We have demonstrated the optical addressability and coherent control of triplet ground-state spins in a series of transition-metal-based molecules. In collaboration with the Freedman Group (MIT), we designed molecular complexes with a central chromium ion coordinated by strong-field ligands in a high symmetry pseudo tetrahedral configuration. These ions have readily addressable microwave-frequency zero-field splittings in the ground state as well as a spin-selective optical interface for initialization and readout. We confirmed the desired energy-level structure of the qubit through magnetic-field-dependent photoluminescence and electron spin resonance (ESR) measurements. We measured optical lifetimes that are much shorter than the spin-lattice relaxation time, as required for optical spin addressability. We performed optically detected magnetic resonance as a function of magnetic field and microwave drive, measuring a zero field splitting and g-factor consistent with ESR measurements. We demonstrated coherent control of the spin state, enabling us to measure the Hahn echo spin coherence time (approaching a microsecond). Finally, we demonstrated that through atomistic chemical control of the molecular structure, we can modify the spin and optical properties of the molecular qubit, leading to shifts in the zero phonon line and changes in the longitudinal and transverse zero-field splitting parameters. These molecular spin qubits show promise for bottom-up chemical design aimed at specific applications in various fields of quantum information processing.

To learn more, please see: S. L. Bayliss, D. W. Laorenza, P. J. Mintun, B. D. Kovos, D. E. Freedman, and D. D. Awschalom, Science 370, 1309 (2020).