Quantum Tunneling: Escaping the Classically Forbidden

Table of Contents

1. Introduction
2. Classical vs Quantum View of Barriers
3. What Is Quantum Tunneling?
4. Mathematical Description
5. Tunneling Through a Rectangular Potential Barrier
6. Transmission and Reflection Coefficients
7. Probability Current and Flux Conservation
8. Tunneling Time and the Hartman Effect
9. WKB Approximation and Tunneling
10. Factors Affecting Tunneling Probability
11. Alpha Decay and Nuclear Tunneling
12. Tunneling in Semiconductors and Electronics
13. Quantum Dots, Josephson Junctions, and STM
14. Tunneling in Chemical Reactions
15. Quantum vs Classical Energy Barriers
16. Philosophical and Conceptual Implications
17. Conclusion

1. Introduction

Quantum tunneling is one of the most striking and non-intuitive phenomena in quantum physics. It allows particles to penetrate and cross energy barriers that they could never overcome classically. This effect plays a central role in nuclear physics, semiconductors, chemical dynamics, and modern quantum technologies.

2. Classical vs Quantum View of Barriers

Classical:

• A particle with energy \( E \) less than a potential \( V_0 \) cannot enter the region \( V(x) > E \)

Quantum:

• The particle’s wavefunction penetrates into and through the barrier
• There’s a non-zero probability of finding it on the other side

3. What Is Quantum Tunneling?

Quantum tunneling is the phenomenon where a particle:

• Encounters a potential barrier
• Has insufficient energy to overcome it classically
• Yet still appears on the far side of the barrier with a certain probability

This arises from the wave nature of particles.

4. Mathematical Description

We solve the time-independent Schrödinger equation across three regions:

\[
-\frac{\hbar^2}{2m} \frac{d^2 \psi(x)}{dx^2} + V(x) \psi(x) = E \psi(x)
\]

Let:

• Region I: \( x < 0 \) (free)
• Region II: \( 0 < x < a \) (barrier \( V_0 > E \))
• Region III: \( x > a \) (free)

5. Tunneling Through a Rectangular Potential Barrier

Assume:

\[
V(x) = \begin{cases}
0, & x < 0 \ V_0, & 0 < x < a \ 0, & x > a
\end{cases}
\]

Solution:

• Region I: \( \psi_I = Ae^{ikx} + Be^{-ikx} \)
• Region II: \( \psi_{II} = Ce^{\kappa x} + De^{-\kappa x} \), with \( \kappa = \sqrt{2m(V_0 – E)}/\hbar \)
• Region III: \( \psi_{III} = Fe^{ikx} \)

6. Transmission and Reflection Coefficients

Apply boundary conditions for continuity and smoothness at \( x = 0 \) and \( x = a \).

Transmission coefficient:

\[
T = \frac{|F|^2}{|A|^2} = \frac{1}{1 + \frac{V_0^2 \sinh^2(\kappa a)}{4E(V_0 – E)}}
\]

For thick barriers:

\[
T \approx e^{-2\kappa a}
\]

Shows exponential suppression of tunneling probability with barrier width and height.

7. Probability Current and Flux Conservation

Define probability current:

\[
j = \frac{\hbar}{2mi} \left( \psi^* \frac{d\psi}{dx} – \psi \frac{d\psi^*}{dx} \right)
\]

• Ensures flux conservation: \( j_{\text{in}} = j_{\text{trans}} + j_{\text{ref}} \)
• Tunneling does not violate probability conservation

8. Tunneling Time and the Hartman Effect

Tunneling time is debated:

• How long does the particle spend inside the barrier?
• Hartman effect: tunneling time becomes independent of barrier width for thick barriers
• Raises questions about superluminal speeds, but does not violate causality

9. WKB Approximation and Tunneling

In the WKB (semiclassical) approximation, tunneling probability is:

\[
T \approx \exp\left(-\frac{2}{\hbar} \int_{x_1}^{x_2} \sqrt{2m(V(x) – E)}\, dx \right)
\]

Where \( x_1 \), \( x_2 \) are classical turning points.

Useful for smooth and slowly varying barriers.

10. Factors Affecting Tunneling Probability

• Barrier width \( a \): wider ⇒ less tunneling
• Barrier height \( V_0 \): higher ⇒ less tunneling
• Particle mass \( m \): heavier ⇒ less tunneling
• Energy \( E \): closer to \( V_0 \) ⇒ more tunneling

Tunneling is sensitive to small changes in parameters.

11. Alpha Decay and Nuclear Tunneling

Alpha particles inside nuclei are trapped by nuclear potential. Tunneling allows escape:

• Explains radioactive decay
• Lifetimes depend exponentially on barrier width and height
• First success of quantum tunneling in explaining a real-world phenomenon

12. Tunneling in Semiconductors and Electronics

• Tunnel diodes: use quantum tunneling for ultra-fast switching
• Flash memory: stores data via tunneling through oxide layers
• MOSFETs: leakage current arises from tunneling in thin oxide layers

13. Quantum Dots, Josephson Junctions, and STM

• Quantum dots: electrons tunnel between confined energy levels
• Josephson junctions: tunneling of Cooper pairs (superconductivity)
• Scanning Tunneling Microscope (STM): measures tunneling current between tip and sample

Tunneling enables atomic-resolution imaging.

14. Tunneling in Chemical Reactions

• Tunneling allows protons or electrons to bypass activation barriers
• Explains low-temperature reactions in astrophysics and biology
• Influences enzyme catalysis and quantum tunneling effects (QTEs) in chemistry

15. Quantum vs Classical Energy Barriers

16. Philosophical and Conceptual Implications

• Challenges classical determinism
• Reveals nonlocal effects of quantum theory
• Raises questions about temporal locality and measurement
• Illustrates that probability amplitudes, not just energy, govern motion

17. Conclusion

Quantum tunneling is a fascinating quantum effect that has profound implications for both theory and technology. It illustrates the deep departure from classical intuition and explains a vast array of natural and engineered phenomena, from nuclear decay to modern electronics. Its understanding is essential for mastering quantum mechanics and leveraging its principles in advanced applications.