Characterizing Excited States of a Copper-Based Molecular Qubit Candidate with Correlated Electronic Structure Methods

Abstract

Molecular spins have a variety of potential advantages as qubits for quantum computation, such as tunability and well-understood design pathways through organometallic synthesis. Organometallic and heavy-metal-based molecular spin qubits can also exhibit rich electronic structures due to ligand field interactions and electron correlation. These features make consistent and reliable modeling of these species a considerable challenge for contemporary electronic structure techniques. Here, we elucidate the electronic structure of a Cu­(II) complex analogous to a recently proposed room-temperature molecular spin qubit. Using active space methods to describe the electron correlation, we show the nuanced interaction between the metal d orbitals and ligand σ and π orbitals makes these systems challenging to model, both in terms of the delocalized spin density and the excited state ordering. We show that predicting the correct spin delocalization requires special consideration of the Cu d orbitals and that the excited state spectrum for the Cu­(III) complex also requires the explicit inclusion of the π orbitals in the active space. These interactions are rather common in molecular spin qubit motifs and may play an important role in spin-decoherence processes. Our results may lend insight into future studies of the orbital interactions and electron delocalization of similar complexes.

Physical Chemistry of Quantum Information Science

“…K. Asnaashari et al describe general approaches to encoding qubits in ultracold polar molecules. A. W. Schlimgen et al investigate the electronic structure of organometallic molecular spin qubits, addressing challenges in modeling due to their complex electronic properties. Furthermore, J. H. Zhang et al present a technique for enhancing accuracy in computing electronic structure energies for large molecular systems by fragmenting the system into overlapping parts, enabling asynchronous processing on classical and quantum hardware.…”