Efficient quantum circuits for quantum computational chemistry
This work introduces the concept of a "qubit excitation", which compared to a fermionic excitation, does not account for fermionics anti-commutation relations, and constructs circuits, optimized in terms of CNOT gates, that perform single and double qubit excitations.
Abstract
Molecular quantum simulations with the variational quantum eigensolver (VQE) rely on ansatz states that approximate the molecular ground states. These ansatz states are generally defined by parametrized fermionic excitation operators and an initial reference state. Efficient ways to perform fermionic excitations are vital for the realization of the VQE on noisy intermediate-scale quantum computers. Here, we address this issue by first demonstrating circuits that perform qubit excitations, excitations that do not account for fermionic anticommutation relations. We then extend the functionality of these circuits to perform fermionic excitations. Compared to circuits constructed with the standard use of "$CNOT$ staircases", our circuits offer a linear reduction in the number of $CNOT$ gates, by a factor of $2$ and $8$ per single and double excitation, respectively. Our results reduce the requirements for near-term realizations of quantum molecular simulations.