4-Quantum Tunneling
Quantum tunneling is an effect where a particle can pass through a barrier it would not normally have the energy to overcome. It can allow a particle to pass through a physical barrier that should be impenetrable, or can allow an electron to escape from the pull of the nucleus without having the kinetic energy to do so. According to quantum mechanics, there is a finite probability that any particle can be found anywhere in the universe, although that probability is astronomically small for any real distance from the particles expected path.
However, when the particle is faced with a small-enough barrier (around 1-3 nm wide), one which conventional calculations would indicate is impenetrable by the particle, the probability that the particle will simply pass through that barrier becomes fairly noticeable. This can be explained by the Heisenberg uncertainty principle, which limits how much information can be known about a particle. A particle can “borrow” energy from the system it is acting in, use it to pass through the barrier, and then lose it again.
Quantum tunneling is involved in many physical processes, such as radioactive decay and the nuclear fusion that takes place in the Sun. It is also used in certain electrical components, and it has even been shown to occur in enzymes in biological systems. For example, the enzyme glucose oxidase, which catalyses the reaction of glucose into hydrogen peroxide, involves the quantum tunneling of an entire oxygen atom.
Quantum tunneling is also a key feature of the scanning tunneling microscope, the first machine to enable the imaging and manipulation of individual atoms. It works by measuring the voltage in a very fine tip, which changes when it gets close to a surface due to the effect of electrons tunneling through the vacuum (known as the “forbidden zone”) between them.
This gives the device the sensitivity necessary to make extremely high resolution images. It also enables the device to move atoms by deliberately putting a current through the conducting tip.