Simple algorithms to test and learn local Hamiltonians

Abstract

We consider the problems of testing and learning an $n$-qubit $k$-local Hamiltonian from queries to its evolution operator with respect the 2-norm of the Pauli spectrum, or equivalently, the normalized Frobenius norm. For testing whether a Hamiltonian is $ε_1$-close to $k$-local or $ε_2$-far from $k$-local, we show that $O(1/(ε_2-ε_1)^{8})$ queries suffice. This solves two questions posed in a recent work by Bluhm, Caro and Oufkir. For learning up to error $ε$, we show that $\exp(O(k^2+k\log(1/ε)))$ queries suffice. Our proofs are simple, concise and based on Pauli-analytic techniques.