Efficient Tomography of Non-Interacting Fermion States

We give an efficient algorithm that learns a non-interacting fermion state, given copies of the state. For a system of $n$ non-interacting fermions and $m$ modes, we show that $O(m^3 n^2 \log(1/\delta) / \epsilon^4)$ copies of the input state and $O(m^4 n^2 \log(1/\delta)/ \epsilon^4)$ time are sufficient to learn the state to trace distance at most $\epsilon$ with probability at least $1 - \delta$. Our algorithm empirically estimates one-mode correlations in $O(m)$ different measurement bases and uses them to reconstruct a succinct description of the entire state efficiently.