Heisenberg limited multiple eigenvalue estimation via off-the-grid compressed sensing

Abstract: Quantum phase estimation is the flagship algorithm for quantum simulation on fault-tolerant quantum computers. We demonstrate that an \emph{off-grid} compressed sensing protocol, combined with a state-of-the-art signal classification method, enables the simultaneous estimation of multiple eigenvalues of a unitary matrix using the Hadamard test while sampling only a few percent of the full autocorrelation function. Our numerical evidence indicates that the proposed algorithm achieves the Heisenberg limit in both strongly and weakly correlated regimes and requires very short evolution times to obtain an $\epsilon$-accurate estimate of multiple eigenvalues at once. Additionally -- and of independent interest -- we develop a modified off-grid protocol that leverages prior knowledge of the underlying signal for faster and more accurate recovery. Finally, we argue that this algorithm may offer a potential quantum advantage by analyzing its resilience with respect to the quality of the initial input state.