Quantum state
A quantum state is the complete description of a quantum system, containing everything that can be predicted about the results of measurements on it. A pure state carries maximal information and is represented by a state vector, equivalently a wave function, in a complex vector space called Hilbert space. A mixed state describes a statistical ensemble, or a system entangled with its surroundings, and is represented by a density operator.
Pure and mixed states
A pure state can still give random measurement outcomes because of quantum superposition, yet it is completely specified. A mixed state reflects classical uncertainty about which pure state the system is in, and cannot in general be written as a single state vector. In quantum computing the state of a Qubit is a two-dimensional quantum state, and many qubits together span an exponentially large state space. The distinction between pure and mixed states is essential to describing entanglement and decoherence.
Sources
- Quantum Mechanics (Stanford Encyclopedia of Philosophy) (Stanford Encyclopedia of Philosophy, 2021)
- Quantum Entanglement and Information (Stanford Encyclopedia of Philosophy) (Stanford Encyclopedia of Philosophy, 2019)
Cite this entry
"Quantum state." postquantum.wiki. Updated July 11, 2026. https://postquantum.wiki/quantum-state@misc{pqwiki-quantum-state,
title = {Quantum state},
howpublished = {\url{https://postquantum.wiki/quantum-state}},
year = {2026},
note = {postquantum.wiki, updated 2026-07-11}
}