Creating and controlling spin qubits through molecular engineering

Spin-based defects within semiconductors are used to construct quantum devices and machines that enable information processing and sensing technologies based on the quantum nature of electrons and atomic nuclei. These materials possess an electronic spin state that can be employed as a quantum bit over a range of temperatures. They have a built-in optical interface in the visible and telecom bands, retain their quantum properties over millisecond timescales or longer, and can be manipulated using a simple combination of light and microwaves. Alternatively, molecular spin systems are attractive building blocks for quantum information science. Through a chemical approach, bottom-up design of qubits enables atomistic tunability of their environment, scalability to multi-qubit architectures using directed assembly, and portability between various host materials and devices. With these functionalities-which are challenging to achieve with semiconductor defects-designer qubits could be synthesized for a diverse range of applications from quantum sensing in biosystems to the creation of nodes in a quantum network. To demonstrate that the enabling features of solid-state defects can be translated into a molecular architecture, we discuss transition metal-based molecular spins with optical addressability. These molecules comprise a central chromium ion coordinated to surrounding ligands, enabling optical initialization and read out, as well as coherent microwave manipulation of the ground-state spin. We also show atomistic tunability of qubit properties by comparing molecules which differ by the placement of a single methyl group on the coordinating ligands, resulting in marked changes in emission and ground state energies, highlighting how molecular qubits can be tailored for specific applications. Overall, this work indicates the potential of a chemical approach for quantum engineering with optically addressable spin systems.