Computational Insights into Electronic Excitations, Spin-Orbit Coupling Effects, and Spin Decoherence in Cr(IV)-based Molecular Qubits

TL;DR

This paper uses computational methods to analyze electronic states, spin interactions, and decoherence in Cr(IV)-based molecular qubits, providing insights for designing more efficient quantum information molecules.

Contribution

It offers detailed computational analysis of electronic and spin properties of Cr(IV) molecules, highlighting how chemical modifications affect qubit performance.

Findings

Zero-phonon line energies match experimental data.

Excited spin states are highly sensitive to chemical changes.

Transverse ZFS reduces hyperfine-induced decoherence.

Abstract

The great success of point defects and dopants in semiconductors for quantum information processing has invigorated a search for molecules with analogous properties. Flexibility and tunability of desired properties in a large chemical space have great advantages over solid-state systems. The properties analogous to point defects were demonstrated in Cr(IV)-based molecular family, Cr(IV)(aryl)$_4$, where the electronic spin states were optically initialized, read out, and controlled. Despite this kick-start, there is still a large room for enhancing properties crucial for molecular qubits. Here we provide computational insights into key properties of the Cr(IV)-based molecules aimed at assisting chemical design of efficient molecular qubits. Using the multireference ab-initio methods, we investigate the electronic states of Cr(IV)(aryl)$_4$ molecules with slightly different ligands, showing that the zero-phonon line energies agree with the experiment, and that the excited spin-triplet and spin-singlet states are highly sensitive to small chemical perturbations. By adding spin-orbit interaction, we find that the sign of the uniaxial zero-field splitting (ZFS) parameter is negative for all considered molecules, and discuss optically-induced spin initialization via non-radiative intersystem crossing. We quantify (super)hyperfine coupling to the $^{53}$Cr nuclear spin and to the $^{13}$C and $^1$H nuclear spins, and we discuss electron spin decoherence. We show that the splitting or broadening of the electronic spin sub-levels due to superhyperfine interaction with $^1$H nuclear spins decreases by an order of magnitude when the molecules have a substantial transverse ZFS parameter.