Quantum Tunneling

Quantum tunneling is a phenomenon in quantum mechanics where particles, such as electrons or other subatomic particles, can pass through energy barriers that classical physics would consider insurmountable. In classical physics, if an object does not have enough energy to overcome a barrier, it would be unable to pass through it. However, quantum tunneling allows particles to "tunnel" through such barriers even when they don't possess sufficient energy to overcome them classically.

Ball and Hill Analogy

FIGURE 1: Contrasting classical (over the barrier) motion vs. quantum (through the barrier) motion.

Consider a ball rolling from one valley to another over a hill (Figure 1). If the ball has enough energy (E) to overcome the potential energy (V) at the top of the barrier between each valley, then it can roll from one valley to the other. This is the classical picture and is controlled by the simple Law of Conservation of Energy approach taught in beginning physics courses. However, If the ball does not have enough kinetic energy (E<V), to overcome the barrier it will never roll from one valley to the other. In contrast, when quantum effects are taken into effect, the ball can "tunnel" through the barrier to the other valley, even if its kinetic energy is less than the potential energy of the barrier to the top of one of the hills.

This phenomenon arises from the probabilistic nature of quantum mechanics. In the quantum world, particles are described by wave functions, which represent the probability distribution of finding a particle in a particular state or location. When a particle encounters an energy barrier, its wave function doesn't suddenly drop to zero at the barrier's edge. Instead, there is a probability that the particle can exist on the other side of the barrier.

Quantum tunneling has been observed and is essential for understanding various phenomena in physics and chemistry. Some practical examples include:

1. Nuclear Fusion: Quantum tunneling plays a role in the fusion of atomic nuclei within stars, allowing them to overcome the electrostatic repulsion between positively charged protons and merge together.

2. Semiconductor Devices: In electronic devices like transistors, tunnel diodes, and flash memory, quantum tunneling is exploited to control the flow of electrons through insulating barriers.

3. Scanning Tunneling Microscopy (STM): Scientists use STM to study surfaces at the atomic and molecular scale. It relies on the quantum tunneling of electrons between the tip of a sharp metal needle and a sample's surface to create detailed images.

4. Nuclear Decay: Quantum tunneling explains the behavior of certain types of radioactive decay, such as alpha decay, where alpha particles (helium nuclei) escape from atomic nuclei.

5. Fusion in Stars: Quantum tunneling enables nuclear fusion reactions in stars, which provide the energy that sustains them for billions of years.

Quantum tunneling challenges our classical intuitions about barriers and energy, highlighting the probabilistic nature of quantum particles. It's a fundamental concept in quantum mechanics and has significant implications for both theoretical physics and practical applications in various fields.

Citations

Libretexts. (2023, March 7). Tunneling. Chemistry LibreTexts. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_