Quantum chemistry simulation of ground- and excited-state properties of
  the sulfonium cation on a superconducting quantum processor
The computational description of correlated electronic structure, and
particularly of excited states of many-electron systems, is an anticipated
application for quantum devices. An important ramification is to determine the
dominant molecular fragmentation pathways in photo-dissociation experiments of
light-sensitive compounds, like sulfonium-based photo-acid generators used in
photolithography. Here we simulate the static and dynamic electronic structure
of the H$_3$S$^+$ molecule, taken as a minimal model of a triply-bonded sulfur
cation, on a superconducting quantum processor of the IBM Falcon architecture.
To this end, we generalize a qubit reduction technique termed entanglement
forging or EF [A. Eddins et al., Phys. Rev. X Quantum, 3, 010309 (2022)],
currently restricted to the evaluation of ground-state energies, to the
treatment of molecular properties. While, in a conventional quantum simulation,
a qubit represents a spin-orbital, within EF a qubit represents a spatial
orbital, reducing the number of required qubits by half. We combine the
generalized EF with quantum subspace expansion [W. Colless et al, Phys. Rev. X
8, 011021 (2018)], a technique used to project the time-independent Schrodinger
equation for ground and excited states in a subspace. To enable experimental
demonstration of this algorithmic workflow, we deploy a sequence of
error-mitigation techniques. We compute dipole structure factors and partial
atomic charges along the ground- and excited-state potential energy curves,
revealing the occurrence of homo- and heterolytic fragmentation. This study is
an important step toward the computational description of photo-dissociation on
near-term quantum devices, as it can be generalized to other photodissociation
processes and naturally extended in different ways to achieve more realistic
simulations.
Mario Motta Gavin O. Jones Julia E. Rice Tanvi P. Gujarati Rei Sakuma Ieva Liepuoniute Jeannette M. Garcia Yu-ya Ohnishi