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The microscopic origin of thermodynamic entropy in isolated systems

A microscopic understanding of the thermodynamic entropy in quantum systems has been a mystery ever since the invention of quantum mechanics. In classical physics, this entropy is believed to be the logarithm of the volume of phase space accessible to an isolated system [1]. There is no quantum mechanical analog to this. Instead, Von Neumann's hypothesis for the entropy [2] is most widely used. However this gives zero for systems with a known wave function, that is a pure state. This is because it measures the lack of information about the system rather than the flow of heat as obtained from thermodynamic experiments. Many arguments attempt to sidestep these issues by considering the system of interest coupled to a large external one, unlike the classical case where Boltzmann's approach for isolated systems is far more satisfactory. With new experimental techniques, probing the quantum nature of thermalization is now possible [3, 4]. Here, using recent advances in our understanding of quantum thermalization [5-10] we show how to obtain the entropy as is measured from thermodynamic experiments, solely from the self-entanglement of the wavefunction, and find strong numerical evidence that the two are in agreement for non-integrable systems. It is striking that this entropy, which is closely related to the concept of heat, and generally thought of as microscopic chaotic motion, can be determined for systems in energy eigenstates which are stationary in time and therefore not chaotic, but instead have a very complex spatial dependence.