Target space entanglement in quantum mechanics of fermions at finite temperature

We consider the target space entanglement in quantum mechanics of non-interacting fermions at finite temperature. Unlike pure states investigated in arXiv:2105.13726, the (Rényi) entanglement entropy for thermal states does not follow a simple bound because all states in the infinite-dimensional Hilbert space are involved. We investigate a general formula of the target space Rényi entropy for $N$ fermions at finite temperature, and present numerical results of the entropy in a one-dimensional model. We also argue the large $N$ behaviors with a comparison to the grand canonical ensemble.