Any [quantum system](Quantum%20systems.md) that is completely isolated from any other _quantum system_ is a _closed quantum system._ What this means is that for a closed quantum system there is no exchange of [information](Quantum%20Information%20(index).md) between that system and its surroundings. ^7e1b39 %%This really needs to be elaborated on - a system can be closed, but subject to external potentials.%% We postulate the following fundamental attributes of closed quantum systems:    # Physical realizations of closed quantum systems In physical realizations of quantum systems, this is an idealization. No quantum system is perfectly closed since _all_ quantum mechanical systems are subject to random interactions with their environment. However, sufficiently isolated quantum systems can be shown to behave as closed systems to within a certain precision and a given time-frame. %%Here at some point you might want to also bring up the universal wavefunction as a potential model for a truly closed quantum system. See discussion of closed quantum systems from Nielsen and Chuang. Discuss unitarity here! unitarity I think is only preserved for closed quantum systems. What about "within a certain precision"? Wouldn't any quantum interaction make something not be a closed system anymore?%% # Dynamics of closed quantum systems [Equations of motion for closed quantum systems](Equations%20of%20motion%20for%20closed%20quantum%20systems.md) #QuantumMechanics/FoundationsOfQuantumMechanics #QuantumMechanics/QuantumStateRepresentations