Host Matrix Engineering of Optically Addressable Molecular Qubits*

Optically addressable molecular spin qubits show promise as tunable, portable, and scalable platforms for quantum sensing [1]. Chemical synthesis methods provide opportunities for bottom-up design of these qubits, a challenge in solid-state spins and quantum dots. In this work, we demonstrate the enhancement of spin coherence and optical contrast properties of chromium (IV)-based molecular qubits through engineering of the host matrix. We detect multiple orientations of the chromium (IV) site through optically detected magnetic resonance. We also measure the optical linewidth of these qubits through all-optical spin sublevel control. We demonstrate improved spin coherence in a non-isostructural host matrix with a significant transverse zero-field splitting which leads to energy levels robust to magnetic-field noise. We model the coherence properties using cluster correlation expansion methods and demonstrate agreement with experimental coherence measurements for four different molecular qubits with varying transverse zero-field splitting values. These host-matrix-engineered molecular systems provide an avenue for coherence-protected sensing in noisy environments.