Quantum Tunneling in Real Devices and Reactions
From abstract theory to technology
Tunneling probability
For a barrier of finite width and height, the transmission probability decreases rapidly as the barrier becomes wider or higher. In many cases it scales roughly like an exponential of the barrier action.
Scanning tunneling microscope
An STM uses electron tunneling between a sharp tip and a conducting surface. The tunneling current depends extremely sensitively on tip-sample separation, allowing atomic-scale imaging.
Alpha decay
Heavy nuclei can decay by tunneling through the nuclear potential barrier. Quantum tunneling explains why some decays are rare yet still observable.
Chemical reaction rates
In some reactions, particles tunnel through activation barriers, especially when light particles such as electrons or protons are involved.
Common estimate
A steeper or wider barrier usually suppresses tunneling much more strongly than a modest increase in particle energy would suggest.
What principle enables the scanning tunneling microscope?
STM measures tunneling current between tip and sample.
Correct answer: Electron tunneling
Why can alpha particles escape a nucleus at all?
The barrier is classically forbidden but not quantum forbidden.
Correct answer: because they can tunnel through the nuclear barrier
How does tunneling current typically depend on distance in STM?
Tunneling is extremely sensitive to separation, often roughly exponential.
Correct answer: Exponential sensitivity
Why are light particles especially likely to tunnel in chemistry?
Quantum effects are more pronounced for small masses.
Correct answer: their quantum wavelengths are larger and barriers can be significant relative to their energies
Broader impact
Tunneling is central in nanoscale electronics, superconducting circuits, nuclear decay, and reaction dynamics.