Doctor Watch the World "Quantum Communication Is Reported? Talk about the Theorem of Uncloning", Fengyun Voice is authorized to reproduce.
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