Friedel Weinert
Technological Applications of Quantum Mechanics
The great American physicist Richard Feynman (1918-1988) once claimed that nobody understood Quantum Mechanics (QM). He did not mean the mathematics but the strange and weird phenomena, which are associated with the theory. These bizarre phenomena cannot be explained by reliance on classical physics. If one tries to explain the behaviour of atomic and subatomic particles by reference to classical mechanics, one gets the wrong answers. That is, the answers do not correspond to the measurable and observable behaviour of quantum particles. “The world of an atom looks nothing like what we think of when we visualize matter” writes Harvard physicist Lisa Randall.[1] Hence physicists were forced to develop a new kind of physics: Quantum Theory or the physics of atomic behaviour. (Atoms are systems, which consists of a nucleus – plural: nuclei – surrounded by electron clouds. The nuclei themselves are made up of positively charged protons and neutral neutrons.) This new theory was developed over several decades, from 1912 to 1925. It is now one of the most successful scientific theories that have ever been invented. As with the General Theory of Relativity the mathematics is complicated but the phenomena can be readily understood. Until the early 1990s Quantum Mechanics was seen as an arcane scientific activity. Then, suddenly, the prospect of technological applications emerged. This article will focus on two applications of QM: lasers and Quantum Information (i.e. quantum cryptography, teleportation, and quantum computation). The working of lasers can be explained by considering the manipulations of photons, i.e. massless particles of light. Quantum Information requires a consideration of unfamiliar states of quantum particles, best described as entanglement and superposition.
- Lasers.
A laser is essentially an optical amplifier. The term laser is an acronym that stands for light amplification by the stimulated emission of radiation. The key terms are amplification and stimulated emission. The technological development of lasers began in the 1950s, but Albert Einstein (1917) laid the theoretical foundations.
Einstein wished to understand how matter interacted with electromagnetic radiation. He distinguished three processes:
- stimulated (or induced) absorption occurs when an atom absorbs a photon;
- spontaneous emission occurs when an atom in an excited state emits a photon of a certain amount of energy;
- the last process, stimulated (or induced) emission, is central for the workings of a laser (fig. 1a).
Stimulated emission was a previously undiscovered form of radiation. In this process an atom is in an excited state. A passing photon (of the right energy) induces the atom to emit a photon and to make a transition to a lower energy state. Two photons emerge (fig. 1b). They can interact with other excited atoms in the system, inducing more excited atoms to release photons. They are all i