Although I have heard the name of quantum computer countless times, like all concepts with the word "quantum", most people don’t know it.

The concept of quantum computer was put forward in 1980s, and it has been put into research and development for 20 years. So far, none of them has really gone out of the laboratory. But it is said that it will be very powerful. Google, IBM, Alibaba and many start-ups are competing to be the first to achieve "quantum hegemony", that is, to make quantum computers faster than traditional computers in a computing task.

After a superficial understanding of the principle of quantum computer, you will find that it is not much different from the computer we are familiar with.

**Quantum computer: a quantum version of the computer**

"Don’t think of the quantum computer as a brand-new system, it is an extended version of the classic computer, imitating the classic computer everywhere." Professor Han Zhengfu, chief scientist of Anhui Wentian Quantum Technology Co., Ltd. and key laboratory of quantum information of Chinese Academy of Sciences of China University of Science and Technology, told the Science and Technology Daily reporter.

Han Zhengfu said that the classic computer represents the number 1 or 0 by the voltage level, which is a bit.

The quantum computer corresponds to a quantum bit (Q-bit), which may be an electron whose spin arrow is up or down, or a light wave that vibrates vertically or horizontally … …

Traditional computing circuits are composed of various "logic gates", which correspond to various "quantum logic gates" of quantum computers. According to certain rules, the 0 and 1 of storage bits are changed.

Han Zhengfu said: "The traditional computer is played like this: a row of memory bits is written with an initial value (such as 10011001). Next, several steps are performed, and at each step, the numbers in the memory are transformed into another set of numbers. Stop after the program and read the numbers inside, such as 00101010, which is the calculation result. " The same is true of quantum computers.

The difference between quantum computers is that Q-bit is particularly flexible and not as rigid as Bit. It is both 0 and 1. For example, it is 60% 0 and 40% 1. This gives it super powers. You have to learn a little quantum mechanics to understand the mystery of Q-bit.

**Fuzzy and accurate Q-bit**

What do you mean, "it’s 60% 0 and 40% 1"?

Take a refresher course in senior high school physics: Experiments at the beginning of the 20th century found that substances are too small to be measured accurately. Because measurement means interference, even if you only look at it. When the object is so small as quantum level, its state will be completely destroyed by observation. By the way, the "touch-and-break" effect is used in quantum communication. Using quantum to carry the password can make it possible for someone to eavesdrop on this password signal and will certainly be found out. ) This is called "uncertainty principle". The smaller things are, the more blurred they are. You go to measure the position of an electron, this time in Beijing and next time in Tianjin. We can only say that a quantum "has a high probability of being in Beijing", "has a high probability of spinning arrows rushing up" and "has a high probability of lying flat and vibrating" … …

These probabilities can be determined by multiple measurements, although the reading of a single measurement is not necessarily.

Therefore, quantum bits are fuzzy and accurate: when the same number is read, 0 is read, and sometimes 1 is read; But after reading it many times, the probability of 0 will tend to a constant value, such as 60%.

**Why is quantum computing faster?**

"In a traditional computer, a high voltage is superimposed on another high voltage, which is still a high voltage; The superposition of qubits is different. " Han Zhengfu said.

Quantum bits store a vector, just like a clock, which corresponds to the probability.

The hour hand can point to zero (the qubit reading is 100% 0) or to three (the reading is 100% 1). Or point to 1: 30 (50% is 0 and 50% is 1), or point to any angle.

Traditional computers store "10011001".

Quantum computers store "bells, bells, bells, bells". Please imagine a row of clocks hanging in the lobby of the hotel.

In traditional calculation, 1 and 0 are superimposed to 1, and then a 1 is superimposed to get 0.

In quantum computing, the superposition of "three points" and "zero point" is "1: 30", and then the superposition of "three points" results in "2: 15".

Q-bit is more expressive than bit. A Q-bit can contain infinitely complex numbers. In this sense, "one is more than one". A Q-bit input transformation is equal to the transformation of multiple digits together, which is called "parallel calculation".

When the potential of parallel computing reaches its limit, the computing power of quantum computer is 2 n ∶ 1 compared with traditional computer.

But it should be emphasized that the results of quantum computers come from probability statistics. Quantum computer is different from tradition, it has to repeat the program again and again, and read it again and again (the result is different every time). Repeatedly, enough times (the reliability of probability exceeds 99.99999%), the ratio of each qubit to 1 and 0 is counted, and that is the required number. So when it comes to less complicated computing tasks, quantum computing may be slower than classical computers.

**Rainbow and zebra**

Before the advent of quantum computers, mathematicians were imagining to use the "rich connotation" of quantum bits to greatly reduce the calculation time. But so far mathematicians have only proved that quantum computing is much faster than traditional computers in two scenarios.

The first is to crack RSA algorithm. RSA is the most commonly used encryption method at present, and its mechanism is the difficulty of factorization — — Multiplying two large prime numbers is very simple, but splitting the product into two prime numbers may take tens of thousands of years for the computer.

Therefore, banks can send a number of thousands of digits publicly and master its two prime factors without worrying about someone working out these two prime factors — — Used to make private digital keys.

But more than 20 years ago, Peter Shor proved that an algorithm based on quantum computer can easily decompose factors, which also greatly increased the interest of academic circles in developing quantum computers.

Another possible application is "searching large unordered databases", or "looking for a needle in a haystack". Traditional computers can only compare targets one by one, while quantum computers can calculate in parallel. If the traditional computer time is t, the quantum computer time is "root t" The former takes 1 million hours, while the latter can be solved in 1,000 hours.

In addition to the above two kinds of calculations, quantum computers are also expected to make great achievements in chemistry, pharmacy and other fields in the future. The reason is: unlike traditional computers, quantum computers are real analog computers, which can reproduce the real nature (physicist Feynman first pointed this out).

The 0 and 1 of traditional bits are equivalent to black and white, and the "hour hand that can point at any angle" of quantum bits is equivalent to a full-color spectrum, which can display any color.

If the traditional memory is zebra, the quantum memory is rainbow. The world is colorful, and it is certainly more direct and convenient to describe the world with rainbows.

**Just getting started**

Quantum is very fragile and will collapse easily.

"To encode information in a very tiny thing, such as an electron or a nucleus, we must first isolate it and let it not work with the surrounding area. This subtle control is very difficult. " Han Zhengfu said.

All kinds of quantum carriers are accompanied by unique difficulties, for example, photons are always advancing, and electromagnetic fields can’t control them, which is very troublesome to control. At present, researchers are probably experimenting with dozens of carriers: electrons, photons and ions in traps … …

Han Zhengfu said: "The Key Laboratory of Quantum Information of the Chinese Academy of Sciences, which belongs to the Chinese University of Science and Technology, now has more than 50 associate professors, 150 doctoral students and nearly 30 postdoctoral students. There are many different groups in a team. Although the things studied are understandable to each other, there are specialties in the field, such as ‘ Make silicon ’ Some people will study the processes that the semiconductor industry cares about, such as exposure and cleaning; ‘ Do light ’ Research laser generators, oscillators, optical fibers and the like. "

"Domestic research on quantum optics began in the early 1980s. There are many more people now, but they are still unpopular. Professionals are scarce. " Han Zhengfu said, "In fact, there are not enough talents in the world. So Google spent hundreds of millions of dollars to dig a team from the University of California, Santa Barbara, mainly studying superconducting quantum computers. "

At present, major companies and research institutions are still improving the quantum bit quantity — — Strive for dozens of quanta to be stable at the same time, and don’t collapse too quickly. Superconducting machines cost millions of dollars to make the environment close to absolute zero. The engineering experimental machine is improving, but it is not known when it will be practical.

Looking back at the first computer ENIAC in 1946, 18,000 electron tubes were used, which were glass tubes in which electrons were evacuated and flew. ENIAC weighs 30 tons and only calculates 5000 times per second. Without the semiconductor revolution more than ten years later, there would be no computers and mobile phones today.

It should be said that the quantum computer has just entered its "electron tube era". (Science and Technology Daily reporter Gao Bo)

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