Witnessing eigenstates for quantum simulation of Hamiltonian spectra by Santagati, R, Wang, J, Gentile, AA, Paesani, S, Wiebe, N, McClean, JR, Morley-Short, S, Shadbolt, PJ, Bonneau, D, Silverstone, JW, Tew, DP, Zhou, X, O'Brien, JL, Thompson, MG