Is Doraemon's portal just around the corner? Use Apple to help you understand quantum entanglement, which won the 2022 Nobel Prize

At 17:45 on October 4, 2022, Beijing time, the 2022 Nobel Prize in Physics was officially announced in Stockholm. French scientist Alain Aspect, American scientist John F. Clauser and Austrian scientist Anton Zeilinger shared the honor. The official reason given by the Nobel Prize was that they "conducted experiments with entangled photons, established the invalidity of Bell's inequality, and pioneered quantum information science."

(Image source: Nobel Prize official website)

The three scientists who won the Nobel Prize this time proved the feasibility of quantum entanglement in theory and are the founders of the field of quantum communication. Their achievements make the implementation of quantum communication and the construction of quantum computers possible, which is a major achievement that may completely subvert the existing scientific and technological system of mankind in the future. The three scientists won the "Wolf Prize", known as the "Little Nobel Prize", in 2010 and have been hot candidates for the Nobel Prize in Physics for more than ten years.

Unlike integrated circuits, blue LEDs and many other tangible results that utilize quantum technology, quantum science itself is highly abstract. This is why the general public tends to be in a state of ignorance about quantum communication. Here, the author has set a small goal for himself - this article will not use difficult technical terms, but will try to use simple examples in life to give everyone a perceptual understanding of quantum science and quantum communication.

Come and take a few minutes to learn about one of the most cutting-edge science and technology of mankind.

An imaginary picture of quantum entanglement (Image source: Nobel Prize official website)

01

My territory, my rules

Welcome to the quantum world

After more than a hundred years of development, the relevant research of quantum science has formed a huge system with flourishing branches and leaves, and its correctness is constantly being verified. However, we have to admit that many laws of quantum science are completely different from our daily experience in the macroscopic world. When I was studying related courses at the master's level, I often felt incredible and even unwilling to believe it. In order to make it easier for everyone to understand the following content, we first use metaphors to give a few examples that can reflect the wonders of the quantum world.

Before giving examples, let us remember some "settings" - when the dimension of the object of physics research itself enters the microscopic field, quantum theory will control various natural laws. In the microscopic quantum world, the first characteristic of the "residents" is quantization. Several physical quantities such as mass and energy will change from continuous changes in the macroscopic world to discontinuous changes, reflecting quantum characteristics.

To give an inaccurate example, apples in the macroscopic world are big and small, and the size of apples can change continuously. For example, if we pick a hundred apples, their size may be any value between 0.1 kg and 1 kg. However, the size of apples in the microscopic world does not change continuously, but is equivalent to an integer multiple of a certain basic apple size. There are no microscopic apples of other sizes.

In the world of quantum mechanics, many physical quantities are integer multiples of a certain basic value (Image source: self-made by the author)

In the quantum world, various microscopic particles are constantly moving in various forms, or in other words, their states are constantly changing. This does not conflict much with our daily experience, after all, movement is eternal and stillness is relative. But in the quantum world, the movement of particles is naturally random. At any moment, if no measurement is made, no one can know where the microscopic particles have moved to.

Let's take apples as an example. In the macroscopic world, apples fall to the ground when they are ripe. If we start calculating from the moment the apple leaves the tree, we can use Newton's laws to deduce the position and speed of the apple at any time before it falls to the ground. However, in the microscopic world, we have no idea where the apple will appear in the next second. We can only give a probability to indicate where it is.

Another amazing thing about the quantum world is that our observations affect the state of microscopic particles. The color of apples in the macroscopic world is always fixed in a short period of time, whether we are staring at them or not. They are either green or red. Whether you close your eyes or not, the color of the apple will not change. What about apples in the microscopic world? Before we observe them, the state of the microscopic apple is random, it may be red or green. Once we open our eyes to observe, the state of the microscopic apple will be fixed, showing green or red.

Portrait of the founders of quantum mechanics (Fifth Solvay Conference, 1927)

(Image source: Public domain)

02

Quantum entanglement: "telepathy" between particles

After talking about some basic principles in the quantum world, let’s take a look at the concepts of quantum entanglement and quantum communication.

Quantum entanglement is a very mysterious phenomenon in the quantum world. Once a pair (or more) of microscopic particles are in an entangled state, they will affect each other. When the state of one particle changes, the other will change instantly. The most amazing thing is that this entanglement is not really like a ball of thread entangled together in the macro sense, but can be achieved over a very long distance.

Let's take an example. If two microscopic apples are in an entangled state, then when one of them is green, the other will instantly turn red. Even if one of them is on Venus and the other on Mars, this state change will still occur instantly across time and space.

Quantum entanglement is so magical that once its concept was proposed, it immediately became the theoretical basis for many scientific fantasies, among which teleportation and quantum communication are more typical. The so-called teleportation is Doraemon's portal, which can teleport people or objects to far away places in an instant. It must be said that although quantum entanglement has a bit of teleportation, it is not the entity that moves, but only the state, and some special states of microscopic particles. Therefore, whether teleportation is possible is not something that quantum entanglement can answer.

A method for preparing entangled photon pairs

(Image source: Wikipedia, author: Lao Chen)

03

Quantum communication makes scientific fantasy a reality

However, quantum communication has indeed become one of the current research hotspots of mankind, and its theoretical basis is quantum entanglement. Imagine that when two particles are in an entangled state, no matter how far apart they are, once the state of one of them is determined by our observation, the state of the other one will also be fixed. In other words, although the entity cannot be transmitted, information can be transmitted. Isn't this what modern communication is doing?

Actual quantum communication is of course very complicated, but we can still use analogies to understand the implementation process of quantum communication as simply as possible.

First, we need to cultivate two small apples that meet the quantum entanglement state in the microscopic orchard. One of them is green and the other is red. When one changes color, the other will change color instantly. Next, we send one of the two apples to Mars, and the other is on Earth. At this time, they do not have the ability to transmit information, so we need to introduce a small apple specifically for control.

When we bring the third apple close to the apple on Earth, the state of the apple on Earth will change, for example, from red to green. At this time, the apple on Mars will instantly sense this change and change from green to red. In this way, a signal transmission is completed. After that, if we take away the third apple on Earth, the first apple will instantly turn back to red, and the second apple on Mars will instantly turn green. Even the two states of red and green can be defined as 0 and 1 in a binary computer. If the speed at which the third apple approaches and is taken away is fast enough, we can transmit rich information in a short time.

It has to be said that from the perspective of ordinary people, quantum entanglement is too "weird", even black holes are not so difficult to accept. However, it is indeed the basic law of the material world. As the Nobel Prize Committee said when announcing the award, quantum entanglement is not the self-entertainment of scientific geeks, nor is it a scientific fantasy. It actually exists around each of us.

It is worth mentioning that the example of China's Micius quantum communication satellite completing an intercontinental quantum communication experiment in 2018 also appeared in the official PPT when the award was announced. This is a great affirmation of China's achievements in this field, and let us look forward to more progress together.

China's Mozi appeared in the official Nobel Prize PPT

(Image source: Nobel Prize official website)

Whether it was yesterday's Nobel Prize in Physiology or Medicine or today's Nobel Prize in Physics, this year's Nobel Prize seems to favor basic research more. Who will win the Chemistry Prize? Let's wait and see.

Produced by: Science Popularization China

Author: Zhang Hao

Producer: China Science Expo

The article is produced by Science Popularization China, written by Zhang Hao, and supervised by China Science Popularization Expo. Please indicate the source for reprinting

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