Linking Chemical Structure and Coherence Times of Molecular Spin Qubits

The electron spin is a natural qubit platform and, when embedded in a molecule, its properties can be tailored via synthetic chemistry. However, the quantum properties can be utilised only for a limited time and lengthening this coherence time is still challenging. Nuclear spin dynamics is one of the principal sources of decoherence, but a complete removal of spin-bearing nuclei is hardly feasible in practice and therefore strategies how to minimise their negative effects are essential. Despite considerable efforts to screen the chemical space for improved molecular spin qubits as well as advances in simulation techniques to understand the physical details of the decoherence processes, a broader picture bringing together these two worlds is still lacking. In this work, we fill this gap by analysing the decoherence induced by a nuclear spin bath of protons through a parameter space screening approach based on the analytic pair product approximation. The analysis reveals the geometric and statistical conditions that lead to strong contributions to decoherence and enables us to identify the nuclear pairs that are most relevant to decoherence effects. This allows to devise new design rules for improved molecular spin quantum bits. As specific cases, we discuss [Cu(dbm)2] and (PPh4)2[Cu(mnt)2] diluted in a non-magnetic crystalline matrix and show systematically how these systems can be improved.