Can somebody clear the fog in front of my eyes: how can a Hamiltonian describing free fermions have a ground state with non-trivial entanglement? My reasoning is: we can build the ground state in the independent electron approximation (which is now exact), hence it is a product state in the position base, hence trivial entanglement!(adsbygoogle = window.adsbygoogle || []).push({});

However, this seems far from true. (An extreme counter-example: the ground state of QHE even has long-range entanglement...)

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# How can a ground state of free particles have entanglement?

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