Quantum communication cannot be deciphered. But before explaining the meaning of this sentence, we need to understand one thing: "quantum communication" here refers to "quantum cryptography".
In the usage of the scientific community, quantum information is an interdisciplinary subject produced by the combination of quantum mechanics and information science, including quantum communication and quantum computing. The applications of quantum communication include quantum cryptography, quantum teleportation and ultra-dense coding. The logical structure of this theme is shown in the figure below.
Content of quantum information discipline
However, all applications of quantum information have high technical difficulties, and most of them are in the stage of laboratory demonstration. Quantum cryptography technology is relatively low in difficulty, so it is close to practicality and becomes the center of media reports. Therefore, when the media reports on "quantum communication", they often actually refer to quantum cryptography, that is, a part of quantum communication rather than the whole. This is what we need to pay attention to when watching the news.
As the name implies, we can immediately understand that quantum cryptography is a secret method, and it is not something incredible like "instant transmission" and "space jump" that many people think. So, what special benefits does it have for confidentiality?
Most popular science works will tell you that quantum codes cannot be deciphered. Good, that's right. But most popular science works go too far and can't explain why, so many readers don't know what to say. The actual reason is simple: the ciphertext encrypted with the "one-time note" key is indecipherable, and the key generated by quantum cryptography is a one-time note.
What is disposable paper money? Shannon, the founder of information theory, proved such a theorem: if the key meets three conditions, then the ciphertext communication is unbreakable. These three conditions are: first, the key is a random string; Second, the length of the key is the same as plaintext, or even longer; Third, the key must be changed every time the ciphertext is transmitted, that is, "one secret at a time". Keys that meet these three conditions are vividly called "disposable notes".
river shannon
You can understand shannon theory after a little thinking. For example, the ciphertext you get is an 8-bit string DHDSBFKF, in which the original text of each bit is another character, and the corresponding rule is "advance x steps in the English alphabet", but x is a separate value for each bit (the key length is required to be at least the same as the original text, that is, the second condition), and it is random (the first condition). For example, the first x = 1, change the C in the original to the D in the cipher, the second x = 3, and change the E in the original to the H in the cipher. If you are the enemy, how can you guess the original text?
Any decoding method must be based on some clues. For example, a common clue is that letters in English are used with different frequencies (the top five are E, T, A, O and I). Therefore, by counting the frequency of each letter in the cipher text, it is expected to find out the key. But this only applies to the case that the transformation rules of each bit are the same (that is, there is only one unified X), where each bit has its own random X, so this trick is not needed. If it is not a secret at a time (the third condition), you can intercept several ciphertexts continuously, and then do this frequency analysis at the same position of multiple ciphertexts. But after adding one secret at a time, even this last hope was dashed. So, what else can you do besides being blind?
Therefore, making unbreakable ciphertext is not a patent of quantum cryptography. In traditional cryptography, as long as both parties get a one-time note key, their ciphertext cannot be deciphered.
You may wonder: in this case, isn't the problem of confidentiality solved? The answer is: no, because the real difficulty lies in the distribution of keys, or distribution.
In traditional cryptography, there are only two ways for both parties to share the key. One is to meet directly-but if it is convenient to meet, which one needs to communicate? The second is through express delivery, but the courier may be arrested or defected. The former is like Li Yuhe in The Red Lantern, and the latter is like Zhigao Pu in The Red Rock. This is the real trouble!