Quantum Mechanics was one of the great revolutions in 20th century physics. It deals, ostensibly with the very small. That is atomic scale or smaller. However there are also macroscopic manifestations such as superconductivity and superfluidity.
In the 1920's two independent but equivalent formulations of the theory were developed.
- Matrix mechanics - by Heisenberg
- Wave mechanics - by Schrödinger
Some of the basic principles of Quantum Mechanics can be understood from an analysis of 2-level systems.