Quantum tunnelling

Tiny bits of matter act like waves.

Imagine a ball rolling toward a hill. If it is too slow, it rolls back. But tiny bits can do something different. They can pass right through the hill! This is called tunnelling.

It happens more easily with very light bits. This helps make flash memory for computers. It also helps stars make energy. Tiny things are full of surprises!

Tiny bits of matter act in strange ways. In our daily lives, things follow simple rules. Imagine a ball rolling toward a steep hill. If the ball is slow, it cannot get over. It will always roll back down. This is how classical mechanics works. But tiny particles like electrons do not follow these rules. They can act like waves.

This wave-like nature allows something called quantum tunnelling. This is when a particle passes through a barrier. It does not have enough power to go over it. Instead, it seems to pass right through.

This happens because a small part of the particle's wave appears on the other side. It is a matter of chance. The chance of this happening changes with the barrier. A taller or wider barrier makes it harder to tunnel. It is also harder for heavy particles to tunnel. This is why it happens most with light electrons.

Tunnelling helps make stars shine through nuclear fusion. It also helps us use flash memory in computers. Scientists even use it to see atoms with a special microscope. This tool is called a scanning tunnelling microscope.

Quantum tunnelling is a strange and amazing thing that happens in the tiny world of atoms. In our normal lives, objects follow rules called classical mechanics. Imagine a ball rolling toward a steep hill. If the ball does not have enough energy, it will always roll back down. It simply cannot get to the other side.

However, very small things like electrons do not follow these rules. They can pass through a barrier that should be impossible to cross. This special event is called quantum tunnelling. It is a key part of how the universe works at its smallest level.

To understand how it works, we must look at how particles act like waves. Scientists use something called a wave function to describe where a particle might be. This wave function shows the probability, or the chance, of finding a particle in a certain spot.

When a particle hits a barrier, most of it bounces back. But a small part of its wave can appear on the other side. This means there is a small chance the particle will just show up past the barrier. This chance gets smaller if the barrier is very high or very wide. It also gets smaller if the particle has a lot of mass.

People have been studying these strange rules for a long time. The Schrödinger equation was published in 1926 to help describe these waves. In 1927, Friedrich Hund used this math to study tunnelling between two areas. Other scientists like Leonid Mandelstam and Mikhail Leontovich found tunnelling on their own in 1928. Around that same time, George Gamow, Ronald Gurney, and Edward Condon used it to explain alpha decay. This was a huge success for the theory.

Later, Leo Esaki, Ivar Giaever, and Brian Josephson won the Nobel Prize in 1973 for their work on tunnelling in solids.

There are many real numbers and names tied to this discovery. In 1957, Leo Esaki showed electrons tunnelling through a tiny barrier. In 1981, Gerd Binnig and Heinrich Rohrer built a scanning tunnelling microscope. This tool uses tunnelling to take pictures of atoms. They won a Nobel Prize in 1986 for this invention. More recently, John Clarke, John M. Martinis, and Michel H. Devoret received a Nobel Prize in 2025. They showed that many particles can tunnel together in a large circuit. They used superconductors, which are materials that carry electricity without any resistance. This showed tunnelling can happen on a much larger scale.

We use quantum tunnelling in many tools we use every day. It is a main part of how flash memory works in computers. It is also used in tunnel diodes, which are special parts in electronics.

Tunnelling even helps stars shine through a process called nuclear fusion. However, it can also cause problems in very small computer chips. If the parts are thinner than 1 nanometer, electrons might tunnel through them when they should not. This can cause heat or waste power. Even with these challenges, tunnelling is a fundamental part of modern technology and science.

Quantum tunnelling, also known as barrier penetration, is a fundamental phenomenon in quantum mechanics. In our everyday world, objects follow the rules of classical mechanics. If you roll a ball toward a steep hill, it will only reach the other side if it has enough energy to go over the top. If the ball lacks that energy, it simply rolls back down. In the quantum world, however, particles like electrons or atoms do not always follow these rules. They can pass through a potential energy barrier even if they do not have enough energy to surmount it. This ability to cross an seemingly impassable barrier is what we call tunnelling.

To understand how this happens, we must look at the wave nature of matter. In quantum mechanics, a particle is described by a mathematical tool called a wave function. This wave function describes the possible states of a physical system. Scientists use equations, such as the Schrödinger equation, to describe how these wave functions change over time. The square of the absolute value of the wave function represents a probability distribution. This distribution tells us the likelihood of finding a particle at a specific position.

When a particle, represented as a wave packet, hits a narrow potential barrier, something strange occurs. Most of the wave packet is reflected back, much like a wave hitting a wall. However, a small portion of the wave function can penetrate the barrier and appear on the other side. This represents a non-zero probability that the particle will be measured outside the barrier. This process is highly sensitive to the physical properties of the system. The probability of transmission decreases exponentially as the barrier height increases. It also decreases as the barrier width increases or as the mass of the tunnelling particle increases. Because of this, tunnelling is most common in low-mass particles like electrons.

Our understanding of tunnelling grew through many important discoveries in the 20th century. The Schrödinger equation was published in 1926, providing the math needed to study these waves. In 1927, Friedrich Hund applied this equation to study tunnelling between two classically allowed regions. Around 1927 and 1928, other scientists like Lothar Nordheim, Ralph Fowler, Leonid Mandelstam, and Mikhail Leontovich also made significant contributions. A major breakthrough occurred in 1928 when George Gamow, Ronald Gurney, and Edward Condon used tunnelling theory to explain alpha radioactive decay. They showed that the half-life of a particle depends directly on the mathematical probability of tunnelling through the nuclear potential.

Experimental work later turned these theories into practical tools. In 1957, Leo Esaki demonstrated electron tunnelling through a semiconductor barrier. In 1960, Ivar Giaever showed that tunnelling also occurs in superconductors. Later, Brian Josephson predicted the tunnelling of superconducting Cooper pairs. These three scientists shared the Nobel Prize in Physics in 1973. In 1981, Gerd Binnig and Heinrich Rohrer developed the scanning tunnelling microscope (STM). This device uses tunnelling currents to create images of surfaces at the atomic level. They were awarded the Nobel Prize in 1986 for this discovery.

Quantum tunnelling is essential to many modern technologies and natural processes. It is a key mechanism in nuclear fusion, which powers stars. In electronics, tunnelling is used to program the floating gates in flash memory. It is also used in tunnel diodes and resonant tunnelling diodes. A resonant tunnelling diode uses two thin layers to create a quantum potential well. This allows electrons to flow like an open wire only at specific resonant voltages.

However, tunnelling also presents challenges for the future of computing. In very-large-scale integration (VLSI) electronics, tunnelling causes current leakage. This happens when insulating layers or transistors become thinner than about 1 nm. When this occurs, electrons tunnel through the layers, causing power drain and heat. Researchers are looking for ways to manage this, such as developing tunnel field-effect transistors. These could potentially reduce gate voltage from 1 volt to 0.2 volts, greatly improving energy efficiency. Most recently, in 2025, Nobel recognition was given to John Clarke, John M. Martinis, and Michel H. Devoret. They demonstrated that tunnelling can be observed on a macroscopic scale using complex electrical circuits with superconductors.