Abstract

Quantum Subspace Methods (QSMs) have recently gained a lot of interest, in particular in the NISQ regime as a promising alternative to variational quantum algorithms. In this talk, we present recent work in which a QSM is used to quantum compute the Green’s function, with encouraging results for the Hubbard model and Dynamical Mean Field Theory. To initialise this approach, as with most QSMs, an initial state preparation is necessary, and to this end we also present a technique to prepare a selected linear combination of Slater determinants on a quantum circuit. This allows for an intuitive way to introduce quantum chemical multireference techniques into quantum computational algorithms, and when applied to a QSM involving Hamiltonian moments to 4th order, results show a surprising impact on accuracy when evaluating the ground state energy of a strongly correlated molecule, despite a low overhead of preparing initial states with accuracy beyond single-reference Hartree-Fock.