In classical physics, if you throw a ball at a solid wall, it bounces back. It will never appear on the other side. In the subatomic world of quantum mechanics, however, the rules are different. It is possible for a particle like an electron to encounter a barrier and instantaneously appear on the opposite side.
This phenomenon is known as quantum tunneling. When charged particles like electrons tunnel through a barrier, their collective passage creates a flow of electricity known as a tunneling current. This occurs not because the particles have enough energy to overcome the obstacle, but due to the probabilistic nature of the quantum realm. Tunneling current highlights a fundamental difference between how matter behaves at the smallest scales versus our everyday experience.
The Quantum Mechanical Explanation
To understand how a particle passes through a barrier it lacks the energy to overcome, we must consider wave-particle duality. In quantum mechanics, particles like electrons behave as both particles and waves. This wave-like nature is a “probability wave” described by a wavefunction, which represents the likelihood of finding the particle at any point in space.
The locations where the wave’s amplitude is highest are where the particle is most likely to be found. When this probability wave encounters an energy barrier, the wave does not stop. Instead, its amplitude decreases exponentially as it penetrates the barrier, but for a thin barrier, the wave does not decay to zero before reaching the other side.
This means a small part of the probability wave extends beyond the barrier. Consequently, there is a finite probability that the particle will be found on the far side, having “tunneled” through it. The particle does not create a physical hole; it appears on the other side because its existence is defined by probability, not certainty.
The likelihood of tunneling is highly sensitive to the barrier’s properties. A thicker or higher energy barrier causes the probability wave to decay more rapidly, reducing the chance of the particle appearing on the other side. This sensitivity is what makes tunneling current useful in certain technologies.
Harnessing Tunneling for Technology
The precision of quantum tunneling has been harnessed for scientific instruments and modern electronics. A primary example is the Scanning Tunneling Microscope (STM), which can create images of surfaces with atomic-level resolution. The microscope works by positioning a sharp conductive tip a few nanometers above a sample’s surface.
A small voltage is applied between the tip and the sample, creating an energy barrier in the gap. Electrons then tunnel across this gap, generating a measurable tunneling current. This current is highly sensitive to the distance between the tip and the surface. By scanning the tip and using a feedback loop to keep the current constant, the microscope maps the tip’s vertical position, producing a topographic image of the atomic landscape.
Another application of tunneling is in flash memory, used in solid-state drives (SSDs) and USB drives. These devices store data by trapping electrons in a “floating gate” insulated by a thin oxide layer. To write data (a ‘0’), a voltage is applied, causing electrons to tunnel through the oxide layer and become trapped. To erase data (a ‘1’), the process is reversed, and the electrons tunnel back out.
Unintentional Tunneling Effects
While tunneling can be exploited, it also presents challenges in engineering, particularly in computer processor design. As manufacturers shrink the size of transistors to make chips faster, the components become smaller. Transistors control electron flow using a thin insulating layer known as the gate oxide.
As transistors shrink to the nanometer scale, this gate oxide layer becomes so thin that electrons can tunnel through it even when the transistor is “off.” This unwanted flow is known as leakage current. Leakage wastes power, generates excess heat, and can reduce battery life in mobile devices or increase cooling needs for computers.
This unintentional tunneling sets a physical limit on how small transistors can be made with current designs. If the insulating layers are too thin, the leakage current makes it impossible for the transistor to function as a reliable switch. Overcoming this leakage is a focus of research in the semiconductor industry, driving the exploration of new materials and transistor designs.
Tunneling in the Natural World
Quantum tunneling is a process that also drives phenomena in the natural world. An example is at the heart of the Sun, where tunneling enables the nuclear fusion reactions that produce its energy. The Sun’s core temperature is not sufficient for protons to classically overcome their mutual electrostatic repulsion, known as the Coulomb barrier.
According to classical physics, the positively charged protons should repel each other too strongly to get close enough for the strong nuclear force to bind them. However, the wave-like nature of protons allows their wavefunctions to overlap even when separated by the Coulomb barrier. This overlap creates a probability that two protons can tunnel through the repulsive barrier and fuse.
Although the probability for any single pair of protons is low, the number of protons in the Sun’s core ensures that fusion reactions occur at a steady rate. This tunneling-driven process converts hydrogen into helium, releasing the energy that powers the Sun. Without quantum tunneling, the Sun would not be able to shine.