Abstract

Co-authors: Alberto Peruzzo (University of Sydney), Peter Shadbolt (University of Bristol), Man-Hong Yung (Tsinghua University), Xiao-Qi Zhou (University of Bristol), Peter Love (Haverford College), Alan Aspuru-Guzik (Harvard University), Jeremy O’Brien (University of Bristol)

Quantum computers promise to efficiently solve important problems that are intractable on a conventional computer. For quantum systems, where the dimension of the problem space grows exponentially, finding the eigenvalues of certain operators is one such intractable problem and remains a fundamental challenge. The quantum phase estimation algorithm can efficiently find the eigenvalue of a given eigenvector but requires fully coherent evolution. We present an alternative approach that greatly reduces the requirements for coherent evolution and we combine this method with a new approach to state preparation based on ans”atze and classical optimization. We have implemented the algorithm by combining a small-scale photonic quantum processor with a conventional computer. We experimentally demonstrate the feasibility of this approach with an example from quantum chemistry: calculating the ground state molecular energy for He-H+, to within chemical accuracy. The proposed appro ach, by drastically reducing the coherence time requirements, enhances the potential of the quantum resources available today and in the near future.