# How useful is entanglement entropy?

In thermodynamics, the entropy $S$ can be used to determine various physical quantities, i.e. the free energy, the temperature etc.

If I calculate the entanglement entropy of a system, is there some analogous quantities that I can derive? Do I know anything more about the full system than I did before, having calculated the entanglement entropy of some sub-system? (Other than the amount of entanglement between the subsystems)

Conceptually, what do I know if I have calculated the EE?

• The entanglement between the parts of the system, given the total state is pure. Mar 21, 2017 at 15:36
• @NorbertSchuch, what does the entanglement entropy represent if the total system is in a mixed state? Dec 14, 2017 at 16:38
• @AndreaPaco If you have a new question you should ask a new question. Dec 14, 2017 at 16:45
• @NorbertSchuch, well, in your previous comment you explained the meaning of EE specifying the assumption "given the total state is pure". I am curious to know the answer also in case the total state is mixed. Do you think that this issue deserve a stand-alone question? If so, no problem. Just let me know. Dec 14, 2017 at 16:52
• @AndreaPaco The vague answer is "it measures classical entropy of the subsystem, classical correlations with the other system, and entanglement". Dec 14, 2017 at 17:21