Quantum theory is a branch of physics that describes the behavior of matter and energy on the atomic and subatomic scale. It is a fundamental theory of nature that is based on the idea that particles, such as electrons and photons, can exist in multiple states at the same time, until they are observed or measured.

One of the key concepts in quantum theory is the wave-particle duality. This means that particles, such as electrons, can exhibit both wave-like and particle-like behavior. This is different from classical physics, where objects are either particles or waves, but not both at the same time.

Another important concept in quantum theory is superposition. This refers to the idea that particles can exist in multiple states at the same time until they are observed. For example, an electron can exist in multiple energy states simultaneously, but when it is observed, it will collapse into a single state.

Quantum theory also introduces the concept of entanglement, which occurs when two particles become correlated in such a way that the state of one particle depends on the state of the other, regardless of the distance between them.

These and other concepts in quantum theory have been confirmed by numerous experiments, and they have revolutionized our understanding of the nature of matter and energy. Quantum theory has many practical applications, including in the development of new technologies such as quantum computing and cryptography.