Shulou(Shulou.com)11/24 Report--

This article comes from the official account of Wechat: back to Park (ID:fanpu2019), author: Dong Weiyuan

The mystery of the quantum world is that the facts it depicts cannot be recognized by human macroscopic perception. In order to explain these counter-common sense things, physicists have put forward many theories and various interpretations. Nowadays, with the development of quantum technology, these technical terms have gradually penetrated into the public view, such as "entanglement", "measurement", "decoherence", and so on, mysterious and obscure. But this paper tries not to use mathematical formulas to explain it, but to incarnate an observer into the two-dimensional world to see what the "coin flip" is like in the quantum world, in order to help us understand quantum mechanics.

God does not play dice with the universe.

-- Einstein

"if you don't make up your mind, quantum mechanics" is as if quantum mechanics is the science of flipping a coin. And the actual situation? Uh. As a matter of fact, it is almost true. Many mysterious expressions of quantum mechanics can be understood through a coin-flipping scene.

The most obvious difference between the quantum world and the classical world is that the different states in the classical world become different aspects of the same state in the quantum world. Like a coin spinning in the air, both positive and back upward states exist at the same time. Or, more accurately, the coins in the air are actually in a "turning state", which is formed by the superposition of a "positive state" and a "back state".

Of course, there is also an inappropriateness in this metaphor, that is, the visibility of "changing dynamics". Even if the ordinary coin has not yet landed, we can still see it spinning in the air. The superposition state of a quantum system is an invisible object. We can only get a visible result by measuring and forcing the quantum coin to stop turning.

In order to make this analogy more appropriate, we temporarily think of ourselves as two-dimensional creatures that only know how to move forward and backward, but not up and down. Then it is difficult for us to intuitively understand the tumbling movement of coins in three-dimensional space. However, if the missing dimensions do not affect us, we can still establish a theoretical model of coin tumbling through experimental observation and logical reasoning. In order to describe it mathematically, we need to use some extra degrees of freedom, which is why the imaginary number I always appears in quantum mechanics. What I represents is the extra degree of freedom that our perception cannot reach and can only be logically inferred. If these extra degrees of freedom still have some symmetry, they are called "gauge degrees of freedom" in physics.

There is a very important gauge degree of freedom, which is the "phase" of the quantum state, which depicts the evolution of the whole quantum state over time. It is also very much like a coin spinning in the air, and its constantly changing posture in the air is also described by the phase of rotation.

Through the analogy of flipping a coin, we can not only imagine the evolution of a single quantum, but also further understand the interaction between the quantum. Specifically, it is the entanglement between the quantum. The most extreme kind of entanglement is equivalent to gluing two coins together. In this way, although they are still turning, the relative state between them has become completely certain. When one of them is face up, the other is always front or back up. However, there are many kinds of entanglement, most of which are not so absolute, but slightly loose adhesions. When one of the coins is facing up, the other can still twist within a certain range.

How are these loose or solid entangled relationships established? The reason is that they exchange energy with each other. Physically speaking, the essence of interaction refers to the meaning of energy exchange. And every piece of energy that passes from A to B can itself be seen as a quantum. So in technical terms, we will see the phrase "transfer interaction through the exchange of bosons". These energy packets that leave A for B carry not only the energy of A, but also the rotational state of A. Therefore, when the energy packet is obtained by B, it more or less affects the rotation state of B. This is how the entanglement relationship is established.

With the help of these images, we can now try to understand the most misunderstood problem of "quantum measurement" in quantum mechanics. At the beginning of the birth of quantum mechanics, it was only known that the quantum state would collapse to the classical state when it was measured, and the collapse process was a kind of probabilistic choice completed in an instant. It seems that this process is very unreasonable, even with the smell of "spirit determines matter".

Although quantum mechanics has been around for more than a hundred years, some people still think that Schrodinger's cat suddenly collapsed into a dead or living state at the moment of being observed, and that as long as it was not observed, the cat was in a superposition of both dead and alive. So a bunch of strange questions were immediately asked:

"if the blind are doing the experiment, will it affect the life and death of the cat?"

"if the cat observes itself, is it a physical measurement?"

"does a rock deep in space that has never been observed really exist?"

"is the universe what it is now because of human observation?"

……

Similar problems arise repeatedly on the Internet, but quantum mechanics is not that weird at all. The disappearance of the superposition state can be simply understood as that the rotating coin falls to the ground, thus stopping the rotation and completely turning into a classical state. That's all.

But why does the rotation of the coin stop? The reason is that measuring this action makes the rotating coin interact with our experimental equipment and establish an entanglement relationship. Don't forget that we are in a two-dimensional world with the experimental equipment, so these sticky entanglements flatten out the coins rotating in three-dimensional space, and finally lie honestly in our two-dimensional world. into a pure classic.

From then on, we know that the process of a particle degenerating from a superposition state to a classical state is not an instant collapse, but a physical process of relaxation time. It's just that the process is so short that we can't perceive it. Moreover, this process has nothing to do with people's cognitive consciousness, as long as there is enough energy exchange, the superposition state will be flattened by the environment and degenerated into a classical state. After understanding this, we should no longer use the misleading "collapse" to describe the process, but should use the word "decoherence".

In addition, this image can also help us understand some other properties in quantum mechanics. The uncertainty principle, for example, is nothing more than replacing coins with dice, and some of its sides are perpendicular to others, so when one side is flattened by our two-dimensional world, the other vertical sides become completely unmeasurable to us.

Finally, it should be noted that the action of quantum measurement will change the state of the object being measured, which is the helplessness that we have to face in the two-dimensional world. When we use qubits to record and process information in quantum computers, it is like using coins that rotate all the time, rather than classical bits lying flat on the ground. At this point, we cannot read the information in bits as casually as we do with classic computers. Because each reading inevitably destroys the rotation of the coin, which destroys the information in the qubits.

The so-called quantum computing process is to carefully adjust its rotation attitude without measurement, that is, to execute the algorithm. The final readings can only be made at one time after the end of all calculation processes. If a qubit inadvertently interacts with the environment in the process, decoherence will occur and quantum information will be destroyed. In order to avoid this situation, researchers are constantly looking for various ways. The common method in the process is to reduce the ambient temperature as much as possible, which can reduce the probability of quantum emission and absorption of energy packets. At the same time, in principle, researchers are also looking for relatively strong quantum states as information carriers, such as topological quantum states, which can keep their information from being destroyed at higher temperatures.

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