Quantum physics is a fundamental theory about how the universe works. It explains a broad range of observations and experimental results. It does this in an extremely detailed, accurate and precise way.
Phenomena that can only be described with quantum theory are most apparent in atomic, nuclear and particle physics, as well as optics, chemistry and material science (also known as condensed matter physics or solid state physics). This contrasts with physics' only other fundamental theory, general relativity, which describes physical systems in the vicinity of strong gravitational fields.
Quantum physics was mathematically formulated by Paul Dirac and John von Neumann. The postulates of the theory relate to: the quantum state of a physical system; how it evolves in time; and what happens when measurements are performed. In the context of quantum computing, these correspond to: initializing the quantum computer; processing the encoded information; and reading out the results of the computation.
Barring general relativity, quantum physics is the theory upon which all others are based. In other words, at the lowest level of reality, the universe is quantum mechanical. Theories like material science, chemistry and optics are, in some sense, high-level versions of quantum physics.
This is important because modern life, with all its technological marvels, crucially relies upon its scientific underpinnings. For example, if we only had a Bronze Age understanding of the world, we would not have smartphones and advanced, life-saving medicines.
A whole range of technologies, from sensing to communication, are now being developed based on quantum phenomena. Most famously, quantum computers are physical devices that exploit superposition and entanglement to efficiently implement novel algorithms.
