Quantum physics is the branch of physics used to describe matter, light, and interactions at atomic and subatomic scales. Its central mathematical framework is quantum mechanics. At these scales, classical intuition is often insufficient: physical systems are described by quantum states, measurable quantities can be quantized, and predictions are fundamentally probabilistic.

What does “quantized” mean?

Some physical quantities in a bound quantum system can take only particular allowed values. An electron in an atom, for example, occupies allowed energy states rather than any arbitrary energy. When a system changes between levels, it exchanges a corresponding amount of energy.

The word quantum refers to a discrete amount associated with a physical process. A photon is the quantum of electromagnetic radiation.

Particles can show wave-like behavior

Experiments show that objects such as electrons and photons can produce interference patterns associated with waves, while also being detected in localized events associated with particles. Quantum mechanics describes this behavior with a wave function or quantum state rather than forcing microscopic objects into an exclusively classical particle-or-wave picture.

Superposition and measurement

A quantum state can be a superposition of different possible outcomes. The theory uses probability amplitudes to predict the chance of obtaining each result when a measurement is performed. Quantum interference arises because amplitudes—including their phase—combine before probabilities are calculated.

Measurement is one of the places where quantum theory differs most sharply from everyday intuition. The mathematical rules predict probabilities with extraordinary accuracy, even though a single measurement has a definite outcome.

Entanglement

Entangled quantum systems share a joint state that cannot be fully described as independent states for each part. Measurements can show correlations stronger than classical local models allow. Entanglement is experimentally established and is used as a resource in quantum information science, but it does not provide a way to send usable information faster than light.

Quantum physics is already part of everyday technology

Quantum mechanics underlies the behavior of atoms, chemical bonds, semiconductors, lasers, LEDs, and many forms of modern sensing. It is not only a theory for exotic laboratory experiments. Quantum computing is a newer engineering application that manipulates quantum information units called qubits.

Where quantum theory is incomplete

Quantum mechanics and quantum field theory successfully describe a vast range of microscopic phenomena, while general relativity describes gravity and spacetime at large scales. A complete tested theory that unifies gravity with quantum physics remains an open problem.

Sources and further reading

Quantum mechanics in plain terms

Quantum mechanics is the framework physicists use to describe matter and light at atomic and subatomic scales. Instead of assigning every particle a single classical path with perfectly defined properties, the theory represents a physical system with a quantum state that determines probabilities for possible measurement outcomes.

Several ideas distinguish quantum mechanics from everyday classical physics. Quantum states can exist in superpositions, measurement outcomes are probabilistic, particles such as electrons show wave-like interference, and separated systems can share entangled correlations. The uncertainty principle also places fundamental limits on how sharply certain pairs of quantities can be specified at the same time.

Why quantum mechanics matters beyond the laboratory

Quantum rules explain atomic structure, chemical bonding, semiconductor electronics, lasers and many properties of materials. Quantum computing is a newer application, but it is only one branch of a much broader theory. The same mathematical framework underlies technologies already used in communication, sensing, imaging and modern electronics.