What is “quantum entanglement,” which troubled even Einstein?

Quantum with particle and wave properties

Quantum is often thought of as an esoteric concept, but this is because quantum is, in the first place, quite far from any everyday experience.

It is generally considered that quantum refers to all very small particles such as electrons, protons, and neutrons, which we cannot see with our eyes. However, it means not only small but also something that has the dual nature of a particle and a wave.

For example, many people think of molecules as particles because they are often represented using spheres like marbles. You can imagine when such marble-like particles roll and collide with another particle, they bounce off in different directions.

On the other hand, light and sound are often depicted as waves, so many people have an image of waves. When such waves collide with each other, they do not bounce off each other like particles, but they strengthen or weaken each other’s amplitude.

In fact, there are techniques to make the sound louder by overlapping them, or conversely, to cancel them out with sound. In other words, waves do not collide and bounce, but they interfere.

We distinguish between the properties of particles and waves as being completely different, based on what we see and experience in our daily lives.

In the quantum world, however, the same thing sometimes exhibits the properties of a particle and sometimes the properties of a wave. Such things are called “quantum.” The double-slit experiment is a clear example of this strange property.

Two gaps (slits) are made in a flat plate, and many quanta are shot into the plate from a distance. If they are particles, the ones that pass through the gaps and travel in a straight line should hit the wall behind the plate and form the same pattern as in the gaps.

However, in reality, the stripe pattern spreads out on the wall. This indicates that the quantum particles spread through the gap like a wave, interfere with each other, and become stronger or weaker, but each quantum that hits the wall and creates the stripe pattern is in fact a particle.

In the microscopic world, it is not the case that a quantum always exhibits the properties of a wave. Very strangely, when one examines which way the quantum is passing through the slit, the interference effect is lost and the pattern on the wall shows particle-like results.

The phenomenon of particles appearing or waves appearing is hard to understand and leaves us feeling uncomfortable. However, the laws of nature are made regardless of whether we can understand them easily or not. Since the world is made of quantum, the world we live in is subject to the natural laws of quantum.

In fact, the semiconductor technology that underpins modern electronics has its origins in quantum mechanics. And it is highly likely that the various technologies that will open up the next generation will also be born from quantum mechanics.

Superconductivity is another mysterious quantum phenomenon

It is known that there are many other strange things in the quantum world. For example, there is the phenomenon of quantum entanglement, in which two entangled quanta are in a certain state depending on the state of one of them.

An entangled state where one state is up and the other is down and simultaneously one state is down and the other is up is allowed in the quantum world. One state can be either up or down in this state.

What is strange about this quantum entanglement is that the entangled state is maintained even when one and the other are separated by the size of the universe. Therefore, the moment you confirm that one is up, you instantly know that the other is down, even though they are about the size of the universe apart.

This phenomenon also troubled Einstein. According to his theory of relativity, no information can travel faster than the speed of light. For this reason, Einstein called quantum entanglement an uncanny remote action.

However, natural laws are not determined by ease of understanding or acceptance, but are supported by experimental facts. In fact, the 2022 Nobel Prize in Physics was awarded to Alain Aspect, John F. Clauser, and Anton Zeilinger for their success in experimentally verifying the existence of quantum entanglement.

In other words, things that are incomprehensible or philosophically unacceptable in our everyday experience can actually occur in accordance with some natural law. A striking example of this is the quantum world.

Certain things trigger the extraordinary quantum world to show its face in our daily lives. The trigger is a phenomenon called phase transition. This is a change in the phase of matter depending on temperature, pressure, and other factors. An example in our daily lives is that liquid water turns into solid ice or gaseous vapor. We know that when a phase changes, the molecules that make up the substance remain the same, but their properties change.

This phase transition sometimes brings the quantum world into our daily lives. An example is superconductivity, a phenomenon in which electrical resistance disappears at ultra-low temperatures such as -200°C.

Most people think of the flow of electricity as such that particles called electrons flow in a copper wire, and the particles sometimes collide with each other, which results in electrical resistance.

However, electrons, which are quantum, sometimes show a wave feature. When a phase transition occurs in a metal and all the electrons are in the same motion all at the same time, the waves are aligned to form a coherent macroscopic wave. Therefore, the waves are not broken when they collide, and no electrical resistance arises. This is the phenomenon of superconductivity, which is similar to a laser beam of light.

What is important here is that the phenomenon of phase transition brought the extraordinary quantum world into the everyday world. There are still many unknown phenomena undiscovered in the world, and these new phase transitions are bringing them to light. It is up to each individual to decide how to make use of the discovered phenomena. If we can make good use of them, the world may change overnight.

The imperfect and diverse world is exciting

In this sense, I am now focusing on the chirality in a microscopic world.

Chirality refers to a mirror-image relationship. In other words, it refers to something whose three-dimensional structure does not overlap with its mirror image, even though its constituent elements are the same. For example, the right hand and its mirrored left hand do not overlap with each other. In fact, the word chirality comes from the Greek word for hand.

Since their components are the same, there should be about the same number of right-handed and left-handed systems in nature. However, for some reason, there are many molecules that are biased in one handedness. This fact is called homochirality.

For example, amino acids have D- and L-forms, which are related to each other like right and left hands. However, living organisms are made up of only L-forms, and sugars are made up of only D-forms. The reason for this is yet to be unraveled.

The fact that organisms have the property of homochirality is very important in industrial applications. For example, it is known that only the L-form of glutamic acid imparts flavor. Also, a good medicine could become a deleterious drug if the right hand and the left hand are mistakenly used. To prevent this from happening, it is necessary to selectively synthesize one hand, and very active research on this aspect is underway.

Furthermore, it is now known that not only molecules but also crystals with chirality can interconvert electric, magnetic, and optical properties. For example, by using chiral materials, we can expect to develop technologies to convert small temperature gradient into electric or magnetic polarizations, or to control these properties with light.

The theoretical concept to understand such as chirality that are related to each other but different from each other is called symmetry. In fact, symmetry and phase transitions are closely related. In other words, symmetry changes when a phase transition occurs. For example, when water freezes and becomes ice, it takes shape. In other words, water that had no special direction becomes ice with proper directions. At the same time, the properties of matter, such as rigidity, are changed.

In fact, there is a concept that the universe in which we live has undergone several phase transitions in the past to reach its current state. In other words, we believe that in the highly symmetric phase of the primordial universe, the forces of nature were one and indistinguishable. It is believed that, as the universe cooled and underwent phase transitions, the symmetry of the universe was lowered and the nature of the forces differentiated, resulting in the four-type of forces of the present universe: the strong force, the weak force, the electromagnetic force, and the gravitational force. In other words, both electromagnetism and gravity are thought to have the same source. On the other hand, if the current universe undergoes further phase transitions, different forces may be generated.

The world of the primordial universe, where all forces were unified into one, is, so to speak, a beautiful world with a high degree of symmetry, like a perfect sphere. It can be said that a phase transition occurred there, and the perfection was lost and the forces differentiated with different roles, giving rise to diversity and functionality such as the electromagnetic force and gravity.

When a well-formed form becomes imperfect, it becomes diverse and acquires different functionalities. What would the world be like if your hands were spherical and there was no distinction between right and left hands? If you try to imagine such a thing, you will realize that the imperfect world is definitely more interesting.

I encourage you to look at things you take for granted in a different way, to wonder at things you do not understand, and to think about them in an interesting way. By doing so, you may discover various possibilities, see mysterious similarities between things you thought were completely different, or suddenly open up a new world.

* The information contained herein is current as of February 2023.
* The contents of articles on Meiji.net are based on the personal ideas and opinions of the author and do not indicate the official opinion of Meiji University.
* I work to achieve SDGs related to the educational and research themes that I am currently engaged in.

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