Constrained Variational Quantum Eigensolver: Quantum Computer Search Engine in the Fock Space

Variational quantum eigensolver (VQE) is an efficient computational method promising chemical accuracy in electronic structure calculations on a universal-gate quantum computer. However, such a simple task as computing the electronic energy of a hydrogen molecular cation, H-2(+), is not possible for a general VQE protocol because the calculation will invariably collapse to a lower energy of the corresponding neutral form, H-2. The origin of the problem is that VQE effectively performs an unconstrained energy optimization in the Fock space of the original electronic problem. We show how this can be avoided by introducing necessary constraints directing VQE toward the electronic state of interest. The proposed constrained VQE can find an electronic state with a certain number of electrons, a certain spin, or any other property. Moreover, the new algorithm naturally removes unphysical kinks in potential energy surfaces (PESs), which frequently appeared in the regular VQE and required significant additional quantum resources for their removal. We demonstrate the performance of the constrained VQE by simulating PESs of various states of H-2 and H2O on Rigetti Computing Inc.'s 19Q-Acorn quantum processor.