When the word "quantum" is mentioned, most people may think of the mysterious and mysterious quantum physics and Einstein's famous assertion that "God does not play dice". In our past impressions, quantum physics is often associated with words such as "uncertainty" and "uncertainty". However, we also know that the most important thing for communication is stability, security, and reliability. So, how are quantum and communication, two seemingly contradictory things, connected together? To understand this, let’s start with why traditional communication requires “quantum”.
Limitations of traditional communication
As we all know, passwords have now become a pervasive part of our lives. For example, online shopping transfers, logging in to WeChat, and even in the transmission of information that we cannot see, passwords are needed because it can ensure the security of communications and transactions. But with encryption, there are codebreakers, and the rivalry between these two sworn enemies has persisted throughout the history of our communications. Especially in war years, the success of decryption can even affect the final battle situation. During World War II, the U.S. military cracked the encryption method of Japanese military telegrams and thus grasped the whereabouts of high-ranking Japanese military officials. It eventually succeeded in killing its naval commander-in-chief Isoroku Yamamoto, laying the foundation for victory in the Pacific War. In addition, direct eavesdropping and interception of information are also very common ways of leaking secrets. For example, the movie "Eavesdropping" tells a series of stories that occur through eavesdropping on other people's communications.
People have been wondering, is there a way to transmit information securely? We can summarize that there are actually two ideas for "making communications confidential". One is physical encryption, such as a dedicated line between A and B. There are sentries in the middle of the dedicated line. Any spy who wants to intercept information must do something on the optical cable to eavesdrop, and this will inevitably be captured by the sentries. In this secure channel, we don't even need to encrypt the information, we can just exchange the information directly in plain text. However, the method of physical isolation is ultimately unrealistic. It has low efficiency, high cost, and limited distance. Only a few important and qualified positions can afford this method.
The other is information encryption, which is to add a password to the encapsulated information and then transmit it through the public channel. This is equivalent to putting it in a small locked box for transportation, even if it is intercepted along the way. It doesn't matter if someone intercepts it, because only the person opposite who has the key can open the box and obtain the information. This method is the traditional encryption method we commonly use today.
It’s just that the existing cryptographic system still ensures security by increasing computational complexity. For example, RSA, the most widely used cryptographic algorithm, uses the product of two very large prime numbers to establish a key. As we all know, there is no better way to factor the product of two large prime numbers than by brute force. Data shows that it would take tens of billions of years to exhaustively crack a 500-bit RSA key using the fastest traditional computer available - almost impossible to crack.
But in theory, any password of limited length can be cracked as long as there is a sufficiently advanced computer. As computer technology updates and iterates, faster and more powerful computers may appear in the future, such as quantum computers under development. At that time, if the corresponding encryption method cannot be upgraded, the original password will no longer be safe, and areas such as the financial system and personal privacy will be completely thrown into chaos.