Hilbert-Schmidt norm estimates for fermionic reduced density matrices

We prove that the Hilbert-Schmidt norm of $k$-particle reduced density matrices of $N$-body fermionic states is bounded by $C_k{N}^{k\mathrm{/}2}$ - matching the scaling behaviour of Slater determinant states. This generalises a recent result of Christiansen (2024) on 2-particle reduced density matrices to higher-order density matrices. Moreover, our estimate directly yields a lower bound on the von Neumann entropy and the 2-Rényi entropy of reduced density matrices, thereby providing further insight into conjectures of Carlen-Lieb-Reuvers (2016,2018).