Why is quantum computer faster than traditional computer?

Quantum computer is a physical device that follows the laws of quantum mechanics, performs high-speed mathematical and logical operations, and stores and processes quantum information. When a device processes and calculates quantum information and runs quantum algorithms, it is a quantum computer. The concept of quantum computer comes from the research of reversible computer. The purpose of studying reversible computers is to solve the energy consumption problem of computers.

In the 1960s and 1970s, it was found that energy consumption would lead to the chip heating in the computer, which greatly affected the integration of the chip, thus limiting the running speed of the computer. It is found that energy consumption comes from irreversible operation in the calculation process. So, does the calculation process have to be completed by irreversible operation? The answer to the question is that all classical computers can find corresponding reversible computers without affecting their computing power. Because every operation in a computer can be converted into reversible operation, it can be expressed by unitary transformation in quantum mechanics. In fact, the early quantum computers were classical computers described in the language of quantum mechanics, and did not take advantage of the essential features of quantum mechanics, such as superposition and coherence of quantum states. In classical computers, the basic information unit is bits, and the operation objects are various bit sequences. Similarly, in a quantum computer, the basic information unit is a quantum bit, and the operation object is a sequence of quantum bits. The difference is that the quantum bit sequence can be not only in the superposition state of various orthogonal States, but also in the entangled state. These special quantum states not only provide the possibility of quantum parallel computation, but also bring many wonderful properties. Different from classical computers, quantum computers can do arbitrary unitary transformation, and after obtaining the output state, they can measure and get the calculation results. Therefore, quantum computing greatly expands the classical computing, which can be regarded as a special kind of quantum computing in mathematical form. The quantum computer transforms each superimposed component, all these transformations are completed at the same time, and they are superimposed according to a certain probability amplitude, and the results are given. This kind of calculation is called quantum parallel calculation. In addition to parallel computing, another important use of quantum computer is analog subsystem, which is beyond the power of classical computer.

Whether it is quantum parallel computing or quantum analog computing, quantum coherence is essentially used. Unfortunately, it is difficult to maintain quantum coherence in practical systems. In the quantum computer, the qubit is not an isolated system, it will interact with the external environment, leading to the attenuation of quantum coherence, that is, decoherence. Therefore, to make quantum computing a reality, a core problem is to overcome decoherence. Quantum coding is the most effective method to overcome decoherence. The main quantum coding schemes are: quantum error correction code, quantum error avoidance code and quantum error prevention code. Quantum error-correcting code is an analogy of classical error-correcting code, and it is the most studied code at present. Its advantage is wide application range, but its disadvantage is low efficiency.

So far, there is no real quantum computer in the world. However, many laboratories around the world are pursuing this dream with great enthusiasm. There are many schemes to realize quantum computing, but the problem is that it is really too difficult to manipulate micro quantum States in experiments. At present, the proposed scheme mainly uses the interaction between atoms and optical cavities, cold trap binding ions, electron or nuclear spin vibration, quantum dot manipulation, superconducting quantum interference and so on. It is hard to say which scheme is more promising, but quantum dot scheme and superconducting Josephson junction scheme are more suitable for integration and miniaturization. In the future, the existing schemes may be useless, and finally a brand-new design will emerge, which is based on a new material, just like semiconductor materials are for electronic computers. The purpose of studying quantum computer is not to replace the existing computer with it. Quantum computer makes the concept of computing completely new, which is the difference between quantum computer and other computers such as optical computer and biological computer. The function of quantum computer is far more than solving some problems that classical computers can't solve. Excerpt from Science and Technology Daily