Verifiably Exact Solution of the Electronic Schrödinger Equation on Quantum Devices

Abstract: Quantum computers have the potential for an exponential speedup of classical molecular computations. However, existing algorithms have limitations; quantum phase estimation (QPE) algorithms are intractable on current hardware while variational quantum eigensolvers (VQE) are dependent upon approximate wave functions without guaranteed convergence. In this Article we present an algorithm that yields verifiably exact solutions of the many-electron Schr\"odinger equation. Rather than solve the Schr\"odinger equation directly, we solve its contraction over all electrons except two, known as the contracted Schr\"odinger equation (CSE). The CSE generates an exact wave function ansatz, constructed from a product of two-body-based non-unitary transformations, that scales polynomially with molecular size and hence, provides a potentially exponential acceleration of classical molecular electronic structure calculations on ideal quantum devices. We demonstrate the algorithm on both quantum simulators and noisy quantum computers with applications to H${2}$ dissociation and the rectangle-to-square transition in H${4}$. The CSE quantum algorithm, which is a type of contracted quantum eigensolver (CQE), provides a significant step towards realizing verifiably accurate but scalable molecular simulations on quantum devices.