Quantum Tunneling

Quantum Tunneling:

Quantum tunneling is a striking phenomenon of modern physics in which particles pass through energy barriers that, according to classical physics, should be insurmountable. Though it may sound abstract, its basic idea can be grasped with simple analogies and real-world examples.

1. What Is Quantum Tunneling?

In classical physics, imagine a ball rolling toward a hill. If the ball does not have enough energy, it cannot climb to the top and will roll back. In the quantum world, however, particles like electrons behave partly like waves. This “wave nature” allows them to “borrow” energy briefly and tunnel through a barrier, emerging on the other side—even if they didn’t have enough energy to climb over it.

2. Why Does It Happen?

• Wave–Particle Duality: Particles at the atomic scale have wave-like properties. These waves extend on both sides of an energy barrier.

• Probability Amplitude: The particle’s wavefunction has a non-zero value inside and beyond the barrier, giving a finite probability of finding the particle on the other side.

• Energy Uncertainty: Short, momentary violations of energy conservation are allowed by the Heisenberg uncertainty principle, enabling “borrowing” of energy to tunnel.

3. A Simple Analogy

Imagine a ghostly mist approaching a locked door. Although the door is closed, part of the mist appears inside the room because its cloud-like nature seeps through cracks and keyholes. Similarly, the electron’s wave penetrates and sometimes passes through the energy barrier.

4. A Concrete Example: Alpha Decay

Inside certain heavy atomic nuclei, alpha particles (helium nuclei) are confined by a strong nuclear force barrier. Classically, they lack sufficient energy to escape. Yet, quantum tunneling allows them to “leak out” over time. This process—alpha decay—is why some radioactive elements emit alpha radiation and transform into lighter elements.

5. Everyday Applications

1. Scanning Tunneling Microscope (STM)

   • Uses tunneling of electrons between a sharp tip and a surface to image individual atoms.

   • As the tip approaches within a nanometer of the surface, electrons tunnel across the gap, producing a current that maps atomic positions.

2. Tunnel Diode

   • A semiconductor device where current flows even at very low voltage, due to electrons tunneling through a narrow energy barrier.

   • Enables extremely fast electronic switching in microwave and high-frequency circuits.

3. Nuclear Fusion in Stars

   • Protons in the Sun’s core lack the energy to overcome their mutual electrostatic repulsion by classical means.

   • Quantum tunneling allows them to fuse, releasing the energy that powers the Sun and sustains life on Earth.

Nobel Prize 2025 Physics

The Hindu