Quantum tunneling is a phenomenon in quantum mechanics where a particle can pass through an energy barrier even if it lacks the energy to classically overcome it. This behavior defies everyday intuition, demonstrating the unusual characteristics of the microscopic world.
The Quantum Leap Past Barriers
In classical physics, an object needs a certain amount of energy to get past a barrier. For instance, a ball rolling towards a hill requires enough kinetic energy to reach the top and continue on the other side. If it lacks this energy, it will simply roll back down. This deterministic view assumes particles are solid objects with precise positions and velocities.
In quantum mechanics, which governs matter at atomic and subatomic scales, particles exhibit wave-like properties, a concept known as wave-particle duality. This duality allows a particle to “leak” through an energy barrier, even if its energy is less than the barrier’s height. The barrier is a region of high potential energy that would classically repel the particle.
When a quantum particle encounters such a barrier, its wave function, which describes the probability of finding the particle at a given location, can extend into and beyond the barrier. This means there is a non-zero, albeit often small, probability of finding the particle on the other side. This contrasts with classical physics, where the probability of passing through an insurmountable barrier would be exactly zero.
Visualizing the Invisible: How it Works
Every quantum particle is associated with a wave function, a mathematical description that represents the probability of locating the particle at any given point in space. When this wave function encounters an energy barrier, it does not abruptly vanish at the barrier’s edge. Instead, a portion of the wave function can penetrate into the barrier.
Inside the barrier, the wave function’s amplitude decreases exponentially. However, if the barrier is sufficiently thin, the wave function can still have a non-zero amplitude on the other side. This non-zero amplitude signifies a measurable probability that the particle will be found beyond the barrier, having “tunneled” through it.
The probability of a particle tunneling is influenced by several factors. A thinner barrier increases the likelihood of tunneling, as the wave function has less distance over which to decay. A lower barrier height also leads to a higher tunneling probability because the energy difference between the particle and the barrier is smaller. The mass of the particle also plays a role; lighter particles, such as electrons or protons, are more likely to tunnel than heavier ones, due to their more pronounced wave-like properties.
Where Quantum Tunneling Shapes Our World
Quantum tunneling is a fundamental process that shapes various natural phenomena and modern technologies. One example is nuclear fusion in stars, including our Sun. The immense temperatures in stellar cores are not hot enough for atomic nuclei to overcome their mutual electrical repulsion, known as the Coulomb barrier, through classical collisions alone. Quantum tunneling allows these nuclei, primarily protons, to “tunnel” through this barrier and fuse, releasing the vast amounts of energy that power stars. Without tunneling, most stars would not have ignited.
Scanning Tunneling Microscopes (STMs) also harness this quantum phenomenon to image surfaces at an atomic level. An STM operates by bringing a sharp, electrically conductive tip within a few angstroms (0.2-0.4 nanometers) of a conductive sample surface. When a small voltage is applied between the tip and the sample, electrons can tunnel across the vacuum gap, creating a measurable tunneling current. The current’s strength is sensitive to the distance between the tip and the surface, decreasing exponentially with separation. By maintaining a constant tunneling current while scanning the tip across the surface, the STM can map the topography and electron density of individual atoms.
Quantum tunneling is important for the functioning of various semiconductor devices. Tunnel diodes, for instance, exploit quantum tunneling for high-speed switching and negative resistance characteristics, finding applications in high-frequency oscillators and switching circuits. Flash memory devices, common in modern electronics for data storage, rely on the tunneling of electrons through a thin oxide layer to charge or discharge a floating gate, representing stored data. This mechanism contributes to flash memory’s non-volatility, high storage density, and low power consumption.
Alpha decay, a common type of radioactive decay in heavy atomic nuclei, is another natural process driven by quantum tunneling. In alpha decay, an alpha particle (consisting of two protons and two neutrons) is trapped within the nucleus by the strong nuclear force, while also experiencing repulsion from the electromagnetic force, creating a potential barrier. Although the alpha particle’s energy is insufficient to classically escape this barrier, it has a non-zero probability of tunneling through it and being emitted from the nucleus. This quantum mechanical explanation accounts for the observed half-lives of radioactive elements.