Quantum-optimal-control-inspired ansätze for variational quantum algorithms

Abstract

A central component of variational quantum algorithms (VQA) is the state-preparation circuit, also known as ansatz. This circuit is most commonly designed to respect the symmetries of the problem Hamiltonian and, in this way, constrain the variational search to a subspace of interest. Here, we show that this approach is not always advantageous by introducing ansätze that incorporate symmetry-breaking unitaries. This class of ansätze, that we call Quantum-Optimal-Control-inspired Ansätze (QOCA), is inspired by the theory of quantum optimal control and leads to an improved convergence of VQAs for some important problems. Indeed, we benchmark QOCA against popular ansätze applied to the Fermi-Hubbard model at half-filling and show that our variational circuits can approximate the ground state of this model with significantly higher accuracy and for larger systems. We also show how QOCA can be used to find the ground state of the water molecule and compare the performance of our ansatz with other common choices used for chemistry problems. This work was recently published under arXiv:2008.01098. *We thank funding from NSERC, the Canada First Research Excellence Fund and the U.S. Army Research Office Grant No. W911NF-18-1-0411.