Quantum tunneling is one of the most fascinating and strange concepts in quantum physics. It explains how a particle can pass through an energy barrier that it classically should not be able to cross.
Let’s understand this step by step in simple language.
Table of Contents
1. The Classical View
In classical physics, if a ball rolls toward a hill, it can only cross the hill if it has enough energy to climb over it.
If it doesn’t have enough energy, it simply rolls back.
Example:
- A ball with low energy cannot go over a high hill.
- Energy barrier stops it completely.
So, classically, the ball can’t pass through the barrier.
2. The Quantum View
In quantum mechanics, particles like electrons don’t behave like tiny solid balls.
They behave like waves — this is called the wave-particle duality.
That means an electron is described by a wave function (ψ), which spreads out in space.
Even if there’s a barrier, the wave function doesn’t stop sharply at the barrier — it extends a little inside it.
3. The Key Idea: “Tunneling”
If the barrier is thin enough, the wave function can continue on the other side.
This means there is a small probability that the electron will be found beyond the barrier, even if it doesn’t have enough classical energy.
This strange effect is called quantum tunneling.
So, the particle doesn’t climb over the barrier — it “tunnels through” it.
4. Simple Example
Let’s take an example of an electron in a metal:
When you apply voltage between two metals separated by a thin insulating layer, classically electrons can’t pass (since insulator is a barrier).
But quantum mechanically, some electrons tunnel through the barrier — this is how tunneling diodes and scanning tunneling microscopes (STM) work.
5. Factors That Affect Tunneling
- Barrier Thickness – Thinner barriers make tunneling more likely.
- Barrier Height – Lower barriers increase tunneling probability.
- Particle Energy – Higher energy particles have a greater chance to tunnel.
6. Real-Life Applications of Quantum Tunneling
- Tunnel Diode – Uses tunneling for high-speed electronic switching.
- Scanning Tunneling Microscope (STM) – Maps atomic surfaces by detecting tunneling currents.
- Nuclear Fusion – In stars, tunneling allows hydrogen nuclei to fuse even at lower energies.
- Quantum Computers – Qubits can tunnel between states, enabling faster computations.
7. In Short
| Classical Physics | Quantum Physics |
|---|---|
| Particle must have enough energy to cross barrier. | Particle can tunnel through even if it doesn’t have enough energy. |
| Deterministic. | Probabilistic (based on wave function). |
| No crossing without energy. | Possible due to quantum wave nature. |
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