Hardware efficient quantum algorithms for vibrational structure calculations

Abstract

Vibrational spectroscopy is at the heart of a wide range of applications from astronomy to medicine as it provides information about molecular structures and dynamics. Its modelization requires going beyond electronic structure calculations and include quantization of the bosonic vibrational modes. Despite the importance of such calculations most interesting systems are still intractable using state-of-the-art computing models. However, with the rise of quantum computers there are now new opportunities to solve this problem with a more favorable scaling. We introduce a framework for the calculation of ground and excited state energies of bosonic systems suitable for near-term quantum devices and apply it to molecular vibrational anharmonic Hamiltonians. Our method supports generic reference modal bases and Hamiltonian representations, including the ones that are routinely used in classical vibrational structure calculations. We test different parametrizations of the vibrational wavefunction, which can be encoded in quantum hardware, based either on heuristic circuits or on the bosonic Unitary Coupled Cluster Ansatz. We also define a novel compact heuristic circuit and demonstrate that it provides a good compromise in terms of circuit depth, optimization costs, and accuracy. We evaluate the requirements, number of qubits and circuit depth, for the calculation of vibrational energies on quantum hardware and compare them with state-of-the-art classical vibrational structure algorithms for molecules with up to seven atoms.