Exploiting subspace constraints and ab initio variational methods for quantum chemistry

Abstract

Variational methods offer a highly promising route to exploiting quantum computers for chemistry tasks. Here we employ methods described in a sister paper to the present report, entitled ab initio machine synthesis of quantum circuits, in order to solve problems using adaptively evolving quantum circuits. Consistent with prior authors we find that this approach can outperform human-designed circuits such as the coupled-cluster or hardware-efficient ansätze, and we make comparisons for larger instances up to 14 qubits. Moreover we introduce a novel approach to constraining the circuit evolution in the physically relevant subspace, finding that this greatly improves performance and compactness of the circuits. We consider both static and dynamics properties of molecular systems. The emulation environments used is QuESTlink; all resources are open source and linked from this paper.

Exploring ab initio machine synthesis of quantum circuits

“…For this task, the evaluation of the cost function is straightforwardly replaced by the Hamiltonian of interest. This idea, among other applications, is explored in the sister paper to the present work [68]. Energy increase considered acceptable for the removal of unnecessary gates.…”

“…Prominently, this is the case for time evolution operators in quantum chemistry, where the number of electrons is a conserved and known quantity, and corresponds to the Hamming weight of the states. The sister paper to the present work [68] explores this application in greater detail.…”