Dynamic ligand fields of molecular qubits: Decoherence through spin-phonon coupling

Abstract

Quantum coherence of S = 1/2 transition metal-based quantum bits (qubits) is often governed by the coupling of electron spin to mol. vibrations. Yet, a general model for understanding coherence of mol. qubits from fundamental chem. principles has not been established. Here a general ligand field theory model is outlined to describe coherence properties of S = 1/2 transition metal complexes. We show that the spin-phonon coupling term for a given vibrational mode is related to the coherence of the complex, and is dependent on: 1) the identity of the metal, 2) the magnitude and gradient in the ligand field excited state energy, and 3) the covalency of the ligand-metal bonds. From an extensive series of d. functional theory (DFT) and time-dependent DFT calcns. calibrated to a range of exptl. data, we show that spin phonon coupling terms of minimalistic high symmetry complexes of Cu(II) and V(IV) translate to and correlate with exptl. obsd. quantum coherence properties of modern mol. qubits of different ligand sets, geometries, and coordination nos. The results can be used to est. the coherence lifetime of any S = 1/2 mol. qubit candidate and guide the discovery of room temp. coherent materials for quantum information processing.