The term _quantum system_ describes any physical system that we may model using [quantum mechanics.](Quantum%20Mechanics%20(index).md) A distinct quantum system may contain either [single](Quantum%20mechanical%20particles.md) or [multiple](Quantum%20mechanical%20particles.md#Multi-particle%20systems) particles. Whether or not something is considered a quantum system depends on whether or not it approaches the [classical limit,](Correspondence%20Principle.md#Classical%20limit) i.e. whether the scale of the system is small enough to be modeled quantum mechanically. %%This is vague and unhelpful. what is small enough? is it related to energies being quantized? some distance approaching a debroiglie wavelength? there are mesoscopic systems as well where QM phenomena manifests n larger systems. What about coherent states?%%   # Mathematical Representations of quantum Systems * [state vectors](State%20vector.md) * [wavefunctions](Wavefunction.md) * [density matrices](density%20matrix.md) # Composite quantum systems %%The term "quantum system" isn't often defined in textbooks. But here are textbooks where it is defined: - Peres A. _Quantum Theory Concepts and methods_ pg. 24 - Woit's _Quantum Theory, Groups and Representations,_ Defines quantum systems in terms of groups and representations on pg 10. Are all composite systems "open" quantum systems? Papers to see: - Hartle, J. B. Quantum Mechanics of Individual Systems%% #QuantumMechanics/FoundationsOfQuantumMechanics #QuantumMechanics/QuantumStateRepresentations