Quantum tunnelling is one of the most intriguing phenomena in quantum physics. Tunnelling may appear to have little relevance to everyday life but it is a fundamental process of nature that is responsible for many things on which life itself is dependent. It has even been hypothesised that the very beginning of the universe was caused by a tunnelling event, allowing the universe to pass from a “state of no geometry” (no space or time) to a state in which space, time, matter, and life could exist.
Imagine a particle approaching a seemingly insurmountable barrier. As per classical physics, it cannot penetrate the barrier due to insufficient energy. To pass through, the particle must overcome the barrier with enough energy else it will bounce back. However, in quantum mechanics, particles exhibit behaviours that defy classical intuition.
Quantum tunnelling occurs when a particle traverses a barrier that it classically shouldn’t be able to cross. This happens without violating the laws of physics as we know them. Instead, quantum mechanics introduces the notion of probabilistic behaviour at the subatomic level.
According to the principles of quantum mechanics, particles, such as electrons, only possess definite positions or energies once they are measured. They exist in a state of superposition, where they can simultaneously occupy multiple states.
When a particle encounters a barrier, its wave function describes the probability of finding it at various positions. Even though classically it lacks the energy to surmount the barrier, there’s a non-zero probability that it can appear on the other side due to its wave-like nature. This probabilistic phenomenon lies at the heart of quantum tunnelling.
The waves associated with particles, called de Broglie waves, increase in size as the mass of particles decreases. That’s why everyday objects don’t exhibit quantum, wave-like behaviours; their mass is too great and their de Broglie waves are too small.
Imagine, a ball thrown by a child without the energy needed to clear the height of the wall. It can’t quantum tunnel to the other side as its mass creates a probability wave too small for there to be any probability of it finding itself on the other side of the barrier.
Common quantum theories predict that the smaller the mass of a particle, the larger the de Broglie wave and the greater the chance of a particle quantum tunnelling through a barrier.
This means when you have lots of lighter particles and lots of heavier particles and a quantum barrier, both will tunnel, but the lighter particles should quantum tunnel faster than the heavy particles.
While quantum tunnelling may sound like an abstract concept with little practical relevance, its implications reverberate across various fields. Here are some of its applications:
In electronics, quantum tunnelling plays a pivotal role in the operation of devices such as tunnel diodes and flash memory. It enables the movement of electrons through barriers, facilitating the functionality of transistors and enabling the miniaturisation of electronic components. Moreover, it serves as the underlying principle in quantum computing, where qubits exploit superposition and tunnelling to perform computations beyond the capabilities of classical computers.
Quantum tunnelling is also integral to nuclear fusion processes in stars and experimental fusion reactors on Earth. In stellar cores, where immense temperatures and pressures reign, it allows hydrogen nuclei to overcome the electrostatic repulsion and fuse into helium, releasing copious amounts of energy in the process. While replicating stellar conditions on Earth remains a significant challenge, scientists harness the principles of quantum tunnelling to advance fusion research and potentially unlock a clean, abundant energy source.
In chemistry, quantum tunnelling influences the rates and pathways of chemical reactions. It facilitates reactions that would otherwise be prohibited by classical mechanics, contributing to diverse processes such as enzymatic catalysis, organic synthesis, and atmospheric chemistry. Understanding and manipulating quantum tunnelling mechanisms can aid in the design of novel catalysts and pharmaceutical compounds, revolutionising industries and enhancing our ability to engineer molecular systems.
Despite its pervasive influence, quantum tunnelling remains enigmatic and continues to stimulate scientific inquiry.
Researchers attempt to unravel its intricacies through theoretical models, computational simulations, and experimental observations. Advanced techniques such as scanning tunnelling microscopy allow scientists to directly visualise and manipulate individual atoms, providing insights into quantum phenomena at the nanoscale.
Moreover, the field of quantum technologies promises to harness quantum tunnelling and other quantum phenomena to revolutionise computing, communication, and sensing.
Quantum tunnelling is a testament to the strangeness and boundless potential of nature’s fundamental laws. It challenges our preconceptions, expands the frontiers of human knowledge, and beckons us to explore the mysteries concealed within the subatomic reality.