Counterintuitive but real! Three amazing quantum effects, how many have you heard of?

In physics, there are many extremely important effects, such as the Doppler effect that is common in life, the famous photoelectric effect, and the Meissner effect behind the superconducting news that has been frequently searched this year. Next, I would like to share with you three effects from the field of quantum physics that sound very strange but actually exist.

Quantum tunneling effect

If you have been following the 2023 Nobel Prize in Physics, you are likely to have read this sentence: "When an electron is trapped by only a narrow potential barrier, quantum mechanics allows it to tunnel through and escape." The "tunneling" here refers to the "quantum tunneling effect" in quantum mechanics, which is also the first magical effect I want to share with you.

If I throw a basketball against a wall, it will bounce back without a doubt. This is common sense. But what if we replace the basketball with an electron and the wall with a "wall" in the microscopic world - a quantum barrier? Quantum mechanics tells us that there is a certain probability that an electron can "pass through the wall". This strange phenomenon is the quantum tunneling effect.

Quantum tunneling allows particles to pass through potential barriers. (Photo/Universität Innsbruck/Harald Ritsch)

In fact, the electron is not passing through a dense solid wall like the basketball, but a set of potential barriers that can restrict its free movement. Quantum tunneling is possible because electrons have wave characteristics. Quantum mechanics gives every particle wave characteristics, and the probability of a wave penetrating a barrier is always limited.

Physicists soon discovered that the ability of particles to tunnel could solve many mysteries. It explains chemical bonds and radioactive decay, as well as how hydrogen nuclei in the sun overcome repulsion to fuse into helium and release photons.

Although the quantum tunneling effect has been discovered for almost 100 years, physicists do not fully understand some details of the quantum tunneling process. For example, physicists are still unable to determine whether the tunneling effect occurs instantly or takes some time. On March 18, 2019, a study published in the journal Nature showed that in hydrogen atoms, electrons tunnel out of atoms in no more than 1.8 attoseconds. This is an extremely short time, which almost means that the tunneling process occurs instantly. However, on July 22, 2020, another study published in the journal Nature showed that atoms stayed in a laser barrier for about 0.61 milliseconds before "jumping" out from the other side. This means that the duration of quantum tunneling is not 0, and the thickness of the barrier and the speed of the atom determine how long the atom stays in it.

Although physicists have not yet figured out all the details, quantum tunneling has long been used as the basis for some technologies, such as quantum computing, scanning tunneling microscopy, and more.

Aharonov-Bohm effect

Next, the second effect I want to mention may sound unfamiliar, that is, the Aharonov-Bohm effect.

In classical electromagnetism, electric and magnetic fields are the fundamental entities responsible for all physical effects. For example, a tiny particle will only be affected by the field, such as speeding up, slowing down, or turning, if it is in direct contact with the electric or magnetic field.
The electromagnetic field can be represented by a quantity called electromagnetic potential, which has a value anywhere in space. The electromagnetic field can be derived from the electromagnetic potential. However, the concept of electromagnetic potential has always been considered to be a purely mathematical concept with no physical meaning.

However, things start to get interesting in quantum physics. In 1959, two theoretical physicists, Yakir Aharonov and David Bohm, proposed a thought experiment that linked the electromagnetic potential to measurable results. In this thought experiment, a beam of electrons is split into two paths, each moving around the two sides of a cylindrical spiral coil, and the magnetic field is confined inside the coil. Therefore, the two electron paths can pass through a region where there is no field, but the electromagnetic potential in this region is not zero.

The Aharonov-Bohm effect is a quantum mechanical effect in which the phase of a particle changes as it moves around a region containing a magnetic field, even if the magnetic field is zero wherever the particle passes. (Image/E.Cohen et al.)
Aharonov and Bohm theoretically demonstrated that the electrons on the two different paths will experience different phase changes, and when the electrons on the two paths recombine, a detectable interference effect can be produced. Since the phase change can be calculated from the strength of the magnetic field, the interference can be explained as the effect that the electrons never actually pass directly through the magnetic field. Today, the Aharonov-Bohm effect has already been verified by many experiments.

Interestingly, in January 2022, a new study published in the journal Science showed that the Aharonov-Bohm effect applies not only to magnetic fields, but also to gravity.

Quantum Hall Effect

Finally, the third effect we want to mention is a very important discovery in condensed matter physics, namely the quantum Hall effect.

We know that when current passes through a metal strip, if the current is measured perpendicular to the direction of the current, it is found that the potential at both ends of the metal strip is usually the same. But in 1879, Edwin Hall, who was only 24 years old, discovered that if a magnetic field acts perpendicularly on the plane of the metal strip, the electrons will be deflected to one side, thereby generating a potential difference on both sides of the metal strip. This phenomenon is called the Hall effect. Just one year after Hall's discovery, he discovered that the Hall effect can be observed in a ferromagnetic material even without an external magnetic field. This is called the anomalous Hall effect.

By the end of the 1970s, researchers began to use semiconductor materials to study the Hall effect under low temperature (close to absolute zero) and strong magnetic field (about 30T). In low-temperature semiconductor materials, electrons have strong mobility, but they can only move in a two-dimensional plane. This geometric restriction leads to many unexpected effects, one of which is to change the characteristics of the Hall effect, which can be observed by measuring the change of Hall resistance with the strength of the magnetic field.

In 1980, German physicist Klaus von Klitzing discovered under similar experimental conditions that the Hall resistance did not increase smoothly and gradually with the change of magnetic field strength as expected, but rather increased in a quantized step-by-step manner. Von Klitzing realized that in this case, the Hall resistance value is related to two basic constants, one of which is Planck's constant h, and the other is the electron charge e, which is a multiple of the quantum physical quantity composed of these two constants. What von Klitzing discovered is the integer quantum Hall effect, which is one of the most important and basic quantum effects in the entire field of condensed matter physics. This discovery also won von Klitzing the Nobel Prize in Physics in 1985.

Integer quantum Hall effect (Image/Public domain)

Just two years after von Klitzing discovered the integer quantum Hall effect, experimental physicists Horst Störmer and Daniel Tsui discovered an even more puzzling phenomenon: at lower temperatures and stronger magnetic fields, they found that the Hall conductance quantized at a fractional multiple of the previously observed value. It was as if the electrons had somehow split into smaller particles, each carrying a fraction of the electron's charge. In 1998, Störmer and Tsui, along with theoretical physicist Robert Laughlin, were awarded the Nobel Prize in Physics for their work on the fractional quantum Hall effect.

We just mentioned that Hall discovered the anomalous Hall effect, so is there also a quantum anomalous Hall effect (QAHE), that is, a quantum Hall effect that does not require an external magnetic field? The answer is yes. However, to observe the quantum anomalous Hall effect experimentally, extremely high requirements are placed on the material. This material must meet three conditions at the same time: first, the material must be ferromagnetic; second, the interior of the material must be insulating; third, the electronic structure within the material must be topological. This means that it is a huge challenge to realize the quantum anomalous Hall effect experimentally. In 2013, Xue Qikun and his team were the first to successfully observe the quantum anomalous Hall effect in Cr-doped (Bi,Sb)₂Te₃ films. On October 24, 2023, Xue Qikun also won the highest award in the field of condensed matter physics, the Buckley Prize, for his research on topological insulators and his discovery of the quantum anomalous Hall effect in topological insulators.

References:

https://www.nature.com/articles/122439a0

https://www.nature.com/articles/s41586-019-1028-3

https://www.nature.com/articles/s41586-020-2490-7

https://www.science.org/doi/10.1126/science.abl7152

This article is a work supported by Science Popularization China Starry Sky Project

Team: Principle

Reviewer: Luo Huiqian, Researcher, Institute of Physics, Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.