Towards Quantum Chemistry on a Quantum Computer

TL;DR

This paper demonstrates a photonic implementation of quantum algorithms for calculating molecular energies, specifically H2, showing progress towards scalable quantum chemistry simulations on quantum computers.

Contribution

It presents the first photonic implementation of quantum phase estimation for molecular energy calculations and discusses how to overcome current non-scalable approaches.

Findings

Successful implementation of iterative phase estimation for H2

High precision and robustness achieved in the experiment

Theoretical groundwork laid for future scalable quantum chemistry simulations

Abstract

The fundamental problem faced in quantum chemistry is the calculation of molecular properties, which are of practical importance in fields ranging from materials science to biochemistry. Within chemical precision, the total energy of a molecule as well as most other properties, can be calculated by solving the Schrodinger equation. However, the computational resources required to obtain exact solutions on a conventional computer generally increase exponentially with the number of atoms involved. This renders such calculations intractable for all but the smallest of systems. Recently, an efficient algorithm has been proposed enabling a quantum computer to overcome this problem by achieving only a polynomial resource scaling with system size. Such a tool would therefore provide an extremely powerful tool for new science and technology. Here we present a photonic implementation for the smallest problem: obtaining the energies of H2, the hydrogen molecule in a minimal basis. We perform a key algorithmic step - the iterative phase estimation algorithm - in full, achieving a high level of precision and robustness to error. We implement other algorithmic steps with assistance from a classical computer and explain how this non-scalable approach could be avoided. Finally, we provide new theoretical results which lay the foundations for the next generation of simulation experiments using quantum computers. We have made early experimental progress towards the long-term goal of exploiting quantum information to speed up quantum chemistry calculations.